Phys203 Exam PDF

Petrology is the study of rocks, and a petrologist is someone who studies rocks.  These words are derived from the Greek root petro- for rock.  For example, the name Peter means rock.  A rock is defined as a naturally occurring solid inorganic object that is an aggregate (a mixture) of minerals.  Si...

Petrology is the study of rocks, and a petrologist is someone who studies rocks.  These words are derived from the Greek root petro- for rock.  For example, the name Peter means rock.  A rock is defined as a naturally occurring solid inorganic object that is an aggregate (a mixture) of minerals.  Since a rock is an aggregate (a mixture) of many different minerals, a rock does not have a definite chemical structure.  This is the most important difference between a rock and a mineral.  Both minerals and rocks are naturally occurring solid inorganic objects.  However, a mineral has a definite chemical structure, while a rock does not have a definite chemical structure, since a rock is an aggregate (a mixture) of minerals.  Since a rock does not have a definite chemical structure, there are significant variations in density, composition, and other properties even for a particular type of rock.  In other words, whereas a particular mineral is unique, a particular rock is not unique.  Petrography is the classification of rocks, the study of different types of rocks.  Petrogenesis is the study of how rocks form.  Broadly speaking, there are three different ways that rocks can form.  Hence, there are three broad categories of rocks: igneous rocks, sedimentary rocks, and metamorphic rocks. If we add sufficient heat to a rock, it melts to become molten rock (liquid rock).  If molten rock cools, it may crystallize (solidify) back into solid rock.  Any rock that forms from the crystallization of molten rock is called igneous rock, one of the three main types of rock.  We can classify igneous rocks based on where they form.  Molten rock deep within the Earth is called magma, while molten rock that has extruded out of the Earth is called lava.  We will discuss shortly a significant difference between molten rock deep within the Earth and molten rock that has extruded out of the Earth that justifies using these two different terms, magma and lava.  For now, we simply mention that different names are often used for the same thing in colloquial languages.  While they are living animals, chickens, turkeys, and ducks are called fowl, but after they have been slaughtered and prepared as food they are called poultry.  While they are living animals, bulls and cows are called cattle, but after they have been slaughtered and prepared as food they are called beef.  While they are living animals, pigs are called swine, but after they have been slaughtered and prepared as food they are called pork.  While they are living animals, deer, elk, moose, and reindeer are called cervids, but after they have been slaughtered and prepared as food they are called venison.  The Spanish word for living fish is pez, but the Spanish word for fish that is slaughtered and prepared as food is pescado.  There are countless other examples in colloquial languages.  Magma that crystallizes into solid rock is called intrusive igneous rock, since it forms deep within the Earth.  Lava that crystallizes into solid rock is called extrusive igneous rock, since it formed from lava that extruded out of the Earth.  Another term for intrusive igneous rock is plutonic igneous rock, and another term for extrusive igneous rock is volcanic igneous rock. Since they form deep within the Earth where it is hot, intrusive/plutonic igneous rocks typically take a long time to cool and crystallize.  This slower, more gradual cooling builds large crystals throughout the rock.  The result is a rock with a coarse-grained texture, meaning that it feels more rough to the touch.  Since they form from lava that has extruded out of the Earth, extrusive/volcanic igneous rocks typically take a short time to cool and crystallize.  This faster, more rapid cooling only permits small crystals to be built throughout the rock.  The result is a rock with a fine-grained texture, meaning that it feels more smooth to the touch.  The term for the coarse-grained texture of igneous rocks is phaneritic, since the Greek root phanero- means visible, meaning that the crystals in a phaneritic igneous rock are large enough to be visible with the naked (unaided) eye.  The term for the fine-grained texture of igneous rocks is aphanitic, since again the Greek root phanero- means visible and the Greek root a- means no or not in words such as apathy, asynchronous, and asymmetrical for example.  In other words, the crystals in an aphanitic igneous rock are too small to be visible with the naked (unaided) eye.  We require at least a magnifying glass, sometimes even a microscope, to see the crystals in an igneous rock with an aphanitic texture.  An igneous rock that takes an extremely long time to cool and crystalize would have an extremely coarse-grained texture.  This extreme form of the phaneritic texture is called pegmatitic.  An igneous rock can take an extremely short time to cool and crystalize.  This virtually instantaneous crystallization is called quenching.  In this case, the igneous rock has an extremely fine-grained texture.  This extreme form of the aphanitic texture is called glassy, since the rock feels as smooth as glass.  The extrusive/volcanic igneous rock obsidian has a glassy texture, since it forms by quenching.  Obsidian is often black in color, which together with its glassy texture makes obsidian a beautiful rock.  After cutting and polishing, obsidian is considered a type of gemstone (or gem).  The extrusive/volcanic igneous rock pumice also has a glassy texture, since it forms by quenching.  However, the abundance of vesicles (cavities) within pumice causes this rock to feel rough to the touch, even though it has a glassy texture.  The abundance of vesicles (cavities) within pumice also gives this rock a density that is usually less than liquid water, permitting this rock to float in liquid water.  In summary, igneous rocks that take an extremely long time to cool and crystallize have a pegmatitic (extremely coarse-grained) texture, igneous rocks that take a moderately long time to cool and crystallize have a phaneritic (moderately coarse-grained) texture, igneous rocks that take a moderately short time to cool and crystallize have an aphanitic (moderately fine-grained) texture, and igneous rocks that take an extremely short time to cool and crystallize (quenching) have a glassy (extremely fine-grained) texture.  Typically, extrusive/volcanic igneous rocks have an aphanitic texture, perhaps even glassy in extreme cases, since they take a short amount of time to cool and crystallize.  Typically, intrusive/plutonic igneous rocks have a phaneritic texture, perhaps even pegmatitic in extreme cases, since they take a long amount of time to cool and crystallize.  