Earth Science 1G03 Exam Notes PDF

Summary

These notes cover the fundamentals of plate tectonics, including historical views, concepts of modern geology, and evidence for continental drift. The text discusses seafloor spreading and the theory of plate tectonics, including different types of plate boundaries. The document also includes information on fossils, geologic evidence, and climate evidence related to the topic.

Full Transcript

Plate Tectonics In 1650, Bishop Ussher claimed that the earth was created in 4004 BCE. The European view (before about 1750) was that all sedimentary rocks were laid down in Noah’s flood. All other surface features were due to intermittent catastrophes. The Law of Superposition suggests that young...

Plate Tectonics In 1650, Bishop Ussher claimed that the earth was created in 4004 BCE. The European view (before about 1750) was that all sedimentary rocks were laid down in Noah’s flood. All other surface features were due to intermittent catastrophes. The Law of Superposition suggests that younger material will land on top of older material in a sedimentary environment. Concepts of Modern Geology: - Uniformitarianism: past geological events can be explained by forces occurring today (contrast: catastrophism) - Gradual Change: could cumulatively account for most of what was known about the earth in the 18th century. Since change is slow, the earth must be very old! - Earth is a dynamic planet: whose surface is constantly changing and its materials being constantly recycled In the 1800s, scientists believed that the earth had been molten and had shrunk. - Evidence: mountain belts like a crumpled skin - Problem: rift valleys seem to be pulling apart, not collapsing inwards In the early 20th century scientists thought that the earth has been expanding from the heat of radioactivity. - Evidence: rift valleys seem to be pulling apart - Problem: mountain belts are compressed, not stretched Until the 1960s, geologists believed that continents and oceans have remained fixed throughout earth’s history. Alfred Wegener proposed that continents ‘drifted’ and had formed from one supercontinent called Pangea (300-200 mya) into two smaller continents: Laurasia and Gondwanaland. However, Wegener could not explain how this could have occurred. Wegener’s Hypothesis was that modern continents move slowly over the globe and have formed from the break-up of Pangel over the last 200 million years. The problem was that there was no known mechanism for the continents to ‘plow’ through the rigid oceanic crust. Climate Evidence: Former glaciers appear to have moved out from the middle of the ocean - but glacial ice doesn’t form in the ocean. The continents fit together into Pangea, and the glacier moved out from the south pole. Coals (former tropical swamps) deposited in the Canadain Arctic at the same time as glaciers moved over N.Africa. Fossil Evidence: Fossils found on the contiences that once compromised Gondwanaland. The Cynognathus was unable to swim long distances due to its body structure, meaning that since its fossils have been found in different continents, they must have been closer together back then. Geologic Evidence: Rock units on different continents with similar structural style (how the rocks have been deformed, pressure, temperature, etc) line up into continuous chains if Pangea is reconstructed. There is much evidence for continental drift, but we still need a mechanism to explain how (and why) the continents have been moving all over the globe. Seafloor Spreading: Until WWII, we had no detailed picture of the ocean which covers 70% of the world’s surface. By the mid-1950s, there were thousands of depth-surrounding measurements which simply told us how deep the bottom was at a giving point. Mid Ocean Ridges (MOR) are submerged mountain regions usually characterized by central rift valleys. Earthquakes associated with MORs as rock bodies slide past one another. These are ‘tensional’ features formed by stretching (or extension) of the oceanic crust. They are long cracks which split basically all over the ocean basins of the world. In Iceland, the Central Rift Valleys have dropped between huge parallel faults - fractures in bedrock along which rocks move more relative to each other. Oceanic crust is pulling apart at mid-ocean ridges (MOR); as it pulls apart, it collapses in the center (central rift valley) and forms fracture zones. The pacific “Ring of Fire” shows the volcanic and earthquake activity now shown to be associated with convergence and destruction of lithospheric plates. Seafloor Spreading Hypothesis: - Oceanic crust is created at the mid-ocean ridges - Oceanic crust spreads laterally - Oceanic crust finally descends back into the mantle at the deep sea trenches The Theory of Plate Tectonics: Regions encircled by seismic (earthquake) belts are thin, rigid slabs of crust, known as plates. Seismic belts are these plate’s boundaries. We know 8 large plates, and numerous smaller ones some only with oceanic crust, ie. Pacific and Nazca plates, others include continental and oceanic crust. Plates, (or lithosphere) are about 100km thick, glide over ‘soft’ semi-molten zone in the upper mantle (atmosphere) The ocean crust is made of salt and gabbro, and mafic igneous rock. The continents, however, are made of more intermediate or felsic igneous rock, so there is a density difference between the ocean crust and the continents. The oceans and the continents are a part of these plates, and those are the things moving around. Plate Boundaries: - Transform Boundaries: two plates are sliding by one another horizontally; crust is neither created or destroyed - When two plates slide past each other horizontally, a lot of earthquakes will occur, with all of the downsides that follow them. No volcanoes or mountain building processes occur. - Part of California (Los Angeles) is on the Pacific plate, while another (San Francisco) is on the North American plate, so, over time, the two cities will continue to move away from one another. - Divergent Boundaries: two plates are moving away from each other (ie. MOR); crust is created. - Volcanoes are active at the site of divergence, magma cools to