Science Notes - Tectonic Plates and Earthquakes PDF

**Tectonic plates** are pieces of Earth\'s crust and uppermost mantle, together referred to as the lithosphere. **Plate Boundary** **Types** - - 1. - - - Example Mid-Atlantic ridge - - 2\. Convergent boundary - - - - - -...

**Tectonic plates** are pieces of Earth\'s crust and uppermost mantle, together referred to as the lithosphere. **Plate Boundary** **Types** - - 1. - - - Example Mid-Atlantic ridge - - 2\. Convergent boundary - - - - - - - Example Himalayas Mountain Range 3\. Transform Plate Boundary - Geologic feature : Fault Example : San Andreas Fault  ***EARTHQUAKE*** ***seismic*** (energy) waves travel through the earth some energy bounces off harder layers called ***reflection*** some energy travels through but gets bent, changing the direction the wave is traveling called ***refraction*** some energy is absorbed as it encounters materials called ***attenuation*** **FACTORS AFFECTING SEISMIC WAVES** **distance**: farther = more attenuation **density**: higher = faster **temperature**: colder = faster ***liquid vs solid*** **solid** = faster; p-waves and s-waves **liquid** = slower; no s-waves **angle of incidence**- controls how much is reflected and how much is absorbed **vertical arrangement** of layers controls the resultant direction of travel **WHAT IS A TSUNAMI?** - **What are seismic waves?** You learned that an earthquake is a vibration of the Earth produced by the rapid release of energy most often because of the slippage along a fault in the Earth's crust. This energy radiates in all directions from the focus in the form of waves ***called seismic waves***, which are recorded in seismographs.The two main types of seismic waves are ***body waves and surface waves***. Surface waves can only travel through the surface of the Earth. They arrive after the main P and S waves and are confined to the outer layers of the Earth. There are two types of surface waves: the Love waves and the Rayleigh Waves. ***Love wave*** is named after A.E.H. Love, a British mathematician who worked out the mathematical model for this kind of wave in 1911. It is faster than Rayleigh wave and it moves the ground in a side-to-side horizontal motion, like that of a snake's causing the ground to twist. This is why Love waves cause the most damage to structures during an earthquake. The other kind of surface wave is the ***Rayleigh wave***. It was named after John William Strutt, Lord Rayleigh, who mathematically predicted the existence of this kind of wave in 1885. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Since it rolls, it moves the ground either up and down or side-to-side similar to the direction of the wave's movement. Most of the shaking felt from an earthquake is due to the Rayleigh wave. Unlike surface waves, body waves can travel through the Earth's inner layers. With this characteristic of the body waves, they are used by scientists to study the Earth's interior. These waves are of a higher frequency than the surface waves. **What are P and S-waves?** The ***P-wave (primary wave)*** is a pulse energy that travels quickly through the Earth and through liquids. The P-wave travels faster than the S-wave. After an earthquake, it reaches a detector first (the reason why it is called primary). The P-waves also called ***compressional waves***, travel by particles vibrating parallel to the direction the wave travel. They force the ground to move backward and forward as they are compressed and expanded. Most importantly, they travel through solids, liquids and gases ***The S-wave (secondary wave or shear wave)*** is a pulse energy that travels slower than a P-wave through Earth and solids. The S-waves move as shear or transverse waves, and force the ground to sway from side to side, in rolling motion that shakes the ground back and forth perpendicular to the direction of the waves. The idea that the S-waves cannot travel through any liquid medium led seismologists to conclude that the outer core is liquid. Scientists gained information about the Earth's internal structure by studying how seismic waves travel through the Earth. It involves measuring the time it takes for both types of waves to reach seismic wave detecting stations from the epicenter of an earthquake. An epicenter is a point in the Earth's surface directly above the focus. Since P-waves travel faster than S-waves, they're always detected first. The farther away from the epicenter means the longer time interval between the arrival of P and S waves. In 1909, Yugoslavian seismologist Andrija Mohorovičić (moh-haw-rohvuh-chich) found out that the velocity of seismic waves changes and increases at a distance of about 50 kilometers below the Earth's surface. This led to the idea that there is a difference in density between the Earth's outermost layer(crust) and the layer that lies below it (mantle). The boundary between these two layers is called Mohorovičić discontinuity in honor of Mohorovičić, and is short termed Moho.  P-waves can travel through liquids while S-waves cannot. During an earthquake, the seismic waves radiate from the focus. Based on figure on the right, the waves bend due to change in density of the medium. As the depth increases, the density also increases. P-waves are detected on the other side of the Earth opposite the focus. A shadow zone from 103° to 142° exists from P-waves as shown in the figure Since P-waves are detected until 103°, disappear from 103° to 142°, then reappear again, something inside the Earth must be bending the P-waves. The existence of a shadow zone, according to German seismologist Beno Gutenberg (ɡuː t ə n bɛʁk), could only be explained if the Earth contained a core composed of a material different from that of the mantle causing the bending of the P-waves. To honor him, mantle--core boundary is called Gutenberg discontinuity. From the epicenter, S-waves are detected until 103o, from that point, S- waves are no longer detected. This observation tells us that the S-waves do not travel