In brief, we can calculate the cooling history of an igneous rock (how long it took the rock to form) by simply feeling its texture, whether it feels more rough to the touch (coarse-grained texture) or more smooth to the touch (fine-grained texture).  Caution: some igneous rocks have both large crystals and small crystals in the same rock.  This occurs when there is an interruption in the cooling history of the rock.  This unusual texture is called porphyritic.  The large crystals in a porphyritic igneous rock are called the phenocrysts, while the small crystals in a porphyritic igneous rock are called the groundmass. We can also classify igneous rocks based on their mineral composition.  Since the vast majority of all minerals are silicates, we classify igneous rocks based on their silicate mineral composition.  Igneous rocks that are composed predominantly of dark silicates are called mafic igneous rocks.  The word mafic is a combination of the words magnesium and ferrum, the Latin word for iron.  As we discussed, dark silicates have the most amount of metals such as iron and magnesium.  Igneous rocks that are composed predominantly of light silicates are called felsic igneous rocks.  The word felsic is a combination of the words feldspar and silica, the term for molten quartz.  As we discussed, feldspar and quartz are two examples of light silicates.  Igneous rocks that are composed predominantly of intermediate silicates are simply called intermediate igneous rocks.  Everything we have discussed about silicate minerals therefore also applies to the igneous rocks that they compose.  In particular, mafic igneous rocks have the most amount of metals, are darkest in color, are most dense, and have the hottest melting temperatures, while felsic igneous rocks have the least amount of metals, are lightest in color, are least dense, and have the lowest melting temperatures.  Intermediate igneous rocks are between these two extremes.  The Bowen reaction series quantifies the melting-temperature spectrum for igneous rocks, named for the Canadian petrologist Norman L. Bowen who discovered this progression of melting temperatures for igneous rocks in the early twentieth century.  According to the Bowen reaction series, as we add more and more heat to igneous rocks, felsic rocks melt first, intermediate rocks then melt at somewhat hotter temperatures, and finally mafic rocks melt at the hottest temperatures.  Conversely, if we extract more and more heat from molten rock, mafic rocks crystallize first, intermediate rocks then crystallize at somewhat lower temperatures, and finally felsic igneous rocks crystallize at the lowest temperatures. The two most important mafic igneous rocks are basalt and gabbro.  Both basalt and gabbro have large quantities of metals, are dark in color, have high densities, and have hot melting temperatures.  The only difference between basalt and gabbro is where they form and thus their cooling histories and hence their textures.  Gabbro forms intrusively/plutonically deep within the Earth and therefore cools and crystallizes over a long period of time.  Thus, gabbro forms with large crystals resulting in a coarse-grained (rough) texture.  Basalt forms extrusively/volcanically on the surface of the Earth and therefore cools and crystallizes over a short period of time.  Thus, basalt forms small crystals resulting in a fine-grained (smooth) texture.  In other words, gabbro is the intrusive/plutonic form of basalt, or basalt is the extrusive/volcanic form of gabbro.  The two most important felsic igneous rocks are rhyolite and granite.  Both rhyolite and granite have small quantities of metals, are light in color, have low densities, and have low melting temperatures.  The only difference between rhyolite and granite is where they form and thus their cooling histories and hence their textures.  Granite forms intrusively/plutonically deep within the Earth and therefore cools and crystallizes over a long period of time.  Thus, granite forms with large crystals resulting in a coarse-grained (rough) texture.  Rhyolite forms extrusively/volcanically on the surface of the Earth and therefore cools and crystallizes over a short period of time.  Thus, rhyolite forms small crystals resulting in a fine-grained (smooth) texture.  In other words, granite is the intrusive/plutonic form of rhyolite, or rhyolite is the extrusive/volcanic form of granite.  The two most important intermediate igneous rocks are andesite and diorite.  Both andesite and diorite have intermediate quantities of metals, are intermediate in color, have intermediate densities, and have intermediate melting temperatures.  The only difference between andesite and diorite is where they form and thus their cooling histories and hence their textures.  Diorite forms intrusively/plutonically deep within the Earth and therefore cools and crystallizes over a long period of time.  Thus, diorite forms with large crystals resulting in a coarse-grained (rough) texture.  Andesite forms extrusively/volcanically on the surface of the Earth and therefore cools and crystallizes over a short period of time.  Thus, andesite forms small crystals resulting in a fine-grained (smooth) texture.  In other words, diorite is the intrusive/plutonic form of andesite, or andesite is the extrusive/volcanic form of diorite. Rocks on the surface of the Earth are subjected to wind, rain, and other natural forces that degrade (weaken and destroy) the rocks, ultimately breaking them into small pieces called sediments.  These natural forces also move these sediments from one location to another.  Layer upon layer of sediments may accumulate at a particular location until the weight of the sediments begins to compress the sediments.  Chemical reactions may cement the sediments together.  Eventually, the sediments become lithified, meaning that they have become rock.  Any rock that forms from the lithification of sediment is called sedimentary rock, one of the three main types of rock.  Sedimentary rocks are classified into three subcategories: clastic sedimentary rocks, chemical sedimentary rocks, and biogenic sedimentary rocks.   A clastic sedimentary rock lithifies through the action of physical forces.  If large sediments are lithified to form a clastic sedimentary rock, then the rock will have a coarse-grained texture, meaning that it will feel rough to the touch.  If small sediments are lithified to form a clastic sedimentary rock, then the rock will have a fine-grained texture, meaning that it will feel smooth to the touch.  The Wentworth scale is a sediment size scale, named for the American geologist Chester K. Wentworth who defined this scale in the year 1922.  According to the Wentworth scale, sediments are classified as gravels, sands, silts, or clay/mud based on their size.  The largest sediments are gravels, which lithify into extremely coarse-grained clastic sedimentary rocks, either conglomerate (if the sediments are rounded) or breccia (if the sediments are angular).  Sands are somewhat smaller sediments that lithify into the moderately coarse-grained clastic sedimentary rock sandstone.  