form new crust, and earthquakes are common here. Magma material comes up, oozes out and becomes new crust, some of it gets cooled along the bottom of the lithospheric plate and eventually dives back down into the mantle (convection cycle) - The Red Sea and the Gulf of California are examples of rift valleys, which split so far apart that oceanic crust is forming in rift (= MOR). If things continue, both the Red Sea and the Gulf of California will continue to widen. - Convergent Boundaries: two plates are colliding with one another; crust is ‘recycled’. Slightly different things occur when convergent boundaries are either ocean-ocean, ocean-continent, or continent-continent. - Ocean - Ocean Convergence: cooler/older/denser oceanic plate subducts underneath island arcs (also oceanic crust), creating a deep sea trench. Melting also occurs, and volcanoes tend to exist on the overlying plate. The lava that comes out tends to be more felsic than what was initially started with. (Felsic material is melted first) - Ocean - Continent Convergence: very similar to ocean - ocean convergence, except it doesn’t matter what state the ocean is at (warm, not dense, etc). The ocean will always be subducted and recycled into the mantle by the continent. You cannot subduct continental lithosphere into the mantle. Earthquakes and volcanoes will occur on bordering continental crust, as water is released from the ocean plate, causing melting to occur. - Continent - Continent Convergence: Neither continent is going to get subducted, so the only way to go is up - creating a mountain belt. This ‘doubling’ of continental crust causes isostatic forces and these high mountain ranges (Himalayas). Here, volcanoes are rare and/or non-existent as there is no water available to initiate melting, and even if there was, the rocks are so thick that it wouldn't even reach the surface. However, earthquakes are very common. Earth is divided into rigid plates which are in constant motion. The North American plate is bound by a divergent boundary to the east and transform boundaries to the west. What Drives Plate Tectonics? Convection currents in the mantle drive plate tectonic movement. Hot material rising in the mantle is found beneath mid-ocean ridges. This material cools and eventually sinks below subduction zones where oceanic lithosphere is destroyed. Four Layers of the Earth The whole earth contains much heavier elements, such as iron and magnesium. - The Crust (outer layer) is 5 - 40km thick, and is solid. The earth’s crust is mainly light elements, such as oxygen and silicon. If the earth was an apple, the crust would have the relative thickness of the apple’s skin - Continental crust: 20 - 40km thick, granitic rocks, rich in silicon, aluminum, sodium and potassium (light elements) - Oceanic crust: 2-10km thick, basaltic rocks, rich in iron and magnesium. The oceanic crust is denser than continental crust - The Mantle is approximately 5 / 40 - 2900km thick, and is solid. The mantle comprises most of earth’s volume and mass, and is the ultimate source of all crustal material, while rich in iron and magnesium - After the load (often glaciers) is taken off, the continent will bounce back up. About 10,000 years ago, the last glaciation has caused North America to continue to bounce back up even today, after the glaciers melted. - The Outer Core is approximately 2900 - 5140km thick, and is molten. Following abrupt changes in seismic velocities, density changes will indicate that the outer core is molten. The outer core is composed mostly of iron and silicon. When molten iron flows around in a convection cycle, this is predicted to have created the earth’s magnetic field - The Inner Core is approximately 1230km thick, and is solid. There is a sharp reflection of seismic waves at 5140km with a deep boundary. The inner core is a metallic core composed mainly of iron and some nickel. The pressure in the centre of the planet is too high for it to be molten Paleomagnetic Evidence: The inclination (tilt) of magnetic minerals depends on the latitude. At the equator, the magnet is pulled equally in opposite directions. The paleomagnetic declination (D) is significantly different from today’s declination. The paleomagnetic inclination (l) is not 0 degrees, as it would be for a rock formed at the equator. The paleomagnetic dipole is indicated symbolically by a bar magnet. In a volcano, magnetic minerals in lava rotate to align with earth’s magnetic field. As lava cools, magnetite minerals ‘freeze’ in place. From rocks, we can reconstruct how far away poles were and in which direction. If this is done in different continents, the magnetic poles are in different places. If you look at the direction of Europe and North America, if their magnetic poles are rotated, they will sit on top of one another. This suggests that they were once next to each other in Pangea. Africa has a slightly different pattern because it moved away from North and South America and also travelled north into Eurasia. Scientists used a magnetometer at mid ocean ridges to calculate the magnetic field strength and determine how much iron was under the ocean floor. They expected to see a positive anomaly all the way across the magnetometer. However, they saw that in other places across the mid ocean ridges, the measurement went to a negative anomaly, with actively weak magnetic fields. Magnetic reversals were detected in bands parallel to the mid ocean ridges. Geologists were then able to assign absolute dates for magnetic reversals.Using radioactive dating, the reversals can be dated from land-based volcanoes and then correlated to the sea-floor records. Long intervals of magnetic reversals are thicker than short periods. For magnetic reversals of short duration, narrow bands of ocean floor were detected and vice versa. The youngest magnetic reversal occurred at the mid ocean ridges, while older reversals were found progressively farther away. Working backwards, from modern igneous rocks, the building pattern of magnetic reversals in the oceanic crust is dated according to the land-based volcanic record. Magnetic reversals provide further evidence that oceanic crust is formed at the mid-oceanic