all throughout the Earth's body. There is a portion inside the Earth that does not conduct the propagation of S-wave. Hence, knowing the properties and characteristics of S-waves (that it cannot travel through liquids), and with the idea that P-waves are bent to some degree, this portion must be made of liquid, thus the outer core. In 1936, the innermost layer of the Earth was predicted by Inge Lehmann, a Danish seismologist. He discovered a new region of seismic reflection within the core. So, the Earth has a core within a core. we can say that the outer part of the core is liquid based from the production of an S wave shadow and the inner part must be solid with a different density than the rest of the surrounding material. The size of the inner core was accurately calculated through nuclear underground tests conducted in Nevada. Echoes from seismic waves provided accurate data in determining its size Thickness of earth's layer in kilometer Layer Thickness in kilometer ------------ ------------------------ Crust 40 mantle 2900 Outer Core 2200 Inner Core 1278 **The Composition of the Earth's Interior** The Earth's composition tells a story about itself. It gives us clues to its past and proofs about the gradual and slow changes that it has undergone for over 4.6 billion years.  **The Crust** The crust is the thinnest and the outermost layer of the Earth that extends from the surface to about 32 kilometers below. Underneath some mountains, the crust's thickness extends to 72 kilometers. The Earth's crust, as gleaned from Figure 5 on page 12, is subdivided into two regions: the continental crust and the oceanic crust. The continental crust is mainly made up of silicon, oxygen, aluminum, calcium, sodium, and potassium. The thickness of the continental crust is mostly 35-40 kilometers. Continental crust, found under land masses, is made of less dense rocks such as granite. The oceanic crust is around 7-10 kilometers thick which its average thickness is 8 kilometers. It is found under the ocean floor and is made of dense rocks such as basalt. The oceanic crust is heavier than the continental crust. The crust consists of two layers. The upper layer is composed of granite and is only found in the continental crust. Below the granite is a layer made mainly of basalt. This is found on both under the continents and the oceans. **Element in the Earth's Crust**  **The Mantle** Beneath the crust is the mantle, which extends to about 2900 kilometers from the Earth's surface. It makes up about 80% of the Earth's total volume and about 68% of its total mass. The mantle is mainly made up of silicate rocks, and contrary to common belief, is solid, since both S-waves and P-waves pass through it. The attempt to study the Earth's mantle extended as far as studying the rocks from volcanoes, simply because they were formed in the mantle. Scientists also studied rocks from the ocean floor. They have determined that the mantle is mostly made of the elements silicon, oxygen, iron and magnesium. The lower part of the mantle consists of more iron than the upper part. This explains that the lower mantle is denser than the upper portion. The temperature and the pressure increase with depth. The high temperature and pressure in the mantle allows the solid rock to flow slowly The crust and the uppermost part of the mantle form a relatively cool, outermost rigid shell called lithosphere and is about 50 to 100 kilometers thick. These lithospheric plates move relative to each other. Beneath the lithosphere lies the soft, weak layer known as the asthenosphere, made of hot molten material. Its temperature is about 300 -- 800oC. The upper 150 kilometers of the asthenosphere has a temperature enough to facilitate a small amount of melting, and make it capable to flow. This property of the asthenosphere facilitates the movement of the lithospheric plates. The lithosphere, with the continents on top of it, is being carried by the flowing asthenosphere. **The Core** The core is subdivided into two layers: the inner and the outer core. The outer core is 2900 kilometers below the Earth's surface. It is 2250 kilometers thick and is made up of iron and nickel. The temperature in the outer core reaches up to 2000oC at this very high temperature, iron and nickel melt. Aside from seismic data analysis, the Earth's magnetic field strengthens the idea that the Earth's outer core is molten/liquid. The outer core is mainly made up of iron and nickel moving around the solid inner core, creating Earth's magnetism. The inner core is made up of solid iron and nickel and has a radius of 1300 kilometers. Its temperature reaches to about 5000oC. The extreme temperature could have molten the iron and nickel but it is believed to have solidified as a result of pressure freezing, which is common to liquids subjected under tremendous pressure. ***The Lithosphere*** which is the rigid outermost shell of a planet (the crust and upper mantle), is broken into tectonic plates. ***What makes the lithospheric plates move?*** 1. - 2. - 3. - 4. - 5. - ***Why does heat transfer happen?*** - 1. - 2. - 3. - During heat transfer, thermal energy always moves in the same direction: **HOT** **COLD** Heat energy only flows when there is a temperature difference from a **warmer** area to a **cooler** area. **WEGNER\'S EVIDENCE FOR CONTINENTAL DRIFT** -  **Alfred Lothar Wegener (1880-1930)** German polar researcher, geophysicist, and meteorologist. He is remembered as the originator of the Continental Drift Theory by hypothesizing in 1912 that he continents are slowly drifting around the Earth and is once a large landmass called **Pangaea**, a Greek word which means *\"All Earth.