Silts are even smaller sediments that lithify into the moderately fine-grained clastic sedimentary rock siltstone.  Finally, the smallest sediments are clay/mud, which lithify into the extremely fine-grained clastic sedimentary rock shale. Whenever natural forces degrade (weaken and destroy) rocks, the resulting sediments always form with irregular, jagged shapes.  The technical term for this irregular, jagged shape is angular.  If the sediments are eroded (moved) over a far distance, the sediments collide with each other.  These collisions tend to smooth out the shapes of sediments.  Liquid water and even water vapor in the air will also contribute to the smoothing of the shapes of the sediments if they are eroded (moved) over a far distance.  The technical term for this smoothed shape is rounded.  If the sediments are eroded (moved) over a short distance, the sediments do not have the opportunity to collide with each other significantly, which would have smoothed out their shapes.  Also, liquid water as well as water vapor in the air will also not have much opportunity to smooth out the shapes of the sediments if they are eroded (moved) over a short distance.  Thus, the sediments retain their original angular shape.  In summary, we can calculate the distance (from how far away) sediments were eroded (moved) from the shape of the sediments, whether the shape is angular (more irregular or jagged) or rounded (more smooth).  For example, the clastic sedimentary rock conglomerate is lithified from gravels with more rounded shapes, while the clastic sedimentary rock breccia is lithified from gravels with more angular shapes.  Therefore, we conclude that the gravels that lithified to form conglomerate were eroded (moved) over a far distance, while the gravels that lithified to form breccia were eroded (moved) over a short distance. Some clastic sedimentary rocks are lithified from poorly sorted sediments, meaning that the sediments are all different sizes (some large and some small).  Some clastic sedimentary rocks are lithified from well sorted sediments, meaning that the sediments are all roughly the same size (all small).  This sorting of sediments within a clastic sedimentary rock reveals the energy of the natural forces that eroded (moved) the sediment.  A major river has a large quantity of energy, meaning that a major river is able to erode (move) large sediments and certainly small sediments.  Hence, the resulting clastic sedimentary rocks will be poorly sorted.  Conversely, a small stream has a small quantity of energy, meaning that a small stream is only able to erode (move) small sediments.  Hence, the resulting clastic sedimentary rocks will be well sorted.  As we will discuss later in the course, a glacier is a giant mass of ice with tremendous energy.  Hence, a glacier is able to move giant boulders in addition to small sediments.  Hence, the resulting clastic sedimentary rocks will be poorly sorted if the sediments were eroded (moved) by a glacier.  As we will discuss shortly, we can determine the history of the Earth from rocks, in particular from sedimentary rocks.  Our current discussion already reveals how this is possible.  By studying the sorting of sediments within a clastic sedimentary rock, we can actually determine the history of the surrounding landscape.  For example, the sorting of sediments within a clastic sedimentary rock may reveal that perhaps there was a small stream in a particular landscape followed by perhaps an ice age with glaciers followed by perhaps a major river, and so on and so forth.   A chemical sedimentary rock lithifies through the process of chemical reactions.  One example of a chemical sedimentary rock is limestone, which is the lithification of the carbonate mineral calcite.  Another example of a chemical sedimentary rock is dolostone, which is the lithification of the carbonate mineral dolomite.  Yet another example of a chemical sedimentary rock is chert, which is the lithification of the tectosilicate mineral quartz. A biogenic sedimentary rock is lithified from organic materials (lifeforms) together with inorganic sediments.  Chalk, coquina, and bituminous coal are three common examples of biogenic sedimentary rocks.  The biogenic sedimentary rock chalk forms from the lithification of microscopic ocean plankton.  Caution: the chalk that is used to write on chalkboards is artificially manufactured from the minerals gypsum and calcite.  The chalkboards themselves are manufactured from slate, which is another type of rock that we will discuss shortly.  Coquina is a gorgeous biogenic sedimentary rock that is lithified from many different types of shells from various invertebrate animals.  The formation of the biogenic sedimentary rock bituminous coal is as follows.  We begin with many layers of dead plants that are compacted with clay/mud.  In addition to compacting the organic matter, the clay/mud also serves to prevent the decomposition of the organic matter.  Over thousands of years, the accumulation of sediments over the organic matter compresses it to high densities until it is classified as peat.  As sedimentary rock continues to lithify over the peat, it is further compressed until it is classified as lignite (or brown coal).  If the lignite (or brown coal) is compressed to even higher densities over millions of years, it is eventually lithified to the biogenic sedimentary rock bituminous coal.  Note that if bituminous coal continues to be subjected to high pressures, it may become anthracite, which is another type of rock that we will discuss shortly.  If anthracite is subjected to further pressures, it may become graphite, one of the mineral forms of carbon.  Graphite is used in writing utensils such as pencils.  Note that the graphite in these writing utensils is often incorrectly referred to as lead because humans used to write with lead, which no one should do since lead is poisonous!  If the mineral graphite is subjected to enormous pressures over millions of years, it is compressed to diamond, another mineral form of carbon that is the most hard mineral on the Mohs scale, as we discussed.  Lignite (or brown coal), bituminous coal, and anthracite are all particular examples of fossil fuels.  All fossil fuels can be divided into three broad categories: petroleum (crude oil), natural gas, and coal.  Natural gas forms when particular microorganisms that generate methane are compacted under modest pressures over millions of years.  As we discussed, coal forms when plants are compacted under high pressures over millions of years.  Geologists continue to debate how petroleum (crude oil) forms.  Older theories claimed that petroleum (crude oil) forms over millions of years like coal and natural gas, but some modern theories claim that petroleum (crude oil) can form in only a few decades.  One form of petroleum is bitumen, which is also called asphalt.  However, the word asphalt is colloquially used for an artificially manufactured substance that is a mixture of bitumen and various minerals.  