ridges and is subsequently pushed away, bearing the magnetic polarity of its time of solidification. The age of the ocean floor increases with distance from mid ocean ridges. The oldest pieces of ocean floor are only 200 million years old. This supports the idea that the sea floor disappears (descends) beneath the deep-sea trenches. Isochrons are contours connecting rocks of equal age. As the oldest sections (blue) are furthest apart from the ocean ridge, and are typically very dense. As a result, they are the most likely to sink down. Mantle Plumes and Hot Spots - The Exception: Mantle plumes are exceptions to the general picture of mantle convection. Plumes of hot mantle rise and form volcanoes on the surface. A hot spot is where this very hot mantle substance rises up, and in some/most cases right through the lithosphere and results in volcanic activity. Instead of coming up as a wall as expected, it comes up as a column (plume) of material. Plates above them continue to move, as well. Oceans are ‘pockmarked’ with volcanoes. Seamounts are volcanoes below sea level, while guyots are flat eroded top, and was/is above sea level. For example, in the Hawaiian Islands, part of them contain a 7000km line of sea-floor volcanoes. Their age increases steadily from Hawaiian to Aleutian trench (65 mya). Only the southernmost Hawaiian volcanoes are still active, due to the fact that the sites are old along the Aleutian trench, and young along the Hawaiian chain. The plates are moving in the direction of the oldest sites, while the plumes remain still, but in the direction of the youngest sites. Volcanoes Volcanoes keep the earth warm. In certain settings, we expect different types of rocks to be found. At the top of a hot spot, we expect mafic materials, (basalts at the surface & gabbros underneath) and the same along the mid ocean ridges. When subduction occurs, under a continent or under another ocean plate, sea water flows into the overlying lithospheric plate, which melts rocks. Since the melted rock is much less dense than the material around it, it moves through the mantle, and what melts is felsic / intermediate material (rhyolite, granite, andosites, diorites, etc). Tectonic Settings of Igneous Activity: Mantle basalts can rise through continental crust. Along with this, mantle continental hotspots under continental crust cause melting of silicon - aluminum crust (rhyolite). When this occurs, many different types of igneous rocks can form, depending on the specific situation. - Arrow one - mafic eruptions: Depending on how fast the plate is moving, along with how the magma is moving up (not moving too quickly), the rock can make it through the crust with little interaction with the material around it, resulting in the creation of basalts. These are known as flood basalts. In flood basalts, the magma releases gas almost as soon as it forms, and this makes the lava really runny and flows extremely easily. Once it reaches the top, it simply floods in sheets over the surface - They can be thick and cover huge areas (ie. the Columbia plateau) - They can also cause columnar jointing by shrinkage of lava cooling (ie. Iceland) - Pahoehoe: Ropy or smooth basaltic lava. In pahoehoe, re-entry of lava may occur in an older lava tube. In a situation with a human, pahoehoe would be more dangerous as it is less viscous - Aa: Blocky basaltic lava which is much stickier than pahoehoe. In a situation with a human, aa would be less dangerous as it is more viscous, although still dangerous as the radiation would be extremely dense which could still burn them - Arrow two - felsic eruptions: On the other hand, if there is a lot of interaction under a continent, rhyolitic eruptions can occur. These eruptions are extremely violent, as there is a lot of gas trapped through silica-rich magma, while the oxygens link up as chains (cages) to create this pressure - Before a big eruption on Mount St. Helens, the mountain looks very similar to others. Along with this, the mountain began developing a bulge on the side of it. This bulge continued to get bigger and bigger, which indicated it was time to evacuate. When the volcano erupted, the bulge bew out of the side. As the gas was released, part of the cone collapsed as well, and a pyroclastic flow came out (pyro - fire, clastic - little pieces). - Pyroclastic Flow: deadly, fast-moving clouds of volcanic ash and gas. It’s speed can be up to 200 km/h, temperatures over 800 degrees, and they are usually a feature of rhyolitic volcanoes. - To predict such an enormous eruption, we now have gps technology which helps us measure any sudden change in size of a volcano at any given time. When these hotspots occur under continents, and really active times are evident, this results in Large Igneous Provinces, and these provinces are typically basalts. Volcanism at Divergent Boundaries: Basalts are also formed at divergent boundaries, and here, light is not typically required. Some photons do make it to the depths of these situations, however this ecosystem does not depend on light. The bacteria that thrive at these chemotrophs thrive on chimneys of mineral particles (sulfide minerals). These minerals are dissolved and when it arrives towards the top, it goes from hundreds of degrees celsius, to four and the minerals begin to precipitate. Types of Lava: - Basaltic Lava: has low viscosity and can flow long distances. - Andesitic Lava: is too viscous to flow far, and tend to break up as it flows - Felsic Lava: is so viscous that it may pile up in a dome-shaped mass Types of Volcanoes: - Basaltic volcanoes are very big and wide, and they are known as shield volcanoes. Here, there is low silica content, low density, and runny lava placid eruptions - Composite volcanoes have high silica content, they are stiff, and the lava is highly explosive. These volcanic eruptions result from a gas buildup due to stiff, viscous, high-silica magmas (rhyolitic). It’s ‘cone’ shape results from debris that fall out near the vent of the volcano Explosiveness of Lava: Viscosity is directly related to silica content and explosiveness. Silica tetrahedral molecules tend to link to others, resulting in a higher viscosity. The 