\"*  EVIDENCES Alfred Wegener collected diverse pieces of evidence to support his theory, including **geological \"fit\"and fossil evidence**. It is important to know that the following specific fossil evidence was not brought up by Wegener to support his theory. GEOLOGICAL "FIT" EVIDENCE Is the matching of large-scale geological features on different continents. It has been noted that the coastlines of South America and West Africa seem to match up, however more particularly, the rock terrains of separate continents confirm as well. Examples include the Appalachian Mountains of eastern North America linked with the Scottish Highlands, the familiar system of South Africa matched correctly with the Santa Catarina system in Brazil, and Brazil and Ghana mountain ranges agreeing over the Atlantic Ocean. **Glaciers** carve rocks and leave marks as they move. In this evidence, scientists can determine the direction of movement of each continent. In addition, the existence of **coal deposits** in Antarctica suggested that it was once located near the region of the Earth where the climate is enough to support complex life forms such as plants and tall trees FOSSIL EVIDENCE The **Mesosaurus** is known to have been a type of reptile, lived in fresh or brackish water, similar to the modern crocodile, which propelled itself through the water with its long hind legs and limber tail has limbs for swimming but was not a strong swimmer. It lived during the early Permian period (286 to 258 million years ago), and its remains are found solely in South Africa and Eastern South America. Now, if the continents were still in their present positions, there is no possibility that the Mesosaurus would have the capability to swim across such a large body of ocean like the Atlantic because it was a coastal animal. The now extinct **Cynognathus** was a mammal-like reptile. Roaming the terrains during the Triassic period (250 to 240 million years ago), the Cynognathus was as large as a modern wolf. Its fossils are found only in South Africa and South America. As a dominant land species, the Cynognathus would not have been capable of migrating across the Atlantic. The **Lystrosaurus**, which translates to \"shovel reptile,\" is thought to have been a herbivore with a stout built like a pig. It survived the Permian Period and was dominant during the early Triassic. Lystrosaurus fossils are only found in Antarctica, India, and South Africa. Similar to the land-dwelling Cynognathus, the Lystrosaurus would not have had the swimming capability to traverse any ocean The most important fossil evidence found in the plant, **Glossopteris**. The Glossopteris fossil is found in Australia, Antarctica, India, South Africa, and South America-all the southern continents. **Glossopteris seed** is known to be large and bulky and possibly could not have drifted or flown across the oceans to a separate continent. Therefore, the continents must have been joined at least one point in time in order to maintain the Glossopteris\' wide range across the southern continents.By about 300 million years ago, a unique community of plants had evolved known as the **European flora Fossils** of these plants are found in Europe and other areas **Seafloor Spreading and Magnetic Reversal** The idea of continental drift circulated in scientific circles until World War II, when sounding gear called SONAR produced new evidence of what the seafloor looked like. The gear, developed in the 1930s, bounced sound waves off the seafloor to determine its depth and features. **Harry Hammond Hess** A geologist from Princeton University. Hess, then in his late thirties, wanted to continue his scientific investigations even while at war. So he left his ship\'s sounding gear all of the time, not just when approaching port or navigating a difficult landing. What Hess discovered was a big surprise. *Ocean floor exploration continued*, and by the 1950s, other researchers had found that a **huge rift ran along the top of the Mid-Atlantic Ridge.** That enabled Hess to understand his ocean floor profiles in the Pacific. **He discovered that the bottom of the sea was not as smooth as expected**, but **full of canyons, trenches, and volcanic sea mountains**. He realized that the Earth\'s crust had been moving away on each side of oceanic ridges, down the Atlantic and Pacific oceans, long and volcanically active. Harry Hess observed that the rate of formation of new seafloor at the mid-ocean ridge is not always as fast as the destruction of the old seafloor at the **subduction zone**. This explains why the Pacific Ocean is getting smaller and why the Atlantic Ocean is getting wider. If the subduction zone is faster than the **seafloor spreading**, the ocean shrinks. **He published his theory in History of Ocean Basins (1962), and it came to be called \"seafloor spreading.\"** **Magnetic Reversal** Further evidence came along by 1963, as geophysicists realized that Earth\'s magnetic field had reversed polarity many times, with each reversal lasting less than 200,000 years. Rocks of the same age in the seafloor crust would have taken on the magnetic polarity at the time that part of the crust formed. Sure enough, surveys of either side of the Mid-Atlantic Ridge showed a symmetrical pattern of alternating polarity stripes. Magnetic reversal happened many times in the past. The occurrence of the magnetic reversal can be explained through the magnetic patterns in the magnetic rocks. These magnetic patterns allow our scientists to understand the ages and rate of movement of the materials from the mid-oceanic ridge.The magnetic reversal, also called the \"magnetic flip\" of the Earth, happens when the North Pole is transformed into the South Pole, and the South Pole becomes the North Pole. This event happens because of the changing direction of the flow of materials in the Earth\'s liquid outer core. By the 1970s, geologists had agreed to use the term \"plate tectonics\" for what had become the core paradigm of their discipline. They used the term \"plates\" because they had found evidence that not just continents move, but so do whole plates of the Earth\'s crust. A plate might include a continent, parts of a continent, and or undersea portions of the crust. Alfred Wegener\'s idea of continental drift had been developed and refined together with the Seafloor Spreading of Harry Hess