This manufactured substance should be called asphalt concrete (or asphalt cement) to distinguish it from naturally occurring asphalt, which should itself be called bitumen to avoid confusion with asphalt concrete (or asphalt cement). Heat, pressure, and chemical reactions can gradually change one rock into a completely new rock.  Any rock that forms by changing a pre-existing rock is called a metamorphic rock, one of the three main types of rock.  The original rock is called the parent rock, while the new metamorphic rock that formed from the parent rock is called the daughter rock.  Contact metamorphism is the process by which metamorphic rocks form primarily from heat, with pressure and chemical reactions being less important processes.  Regional metamorphism is the process by which metamorphic rocks form primarily from pressure, with heat and chemical reactions being less important processes.  Hydrothermal metamorphism is the process by which metamorphic rocks form primarily from chemical reactions, with heat and pressure being less important processes.  The adjective hydrothermal is derived from the Greek root hydro- meaning water, since the chemicals are almost always dissolved within water. Metamorphic rocks can be subclassified based on their shape.  Metamorphic rocks that have a folded shape resulting from asymmetrical regional metamorphism (asymmetrical pressures) are called foliated metamorphic rocks, while metamorphic rocks that do not have a folded shape are called non-foliated metamorphic rocks.  Non-foliated metamorphic rocks may form from symmetrical regional metamorphism (symmetrical pressures), but non-foliated metamorphic rocks may also form from contact metamorphism (heat) or from hydrothermal metamorphism (chemical reactions).  If we begin with siltstone or shale as the parent rock and apply asymmetrical pressures, the result is the daughter rock slate, a foliated metamorphic rock.  We can begin with slate itself as the parent rock and apply further asymmetrical pressures to yield the daughter rock phyllite, a more severely foliated metamorphic rock.  We can begin with phyllite as the parent rock and apply even further asymmetrical pressures to yield the daughter rock schist, an even more severely foliated metamorphic rock.  We can begin with schist as the parent rock and continue to apply asymmetrical pressures to yield the daughter rock gneiss, a severely foliated metamorphic rock.  If we begin with the biogenic sedimentary rock bituminous coal as the parent rock and apply symmetrical pressures, the result is the daughter rock anthracite, a non-foliated metamorphic rock.  Another non-foliated metamorphic rock is marble, the daughter rock to the chemical sedimentary rocks limestone and dolostone.  Yet another non-foliated metamorphic rock is quartzite, the daughter rock to the clastic sedimentary rock sandstone.  Another non-foliated metamorphic rock is hornfels, the daughter rock to the clastic sedimentary rock shale. Metamorphic rocks that form deep within the Earth may be subjected to sufficient heat to melt them.  That molten rock may later cool and crystallize into an igneous rock.  Metamorphic rocks may also be thrust to the surface of the Earth by a violent event, such as an earthquake.  This metamorphic rock would now be subjected to wind, rain, and other natural forces that will degrade (weaken and destroy) the rock, ultimately breaking it into sediments, which may later lithify into a sedimentary rock.  An intrusive/plutonic igneous rock that formed deep within the Earth is subjected to heat, pressure, and chemical reactions that may gradually change the igneous rock into a metamorphic rock.  Sedimentary rocks can be thrust to the deep interior of the Earth by a violent event, such as an earthquake.  These sedimentary rocks could now be subjected to sufficient heat to melt them, and that molten rock may later cool and crystallize into an igneous rock.  In summary, rocks are continuously changing from one type to another, and any rock can become any other type of rock.  The principle that rocks are continuously changing from one type to any other type is called the rock cycle.  It is difficult for us to believe that the rock cycle actually occurs, since in most cases we do not witness rocks change before our eyes.  Most rocks change very slowly over long periods of time, although there are rare cases when we can witness before our very eyes rocks forming in a short period of time, such as quenching resulting in igneous rocks with a glassy texture.  Throughout this course, we will discuss innumerable manifestations of our dynamic planet Earth.  The word dynamic means continuously changing.  The opposite of the word dynamic is the word static, meaning not changing.  Our planet Earth is not static.  Our planet Earth is dynamic since the Earth is continuously changing, and the rock cycle is one of the many processes we will discuss in this course that reveals that our planet Earth is dynamic.  If heat changes a parent rock into a metamorphic daughter rock, that heat must not melt the rock.  If the rock were to melt and recrystallize into a solid rock, then we must classify the rock as an igneous rock, not as a metamorphic rock.  The rock migmatite forms from heat that melts some parts of the rock which then recrystallize into solid rock, but the heat also changes other parts of the rock without melting those parts of the rock.  Some petrologists classify migmatite as igneous, while other petrologists classify migmatite as metamorphic.  Still other petrologists classify migmatite as both igneous and metamorphic, and yet other petrologists actually place migmatite in a unique category of rock that is intermediate between igneous and metamorphic.  There will never be a consensus among petrologists on the classification of migmatite.  Therefore, migmatite is a rock that cannot be classified.  We now realize that not all rocks can be classified as either igneous, sedimentary, or metamorphic.  As another example, the rock tuff partly forms from the crystallization of molten rock and partly forms from the lithification of sediment.  Some petrologists classify tuff as igneous, while other petrologists classify tuff as sedimentary.  Still other petrologists classify tuff as both igneous and sedimentary, and yet other petrologists actually place tuff in a unique category of rock that is intermediate between igneous and sedimentary.  Again, there will never be a consensus among petrologists on the classification of tuff.  Therefore, tuff is another rock that cannot be classified.  A more subtle example is the sedimentary rock marlstone, which is intermediate between the sedimentary rocks limestone and shale.  Although both limestone and shale are sedimentary rocks and therefore marlstone is also a sedimentary rock, limestone is a chemical sedimentary rock while shale is a clastic sedimentary rock.  Therefore, marlstone is a sedimentary rock that is intermediate between chemical sedimentary and clastic sedimentary.  