25% increase in silica from basalt to rhyolite causes an increase in viscosity of 5 orders of magnitude. Intrusive Igneous Rock: If an intrusion runs parallel to any layering in the rock, it is considered a sill, aka concordant intrusion. Sills take advantage of the bedding layers of the sediment. On the other hand, if it cuts across the layering, it is considered a dike, aka discordant intrusion. Batholiths are huge features, obvious at the crustal scale. In individual features such as volcanoes, sills and dikes are comparatively local features that cannot be seen on a map. Earthquakes Earthquakes are shock waves or vibrations within the earth. They are triggered by the sudden slippage of rock along fault planes in the crust or mantle. They exist due to the release of accumulated strain energy. The deformation that an object undergoes in response to stress, the force per unit area, is known as the strain. Local stress is usually caused by regional: - Shear stress - Tensional stress - Compression stress in the crust Strain energy accumulates at ‘rough’ spots on the plates. Foreshocks are small, early quakes as rocks begin to fracture, while aftershocks occur when rocks along fault adjust and transfer their strain. Not all earthquakes come with foreshocks, they can come out of the blue. Seismic waves are the energy released during earthquakes that pulverizes the rock, generate heat, and cause vibrations. These waves originate in the earthquake focus, directly beneath the epicenter. All types of waves are created the moment the rock breaks, and they go in their separate directions. - Body Waves travel through liquids and/or solids throughout earth, and they do not cause most of the damage - Primary Waves: the fastest waves that move and are compressive (P waves). If a primary wave is coming through and an individual was at the epicenter, the camera would move closer to them, and farther away, and on. In the meantime, the camera would push up on whatever is behind it. As primary waves travel through liquids and solids, rock vibration is parallel to transport direction. Primary waves include series of expansions / compressions. - Secondary Waves: the waves that arrive second (S waves). If a secondary wave is coming through and an individual was at the epicenter, they would be lifted up while the camera drops and vice versa. As secondary waves travel through solids only, rock vibration is perpendicular to transport direction. Secondary waves are slower than primary waves, and are also referred to as shear waves - Surface Waves cause most of the damage, as they travel near the surface of the earth - Love Waves: horizontal snake-like waves. If a love wave is coming through and an individual was at the epicenter, they would go one way while the camera goes another, and vice versa - Rayleigh Waves: vertical ocean-like waves. If a rayleigh wave is coming through and an individual was at the epicenter, the camera would go up and towards the individual, and slide back down again (ocean wave going the wrong direction) Seismic wave velocities depend on the elastic properties and densities of the rocks. Primary waves travel fast through solids and liquids, while secondary waves do not travel through liquids at all, as liquids cannot support shear stress. Secondary waves do not penetrate beyond 2900km of depth. Whenever a wave encounters a different density material, it bends. We know that the mantle is solid because the primary waves can go through it, and that the outer core is liquid because secondary waves can go through it. A seismograph records seismic waves in seismograms, and seismic waves travel at different speeds. Before an earthquake, the seismograph still may not be completely straight due to human movement, subways, etc. Locating an Earthquake’s Epicentre: While primary waves arrive before secondary waves, the difference in arrival time increases with distance from the epicenter. To locate an epicentre, seismographs from three stations are required, and wherever all three circles intersect, the epicenter lies. Distance is estimated by the time difference between wave intervals (v = d/t). Primary waves outrun the secondary waves through the interior - the farther the distance, the greater the discrepancy. The epicentre is not where the earthquake took place (focus), but it is the point on the surface directly above the focus. The magnitude of an earthquake is proportional to the maximum amplitudes on seismograms. The Richter magnitude scale compensates for decreasing amplitude with increasing distance from the epicenter. The Richter scale is logarithmic, as magnitude 5 is 10 times larger than magnitude 4. The energy that is released increases 32 times between each magnitude. Great earthquakes are between 7.0 and 8.0 on the Richter scale, while strong earthquakes are around 5.0 and 6.0 on the scale. To determine the magnitude on the Richter scale, a scientist would take data plots A and C, with A as distance vs time between the P and S waves, and with C as the amplitude of the waves. By connecting the points in a line, they intersect at the magnitude scale. The moment magnitude is related to the size of the fault rupture and amount of seismic energy that was released. It is similar to the values on the Richter scale, but they are more accurate measurements on seismograms and can be measured in a field. Magnitude does not equal destructiveness. The extent of destruction depends on the depth of focus, rock types available, proximity to population centres, types of structures around, utilities affected, time of day, etc. The Mercalli intensity scale classifies the destructiveness of earthquakes in 12 categories. Earthquakes and Plate Tectonics: The epicentres of 99% of earthquakes are confined to seismic belts; the depths of their focus, and sense of faults are consistent with the theory of plate tectonics. Shallow earthquakes reach depths of 30 - 40km, while deep earthquakes reach depths down to 700km. - Mid ocean ridge earthquakes are shallow, resulting in tensile stress - Transform boundary earthquakes are shallow, resulting in shear stress As the energy of earthquakes dissipate with distance, we are typically in more danger the shallower the earthquake is. The higher the breakage of rocks is in the crust, the closer it is to where we live, and ground motion increases. In British Columbia, earthquakes occur along mid ocean ridges, transform faults, as well as subduction zones. In North America as a whole, earthquakes are concentrated along plate boundaries. Some earthquakes are in eastern North America due to residual weakness from an old rift system. The continent was at one point pulling apart, but it never totally separated. Most of these earthquakes are not felt by humans. Minimizing Earthquake Damage: - Earthquake-resistant building construction - Liquefaction of sediments is potential danger During liquefaction, saturated sediment or soil (not hard rock) is transformed to liquid when ground shaking causes particles to lose contact. Of soil in Niigata, Japan, liquefaction causes earthquake-resistant buildings to fall over. - Tsunami zones Tsunamis can be caused by submarine earthquakes / landslides. The waves are created over the epicenter, and they begin to slowly rise as they ‘touch bottom.’ When the wave touches the bottom, the friction causes it to slow down, however the wave behind it is still moving fast. This causes them to build on each other and create a big wave. Water then pulls back from the shorelines before the tsunami arrives. - Avoid landslide prone regions - Control unsafe dams Geologic Structures Ductile (plastic) response of rock layers results in folds, also known as permanent wavelike deformations in layered rocks. A fold consists of two limbs which are divided by an imaginary surface called the axial plane. The line is formed by the intersection of the axial plane and the surface of a rock layer is called the fold axis. An anticline is a fold with the convex side upward, therefore the oldest layers are in the middle, while a syncline is a fold with the concave side upward, therefore the youngest layers are in the middle. Usually, anticlines and synclines occur together and alternate in the field. Special Folds: A structural dome is an anticlinal circular structure. A structural basin is a synclinal circular structure. Brittle response to stress results in faults, also known as a fracture in bedrock along which rocks on one side have moved relative to the other side. When such movement is absent, the fracture is called a joint. Several joints are known as joint sets. In a normal fault, the hanging wall is the portion that lies above the fault, while the footwall is the portion that lies below the fault. The hanging wall comes down, and the footwall comes up. This can be known as tension. In a reverse fault, the hanging wall is the portion that lies above the fault, while the footwall is the portion that lies below the fault. The hanging wall comes up, while the footwall comes down. This can be known as compression. Common in mountain belts, a reverse fault called a thrust fault exists at a very small angle. In a strike-slip fault, rock units slide past each other. There is only motion along the fault’s strike only, and there is no dip-slip component. On a left lateral fault, both the hanging and footwalls slide to the left. On a right lateral fault, both the hanging and footwalls slide to the right. Strike-slip faults (or transform faults) form long linear valleys usually indicated by streams or long lakes. Normal faults case extension in the crusts, also known as rifts. Basin and range topography are often caused by heating of deep crustal rocks. The Canadian Rocky Mountains’ structural style changes with rock types and East-West position. Series of small thrust faults exist and ride on top of larger thrust faults. How a rock behaves depends on what it’s made of. The rock response to stress is influenced by a number of factors: - Type of stress - Type of rock - Temperature - Pressure - Fluids - Length - Magnitude of stress applied Deeper in the crust, there are warmer conditions. Within a depth of ~15 km, all rocks will tend to bend instead of break. Up near the surface, temperatures are colder, and rocks become more brittle and tend to break. Latitude and Longitude: The units of measurement for latitude and longitude are angular shown in degrees, minutes, and seconds. When determining the longitude and latitude values for a certain place, count to the last full minute mark before the target and record that number as the minute value. Geologic Time Relative Time: The relative age is the age of an object/event relative to the age of other objects/events in the absence of time units. Relative time usually remains constant.Relative ages help to observe physics relationships of rock layers: - Fossils: physical evidence of past life preserved in rocks - Faunal Succession: fossils succeed one another in definite and recognizable order Stratigraphic correlation uses the principle of faunal succession (W. Smith). Each formation contains a unique assemblage of fossils and the fossil assemblages succeed one another. Rocks containing identical fossils are identical in age. Index fossils existed only for a short time interval but had a wide geographical distribution. Implicit in the application of faunal succession is that the evolution of organic life is the explanation for fossil records. This also allows us to correlate rocks on one continent with those on another. Often, igneous, metamorphic, and sedimentary rocks occur in the same outcrop or region. In this case, geologists study cross-cutting relationships: the principle that an intrusion of fault is younger than the rock that it cuts/intrudes (ie. new things disrupt older things). Unconformities are a gap in the sedimentary record. Not all layers have been deposited continuously over time. This ‘time gap’ (missing rock) can only be measured by fossils and radiometric dating. Absolute Time: The absolute age is the age of an object/event in years. Radioactive isotopes are used to determine absolute ages of rocks. Radioactive parent isotopes are transformed into (non-radioactive) stable daughter products/isotopes. Half-Life Principle: Half-life is the time required for half of a given parent isotope to decay into its daughter product. The rate of decay