There will never be a consensus among petrologists on the precise classification of marlstone.  Therefore, marlstone is yet another rock that cannot be classified. The Structure and the Composition of the Geosphere The Earth is mostly covered with oceans, while a small fraction of the surface of the Earth is continents.  The continents are composed of mostly felsic igneous rock, for reasons we will make clear shortly.  On the surface of the continents is a thin layer of sedimentary rock, having formed from the lithification of sediments that themselves formed from natural forces degrading rocks into sediments.  Therefore, if we were to drill into the continent, we would first drill through a layer of sedimentary rock followed by rhyolite (extrusive/volcanic felsic rock, having crystallized near the surface of the Earth) followed by granite (intrusive/plutonic felsic rock, having crystallized deep within the Earth).  The ocean basins (at the bottom of the ocean) are composed of mostly mafic igneous rock, for reasons we will make clear shortly.  On the surface of the ocean basins (at the bottom of the ocean) is a thin layer of sedimentary rock, having formed from the lithification of sediments that themselves formed from natural forces degrading rocks into sediments.  Therefore, if we were to drill into the ocean basins (at the bottom of the ocean), we would first drill through a layer of sedimentary rock followed by basalt (extrusive/volcanic mafic rock, having crystallized near the surface of the Earth) followed by gabbro (intrusive/plutonic mafic rock, having crystallized deep within the Earth). The geosphere (the solid part of the Earth) is layered.  The most dense layer of the geosphere is the core at its center.  The core is composed primarily of metals, such as iron and nickel.  The next layer of the geosphere surrounding the core is the mantle, which is less dense than the core.  The mantle is composed primarily of rock, which is itself composed primarily of silicate minerals.  There are also fair amounts of metals in the mantle, such as iron.  In summary, the mantle is composed of iron-rich silicate rock.  The outermost layer of the geosphere surrounding the mantle is the crust.  The crust is the least dense layer of the geosphere and the thinnest layer of the geosphere.  The crust is also composed of silicate rock, but there are fewer metals such as iron in the crust as compared with the mantle.  Therefore, the crust is composed of iron-poor silicate rock.   The Earth's core is itself layered.  At the very center of the geosphere is the inner core, composed primarily of metals such as iron and nickel.  The temperature of the Earth's core is very hot, for reasons we will discuss shortly.  These hot temperatures should be sufficient to melt metals into the molten state.  However, the pressure of the inner core is so enormous that the metals are compressed into the solid state even though they are at temperatures where they should be in the molten state.  Since the inner core is composed of solid metal, the inner core is also called the solid core.  The layer around the solid (inner) core is the outer core, which is also composed primarily of metals such as iron and nickel.  Again, the temperature of the outer core is sufficiently hot to melt metals into the molten state.  Although the pressure of the outer core is enormous by human standards, the pressure is nevertheless not as high as the pressure of the inner core.  Therefore, the pressure of the outer core is not sufficient to compress metals into the solid state.  The outer core is therefore molten, as metals should be at these hot temperatures.  Since the outer core is composed of molten metal, the outer core is also called the molten core.  Surrounding the molten (outer) core is the first layer of the mantle: the mesosphere.  The Greek root meso- means middle.  For example, Central America is sometimes called Mesoamerica, as in Middle America.  Therefore, the word mesosphere simply means middle sphere or middle layer.  The next layer of the mantle that surrounds the mesosphere is the asthenosphere.  This layer of the mantle is composed of weak rock.  Indeed, the Greek root astheno- means weak.  The asthenosphere is composed of weak rock that is still mostly solid, although parts of the asthenosphere are composed of rock that is partially molten.  Finally, the rest of the mantle together with the entire crust is called the lithosphere.  The lithosphere is of varying thickness.  Some parts of the lithosphere are so thick that they protrude out of the oceans.  These thick parts of the lithosphere are called continents.  The parts of the lithosphere that are thin are the ocean basins at the bottom of the ocean. Since density is defined as mass divided by volume, density is inversely proportional to volume.  Therefore, more dense rock must occupy a smaller volume, while less dense rock must occupy a larger volume.  Some parts of the lithosphere are so thick that they protrude out of the oceans; these are the continents.  Since the continental parts of the lithosphere are thick occupying a larger volume, they must be composed of less dense rock.  This reveals why the continents are composed primarily of felsic rock, since felsic rock is the least dense igneous rock.  Other parts of the lithosphere are so thin that they do not protrude out of the ocean; these are the ocean basins at the bottom of the ocean.  Since the oceanic parts of the lithosphere are thin occupying a smaller volume, they must be composed of more dense rock.  This reveals why the ocean basins are composed primarily of mafic rock, since mafic rock is the most dense igneous rock. How have geophysicists determined the layers of the geosphere, their thicknesses, their compositions (metal or rock), and their physical states (solid or molten)?  It is a common misconception that we have drilled to the center of the Earth and directly studied the interior of the Earth.  This is false.  We have come nowhere near drilling to the center of the Earth.  We have not even drilled through the Earth's crust, which is by far the thinnest layer of the geosphere.  We will never have technology advanced enough to drill far beyond the crust, and reaching the core is out of the question.  Geophysicists have determined the layers of the geosphere, their thicknesses, their compositions (metal or rock), and their physical states (solid or molten) using seismic waves.  We will discuss earthquakes in more detail shortly.  For now, earthquakes cause waves that propagate (travel) throughout the entire geosphere.  These waves are called seismic waves, and a seismometer is a device that detects seismic waves.  There are millions of earthquakes on planet Earth every day, as we will discuss shortly.  Most earthquakes are so weak that humans cannot feel them, but seismometers sensitive (accurate) enough can detect incredibly weak seismic waves.  Geophysicists have placed thousands of seismometers all over planet Earth to detect these seismic waves.  Surface seismic waves propagate (travel) along the surface of the geosphere, while body seismic waves propagate (travel) throughout the interior of the geosphere.  There are two different types of body seismic waves: pressure waves and shear-stress waves.  A pressure is a force exerted directly onto (perpendicularly onto) an area.  Consequently, a pressure wave can propagate (travel) through solids, liquids, and even gases, since all that is necessary for a pressure wave to propagate is for atoms or molecules to collide with other atoms or molecules in the direction of propagation.  A shear-stress is a force exerted along (parallel across) an area.  Consequently, a shear-stress wave can propagate (travel) through solids but not through liquids or gases, since there must be strong chemical bonding for atoms or molecules to pull other atoms or molecules along (parallel across) an area.  Also, pressure waves propagate faster than shear-stress waves.  Therefore, a seismometer will always detect pressure waves first.  For this reason, pressure waves are also called primary waves.  A seismometer will always detect shear-stress waves second, since they propagate slower than pressure (primary) waves.  For this reason, shear-stress waves are also called secondary waves.  By an amazing coincidence, the words pressure and primary both begin with the same letter.  Therefore, these waves are also called P-waves.  By another amazing coincidence, the words shear, stress, and secondary also all begin with the same letter!  Therefore, these waves are also called S-waves.  We can calculate how distant an earthquake occurred from a seismometer from the arrival times of the P-waves and the S-waves.  If a seismometer detects the S-waves a long duration of time after the P-waves, the earthquake must have occurred far from the seismometer, since the P-waves had sufficient distance to propagate (travel) far ahead of the S-waves, resulting in a long delay between them.  If the seismometer detects the S-waves immediately after the P-waves, the earthquake must have occurred near the seismometer, since the P-waves did not have sufficient distance to propagate (travel) that far ahead of the S-waves, resulting in a short delay between them.  In summary, we calculate the distance the seismic waves propagated from the arrival times of the P-waves and the S-waves.  We now have the distance of propagation, and we certainly know the time of propagation, since we know when the earthquake occurred and when the seismometer detected the seismic waves.  We can then calculate the speed of the seismic waves, since the speed of anything equals its distance traveled divided by its time of travel.  Once we have the speed of the seismic waves, we can determine the properties of the materials that would cause that speed of propagation, whether the material was solid metal, molten metal, solid rock, or molten rock.  We can program a computer with all of these data, and the computer can calculate the layers of the geosphere, their thicknesses, their compositions (metal or rock), and their physical states (solid or molten) from all of these data.  Also, we are certain that the interior of the Earth is at least partially molten, since seismometers on the opposite side of planet Earth from an earthquake do not detect S-waves; these seismometers only detect P-waves.  As we discussed, S-waves cannot propagate through liquids; they can only propagate through solids.  However, P-waves can propagate through either solids or liquids, as we discussed.  The opposite side of planet Earth from any earthquake is called the shadow zones of that particular earthquake.  This term comes from the idea that the molten (outer) core casts a shadow, preventing those seismometers from detecting S-waves.  In actuality, the S-waves cannot propagate through the molten (outer) core, while the P-waves can propagate through the molten (outer) core.  Hence, seismometers in the shadow zones only detect P-waves from earthquakes on the opposite side of planet Earth.  In summary, by using thousands of seismometers all over planet Earth that detect seismic waves from millions of earthquakes each day and by running computer simulations, geophysicists have determined the layers of the geosphere, their thicknesses, their compositions, and their physical states.  One of the earliest examples of the successful use of these techniques was when the Croatian geophysicist Andrija Mohorovičić discovered the boundary between the crust and the mantle in the year 1909 using seismic waves.  Consequently, the boundary between the crust and the mantle is called the Mohorovičić discontinuity in his honor. It cannot be accidental or coincidental that the geosphere (the solid part of the Earth) is layered according to density.  Why are the inner layers more dense and the outer layers less dense?  To explain this layering, we must discuss the formation of the Earth.  The Earth, its Moon, the other planets and their moons, the Sun, and the entire Solar System formed roughly 4.6 billion years ago.  The four planets closer to the Sun (Mercury, Venus, Earth, and Mars) were born as small, dense planetesimals (baby planets) that grew larger through accretion, which is the gaining of mass through sticky collisions.  During a sticky collision, a significant fraction of the kinetic energy (moving energy) of the colliding objects is converted into thermal energy (heat energy).  Thus, objects that suffer from sticky collisions become significantly warmer.  Therefore, as the Earth was forming and growing larger through accretion, it became warmer.  Eventually, the Earth became so hot that it became almost entirely molten.  While the Earth was almost entirely molten, more dense materials were able to sink toward the center of the planet while less dense materials were able to rise toward the surface of the planet.  Most metals are more dense than most rocks.  That is, most rocks are less dense than most metals.  Therefore, most of the metals sank toward the center of the planet, forming the core.  Most of the rocks rose toward the surface of the planet, forming the mantle and the crust.  The process by which any planet separates materials according to density is called differentiation, and the planet is said to be differentiated.  A planet larger than the Earth would be more severely differentiated than the Earth, since it would have more mass and therefore stronger gravity that would pull the metals towards the center of the planet more strongly.  A planet smaller than the Earth would be less severely differentiated than the Earth, since it would have less mass and therefore weaker gravity that would pull the metals toward the center of the planet less strongly.  For example, planet Mars is smaller than the Earth.  Therefore, Mars has less mass and thus weaker gravity as compared with the Earth.  Hence, Mars is less severely differentiated as compared with the Earth.  Of course, planet Mars is still differentiated.  It has a dense core composed primarily of metals such as iron, and it has less dense outer layers composed primarily of rock.  