is assumed to be constant. The effective time range is the interval order which a radioactive isotope yields useful dates (very, almost too small to see). It is approximately 10 times the half-life. Radioactive carbon is created in the atmos[here from cosmic radiation. It is incorporated into all living tissues. When an organism dies, radioactive 14C decays to stable 14N. So, the ratio of 14C to 14N is directly related to the time of death. Igneous minerals usually yield the most accurate radiometric ages for rocks. In contrast, sedimentary rocks are often problematic to date with radiometric techniques. Bracketing combines the determination of radiometric dating of igneous intrusions with the principle of cross-cutting. By applying these methods, geologists have dated the entire geological time scale. Age of Earth: The oldest known minerals (zircons) were found in Australia and are, according to their radiometric date, 4.3 billion years old. Based on the observations: - Many meteorites have an age of 4.6 billion years - Lunar soils yielded dates of 4.55 billion years - Comparison of the current lead-207/lead-206 ratio of the Earth with that of meteorites Geologists concluded that the planet Earth is approximately 4.6 billion years old. Anthropocene: Anthropocene is a potential new time unit, as geological evidence suggests human-induced environmental changes. Here, a typical ‘index fossil’ could be plastic, and part of the Holocene unit or maybe the Holocene unit has ended, whereas Anthropocene has begun. Plastiglomerate is a new type of rock robled together from plastic, volcanic rock, beach sand, seashells, and corals has begun forming on the shores of Hawaii. Topographic Maps Magnetic declination describes the movement of magnetic north (MN) away from true north (TN). It is the measurement of the angle shown as X, and is expressed in degrees and minutes. Slope Gradient (*NO UNITS*): - Use your coordinates to locate your start and end points - Use the first contour line you come to as the elevation for each point and calculate the difference in their elevations - Calculate the ‘real world’ distance between them using a ruler and the map scale Fossils Fossilization is rare. In order to occur, it needs to have hard parts (bone, shell, etc), exist in anoxic (low oxygen) conditions, and must be buried rapidly in a low energy environment. Uniformitarianism: In modern day shallow-water environments, 30% of organisms have sturdy shells, 40% of organisms have fragile shells, and 30% of organisms are soft-bodied. Types of Preservation: - Preserved / replaced bones - Permineralization (petrified wood) - Molds & casts (molds and casts of a shell) - Carbonization (carbonized impressions of fern fronds in a shale) - Trace fossils (dinosaur footprints in a mudstone) Trace fossils provide information about the depth of water, and can be categorized based on shape and orientation, even without seeing the actual animal. Index Fossils: Mass Movement/Wasting Mass movement is the downhill movement of Earth materials due to gravity. It is an important part of erosion and weathering, as its movement is caused by oversteepening of slopes due to erosion by streams, glaciers, waves, etc. The angle of repose involves unconsolidated material at gravitational equilibrium. The angle depends on particle size, angularity, sorting, and whether or not the material is wet or dry. The angle of repose varies from 35 to 45 degrees. Surface tension of water causes cohesion of grains only if a film is present. Failure of a slope occurs when the external (driving) forces exceed the shear strength. Small amounts of water increase shear strength, and large amounts of water decrease shear strength. Flows (mudflows, debris flows, earthflows) are unconsolidated and sometimes contain water-saturated materials. Catastrophic mudflows occur due to Hurricane Mitch in Honduras and other Central American countries. Flows are often associated with volcanic complexes (Lahars - observed at Mt. St. Helen’s) A creep is the slowest form of mass movement, where unconsolidated material slowly drifts downhill. Frost wedging contributes to rock falls by weakening joints. Here, water gets into cracks of rocks and freezes when it gets cold. The wedges crack open every winter season, and eventually the strength overcomes and the rocks fall. Rock falls result from steep slopes, and rubble collect at the base at an angle of repose (~35 degrees), known as ‘talus.’ Rock slides are a movement of blocks of bedrock (ie. the Frank slide). Triggers for mass movement can include: - Unfavourable geologic structures: Frank Slide, Gross Ventre Slide, Vaiont Slide - Heavy rainfall or melting snow: Gros Ventre - Earthquakes: Frank slide (?) - Unfavourable artificial structures: Vaiont Slide Gros Ventre/Vaiont Slide: Here, there is a layer of shale with sandstone on top of it. The initial weight of the sandstone was heavy, but not heavy enough to slide. The groundwater levels grew higher due to melting snow and rain. Once water got involved, the sandstone began to erode, and shale began to weaken and break. The debris from the erosion filled the valley, which blocked the stream and formed the side lake. The scar remained on the hillslope. Vaiont Dam: The dam increased groundwater level, and increased pore water pressure caused the slope to move, ultimately creating a rockslide. The rocks slid into the reservoir pushed water into the dam reservoir over the dam and ultimately into communities downstream. After this disaster, rock bolts had been placed that pulled sandstone layers together. Frank Slide: If conditions do not change, slides will continue to occur in the same place. Landslide hazards can include: - Houses and cars add weight to the slope - Cesspools and lawn sprinklers add water to the slope - Roads cut the undermine slope - Natural vegetation that is removed and lawns planted in its place - Streams undercut the slope Recognizing these hazards can be done by considering the: - Relationship of the bedrock to the topography - Condition of the bedrock - Composition and steepness of the slopes - Groundwater conditions - Tectonic history - Location of roads, dams, bridges, and