Nevertheless, Mars is less severely differentiated as compared with the Earth, meaning that not all of the metals sank toward the center of Mars to form its core.  This is also the case with the Earth.  Although most of the metals sank toward the center of the Earth to form its core, there are still fair amounts of metals in the mantle and small amounts of metals in the crust.  Since Mars is less severely differentiated as compared with the Earth, we find more iron on the surface of Mars as compared with the amount of iron on the surface of the Earth.  The abundance of iron on the surface of Mars has oxidized.  As we discussed, iron oxide is commonly known as rust, which has a reddish color.  This is why Mars is red.  In fact, the nickname of planet Mars is the Red Planet.  Mars even appears red to the naked eye (without the aid of a telescope or even binoculars) in our sky.   The Earth has a magnetic field, but we do not completely understand how this magnetic field is generated.  According to older theories, the Earth's magnetic field is generated by its solid (inner) core.  This theory may seem reasonable, since the solid (inner) core is composed of ferromagnetic metals such as iron and nickel.  However, we now realize that this old model is too simplistic.  According to more modern theories, the Earth's magnetic field is created by its molten (outer) core.  This more modern theory also seems reasonable, since the molten (outer) core is also composed of ferromagnetic metals such as iron and nickel.  These modern theories claim that the rotation of the Earth causes circulating currents of molten metal in the outer core.  These circulating currents of molten metal in turn generate the Earth's magnetic field.  These more modern theories seem reasonable, but nevertheless these theories are not fully developed.  If these models are correct, then the two equally important variables that create a metallic-rocky planet's magnetic field is a metallic core that is at least partially molten and reasonably rapid rotation.  Indeed, among the four planets closer to the Sun (Mercury, Venus, Earth, and Mars), the Earth has the strongest magnetic field, since it is the only one of these four planets that has both a partially molten metallic core and reasonably rapid rotation.  For example, Venus probably has a partially molten metallic core, but Venus has very slow rotation.  Hence, Venus has a very weak magnetic field.  As another example, Mars has reasonably rapid rotation, but Mars has a metallic core that is no longer partially molten.  Hence, Mars also has a very weak magnetic field.  Caution: we are only comparing the magnetic fields of the four inner planets closer to the Sun.  Although the Earth has the strongest magnetic field among the inner planets, its magnetic field is still weak compared with all four outer planets further from the Sun (Jupiter, Saturn, Uranus, and Neptune). The overall structure of the Earth's magnetic field is very much similar to the magnetic field created by a bar magnet, such as a refrigerator magnet.  In fact, the only difference between the Earth's magnetic field and a bar magnet's magnetic field is strength.  The Earth's magnetic field is thousands of times weaker than a bar magnet's magnetic field.  That is, a bar magnet's magnetic field is thousands of times stronger than the Earth's magnetic field.  Students often cannot believe that a small refrigerator magnet could create a stronger magnetic field than an entire planet, but this stands to reason actually.  A refrigerator magnet's magnetic field is strong enough to lift paper clips for example, but the Earth's magnetic field is not this strong.  The Earth's magnetic field is everywhere around us, yet we do not see paper clips floating around us!  A refrigerator magnet's magnetic field is strong enough to lift paper clips against planet Earth's gravitational field, but planet Earth's magnetic field is not strong enough to lift paper clips against its own gravitational field.  Hence, the Earth's magnetic field is indeed thousands of times weaker than a bar magnet's magnetic field. The Earth's magnetic field reverses itself once every few hundred thousand years.  We do not understand how or why this occurs.  We do know that it does happen from the magnetization of iron within rocks.  The uppermost layer of rock has its iron magnetized in the same direction as the Earth's magnetic field; this is called normal polarity.  However, a deeper layer of rock has its iron magnetized in the opposite direction of the Earth's magnetic field; this is called reverse polarity.  An even deeper layer of rock has normal polarity again, and an even more deep layer of rock has reverse polarity again.  In other words, the magnetization of iron within rock (the polarity) alternates from normal to reverse and back again.  The reason for this is clear.  When new rock forms, the iron within that rock will become magnetized in whichever direction the Earth's magnetic field happens to point at the time of that rock's formation.  The Earth's magnetic field must reverse itself periodically to cause the alternating polarity of rock layers.  Therefore, we are certain that the Earth's magnetic field reverses itself once every few hundred thousand years, but again we do not understand how or why this occurs. It is a common misconception that the Earth's magnetic field begins at the north pole and ends at the south pole.  This is false for a couple of reasons.  Firstly, it is a basic law of physics that magnetic field lines are not permitted to begin or end anywhere; magnetic field lines must form closed loops.  The magnetic field lines of a bar magnet, such as a refrigerator magnet for example, do not begin at the north pole of the magnet, nor do they end at the south pole of the magnet.  The magnetic field lines of a bar magnet go straight through the magnet, coming out of its north pole, circulating around to go into its south pole, going straight through the magnet, and coming out its north pole again.  Similarly, the magnetic field lines of planet Earth go straight through the planet, coming out one end, circulating around to go into the opposite end, going straight through the planet, and coming out again.  Moreover, the Earth's magnetic field lines do not come out from nor do they go into the geographical poles.  The Earth's magnetic field lines come out from and go into the magnetic poles, which are different from the geographical poles.  Indeed, a magnetic compass does not point toward geographical north as is commonly believed.  A magnetic compass points toward magnetic north, which again is different from geographical north.  Admittedly, the Earth's magnetic poles are somewhat close to the planet's geographical poles, but there is no reason to expect that the magnetic poles should be the same or even close to the geographical poles.  The solid (inner) core is literally floating within the molten (outer) core.  Therefore, the solid (inner) core is actually detached from the rest of the planet.  Consequently, there is no reason to expect that the solid (inner) core is rotating in precisely the same direction as the rest of the planet.  In addition, there are circulating currents of molten metal within the molten (outer) core.  In brief, the rotation of the Earth is complicated; the entire planet does not rotate together as one solid unit.  Therefore, there is no reason to expect that the magnetic poles should be the same or even close to the geographical poles.  To ask why the Earth's magnetic poles are different from the geographical poles is a wrong question to ask, since there is no reason to expect that the magnetic poles should be the same as the geographical poles.  The correct question to ask is why are the Earth's magnetic poles even close to the Earth's geographical poles.  We do not understand why this is the case.  Indeed, other planets have magnetic poles that are completely different from their geographical poles. The solar wind is a stream of charged particles from the Sun composed primarily of protons and electrons.  This solar wind is capable of substantially ionizing the Earth's atmosphere in a fairly short amount of time.  Fortunately, the Earth's magnetic field, although weak compared with the magnetic field of bar magnets, is sufficiently strong to deflect most of the Sun's solar wind.  Some of the charged particles in the solar wind do however become trapped within the Earth's magnetic field.  These charged particles execute helical trajectories around the Earth's magnetic field lines.  These regions of the Earth's magnetic field are called the Van Allen belts, named for the American physicist James Van Allen who discovered these belts in the year 1958.  The charged particles within the Van Allen belts execute helical trajectories while drifting along the Earth's magnetic field lines toward the magnetic poles.  The charged particles eventually collide with the molecules of the Earth's atmosphere, surrendering their kinetic energy by emitting light.  The result is gorgeous curtains of light or sheets of light across the sky near the Earth's magnetic poles.  This is called an aurora.  Near the Earth's north magnetic pole it is called aurora borealis (or more commonly the northern lights), and near the Earth's south magnetic pole it is called aurora australis (or more commonly the southern lights).  If the Sun happens to be less active, its solar wind would be weaker, the resulting aurorae would appear less spectacular, and we would only be able to enjoy them near the Earth's magnetic poles.  If the Sun happens to be more active, its solar wind would be stronger, the resulting aurorae would appear more spectacular, and we would be able to enjoy them further from the Earth's magnetic poles.  For example, the Battle of Fredericksburg in December 1862 during the American Civil War was interrupted by the appearance of the aurora borealis in the sky, even though Fredericksburg, Virginia is quite far from the Earth's north magnetic pole. The interior of the geosphere is hot due to geothermal energy.  This geothermal energy drives geologic activity, as we will discuss shortly.  The source of this geothermal energy is primarily radioactive decay.  There are certain atoms that have an unstable nucleus, since the nucleus has too much energy.  To stabilize itself, the nucleus will emit particles to decrease its own energy.  These atoms are called radioactive atoms, and the e6mission of these particles is called radioactive decay.  A certain naturally occurring fraction (percentage) of all the atoms in the universe are radioactive.  A certain fraction (percentage) of the atoms that compose the geosphere are radioactive.  When these atoms suffer from radioactive decay, the emitted particles are themselves a source of energy.  This is the source of the Earth's geothermal energy.  A planet larger than the Earth would have a longer geologic lifetime than the Earth, since it would have more mass and therefore more radioactive atoms and hence a greater supply of geothermal energy.  A planet smaller than the Earth would have a shorter geologic lifetime than the Earth, since it would have less mass and therefore less radioactive atoms and hence a smaller supply of geothermal energy.  Billions of years from now, most of the radioactive atoms of planet Earth will have decayed.  With very little radioactive atoms remaining to provide geothermal energy, geologic activity will end, and the Earth will become a geologically dead planet.  This geologic death has already occurred for other planets smaller than the Earth.  The planets Mercury and Mars for example are geologically dead planets, since they are significantly smaller than the Earth.  However, planet Venus has almost the same size as the Earth.  Presumably, the geologic lifetime of Venus is roughly the same as the geologic lifetime of the Earth.  Indeed, both Venus and the Earth are currently geologically alive. The three mechanisms by which heat (energy) is transported from one location to another are conduction, convection, and radiation.  Conduction is the transfer of heat (energy) from one object to another because they are in direct contact with one another.  For example, our hand becomes hot when we grab a hot object because our hand is in direct contact with the hot object when we grab it.  The heat (energy) is transferred from the hot object to our hand by conduction, since our hand is in direct contact with the hot object.  Convection is the transfer of heat (energy) from one object to another by moving materials.  It is commonly known that hot air rises and cold air sinks.  This is not just the case for air; this is true for any gas and for any liquid as well.  In physics, gases and liquids are both considered fluids.  Caution: in colloquial English, the word fluid refers to liquids only, but in physics the word fluid may apply to both liquids and gases.  Convection is the transfer of heat (energy) by moving fluids, since hot fluids rise and cool fluids sink.  For example, a heater on the first floor of a house also warms the second floor by convection.  The heater on the first floor warms the air on the first floor; since hot fluids rise, the hot air rises to the second floor and warms the second floor.  When the air that has risen to the second floor loses its heat and cools, it sinks back to the first floor, where it is warmed again by the heater.  Radiation is the transfer of heat (energy) without direct contact and without moving fluids.  Radiation typically transports heat (energy) using electromagnetic waves.  For example, heat (energy) is not transported from the Sun to the Earth by conduction, since the Earth is not in direct contact with the Sun, thank God!  Heat (energy) is not transported from the Sun to the Earth by convection, since there is no fluid in outer space that takes the Sun's heat (energy) and moves it to the Earth.  The Sun's heat (energy) is transported to the Earth across outer space by radiation, through electromagnetic waves.