towns with respect to any hazards Slope Stability Map: Quickclay is found in places that used to be glaciated, and it's deposited in saltwater in the ocean. It is very stable if the water in its core space remains salty. When the glaciers pushed down on North America which caused the quickclay to become above sea level. Groundwater is now able to approach this clay, and the salt dissolves and is removed. Quickclay is strong, unless it is overloaded with weight that it cannot handle, and it ultimately becomes a liquid consistency. Streams Water that falls within a particular area of a continent eventually reaches the ocean via rivers and their tributary streams. The total area drained is known as the drainage basin. A dissolved load (potassium, bicarbonate, etc) occurs where dissolved ions are carried in water solution (~ 35% of suspended load). A suspended load occurs where particles are carried in suspension in flowing water, and most material is transported in suspension. A bed load occurs where particles are dragged along the stream bottom. The capacity is the amount of material a stream can carry. As velocity decreases, sediment comes out of suspension and settles on the bottom and sides of the channel. Slower streams deposit finer material. The discharge (length x width x velocity = m3/s) is the volume of water moving past a given point per unit of time. This is a product of the cross section area and the stream’s velocity. Increasing cross-sectional area or the stream’s velocity will increase discharge. A river profile is a concave profile which is different for each river. The gradient is the steepness of the channel over a specified length, where the long profile is the cross section along a channel showing gradients from the source to the mouth. The base level is the limiting level below which stream that can not erode (usually ocean/lake). Changing base levels can be done using dams. Building dams causes a new base level to form upstream of the dam. Eventually, sediment must be dredged in order to continue using the dam. When the water goes from the stream into the reservoir, the velocity becomes 0, and the debris settles and the sediment comes out of suspension. There are screens located at the opening of the dam that cause ONLY water to flow out. As a result, an amount of material builds up within the reservoir. Eventually, there will be no space left for water to remain and flow through. However, because this occurs, there are issues downstream; erosion will occur because there is no sediment coming through the channel. Channels of groundwater are constantly in motion as they are choked by the sediment load and new channels are ultimately established, known as braided streams. So much water is melted off of the glaciers, but it isn’t enough water to deal with all the sediment being melted out as well. This can also occur when water is travelling downward at high velocities and high elevations, they end up dumping their sediment at the flatter valley to cause the braids. Eventually, the sediment will clear but it is a very slow process. Meandering streams occur when deflection of the current causes erosion on one bank (cutbank). An increase of current increases erosion on that bank and a decrease of current on the other bank increases deposition and causes sandy point bars. Clay and silt size particles that settle cause floodplains to become very fertile and ultimately aid agricultural lands. Climate change causes more extreme events to occur at smaller intervals of time. Groundwater In groundwater zones, water is always on the move. An unsaturated zone is full of water and air fill pores, while a saturated zone is full of pores of rock/sediment which are filled with water. The water table is the upper limit of the saturated zone. During wetter years, the water table may rise and the discharge area will expand, causing a wider and taller stream. During drier years, the water table may fall and the water stream may also fall. In general, only water in the saturated zone is considered ‘groundwater.’ Water infiltrates from rainfall etc. through the soil, and then percolates down to the water table, and the position of the water table depends on seasonal factors. Groundwater flows from recharge to discharge areas. The recharge area is an area in which surface water enters the groundwater zone, and it generally located in upland areas. The discharge area is an area in which groundwater emerges at the surface, and it is typically located in lakes, ponds, streams etc. The water table is not flat, as it tends to follow topography. Most streams fed by groundwater are called effluent or gaining streams. In arid areas, streams may flow on the surface or above the water table; these are known as influent or losing streams. Porosity is the fluid storage capacity of any rock or sediment, while permeability is the ability to transmit that said fluid, along with the volume of water that can move through a given cross section per unit of time; it can also refer to the connectedness of fractures in a non porous igneous rock. Very fine sediments like clay typically have a low permeability, while very coarse sediments have excellent permeability. An aquifer is a body of sediment that is both porous and permeable. Aquifers can also be in fractured rocks if the fractures are well-connected. Pumping Water from Wells: Water generally can be pumped from a well faster than the aquifer can recharge. Before pumping, the water table is reasonably flat, but as pumping proceeds, the water table drops. A cone of depression forms at each well, and this lowering of the water table can make previously productive wells go dry. Small wells have small cones, while big wells have big cones. An unconfined aquifer has access to the surface and water can percolate freely from rainfall into the aquifer, while a confined aquifer is bound by low permeability beds above and below and can only get water from surface exposures. To access the unconfined aquifer, you will not have to dig as deep to reach the water source, which makes an unconfined well cheaper to produce. However, the risk for pollution increases within the unconfined aquifer. An artesian well occurs when water under pressure flows at the surface. Along with this, many geological factors can cause the occurrence of springs, but generally need to provide a conduit to bring the water to the surface. Groundwater pollution can occur when the groundwater table rises under the heaped up landfill. Here, a plume of contaminated water flows in the direction of regional flow. A common problem is the leaking of old gasoline tanks. Throughout Ontario, there have been a number of gas stations that have been closed to clean up the gas leaking into groundwater. Toxic chemicals such as carcinogenic ones are denser than water, and they travel to the very bottom of the groundwater supply. In Walkerton, Ontario, coliform and E. coli bacteria from agricultural activity had infiltrated the unconfined aquifer. As groundwater flow rates are fast in this area, the bacteria was carried to the town relatively quickly. Salt Water Encroachment: Seawater is denser than freshwater, so salt water usually lies under the freshwater in coastal aquifers. Pumping of fresh water lowers freshwater hydraulic potential and seawater moves into the freshwater zone. Since seawater has loads of sulfate, this can lead to a very smelly water supply as well as salt contamination. If more water is pumped through the well than can be replenished, the surface that divides the saltwater and freshwater is eventually pumped and contamination ultimately occurs. Land Subsidence: Land subsidence involves the mining of groundwater, and is mainly a problem in highly populated areas with high demands for water due to agriculture, populations, etc. Once an aquifer begins to collapse, there is no going back, as adding water does not do anything. Karst and Caves: Caves are formed in limestone by dissolving CO2 in groundwater. Caves form just below the water table. When old caves are empty, speleothems grow. When the water table sinks, new caves form. When caves collapse, karst landscapes develop. When a cave is dissolved under the water table and the water table falls, the cave is left. As groundwater infiltrates through the cave, stalactites and stalagmites are precipitated out of water. In karst environments, sinkholes are features that occur when the cave has collapsed. Disappearing streams and springs are also common in these areas. As sinkholes connect and roofs fall from caves, karst landscapes occur in the absence of the roof. Oceans & Coastlines Surface currents in an ideal ocean (with no continents) would simply follow global wind patterns, while surface currents in a real ocean follow wind patterns but are interrupted by the continents. However, the current ocean currents are relatively new, as the continents have not always been where they are today. Another slower moving density dependent current type system moves in the z-axis, where water goes both up and down instead of just left and right; known as the oceanic conveyor belt. Dense water is cold and/or salty, while light water is warm and/or fresh. Freezing occurs in the North Atlantic, and sea ice forms, and this ice tends to be more salty than the water that it formed from. This causes the water remaining below it to become saltier and colder, and eventually the water begins to sink forming nutrient rich bottom water, showing a huge density difference. Waves are created by wind, and height as a function of wind speed, duration of wind, and the distance over which the wind blows (fetch). When the wave approaches shore, the ocean bottom interferes with particle motion. The wave slows and rises, eventually ‘breaking’ in the surf. The motion of the water near shore is usually strong enough to carry sand and pebbles. Since natural shore lines are irregular, and winds change direction over time, we almost never have a perfectly straight coastline and perfectly perpendicular wave movement. As a result, sand grains move away from and towards the water, and the overall movement is along the shore. This is known as longshore drift. Longshore drift spits (curling shape of sand) barrier islands, lagoons, and baymouth bars. Baymouth bars can be cut off with the help from storm weather. Controlling beaches occur when groins are built to catch the longshore drift and stop it from moving down the coast. The problem with this however is that erosion occurs between the groins due to the increased water energy. With erosion, water begins to flood towards the communities near the beach. Many beaches in North America are shrinking due to dams catching river sediment and rivers are straightening. In controlling beaches, retaining walls have been placed to protect buildings near the coast from wave action. The problem with this is that the wave energy is directed at the beach instead and is eroded away during storms. In sandy coastlines, rip currents are typical where water flows straight out to sea. The return of water occurs to open the ocean after being pushed up on the beach. These fast currents can be very dangerous. Rip currents carry sediments out to open water, and can be spotted by looking for a sediment cloud. Cliff erosion occurs when waves start touching the bottom and slow down closer to the headland. They move at different speeds which causes the waves to rotate towards the headland. As a result, the cliffs (headlands) sticking out into the ocean are eventually eroded. The wave erosion at the base of a cliff contributes to mass movement. Over time, the headbands are eroded and the material is deposited in beaches between headlands. As a result, sea arches and sea stacks are formed. For the Scarborough bluffs in Ontario, toe berms have been installed to prevent cliff erosion, and there are indentations in these berms used for fish. Vegetated coastlines have vegetation that has adapted to either salty or brackish water. They protect the coastlines as they take all of the water energy before it reaches the coast. On natural beaches, storms push sand from beaches farther up onto shore, and eventually, the sand will make it back to the beach. As a result, the beach will continue being wide and protective. Sea walls etc. prevent the washing up of the sand, and so the sand will simply wash offshore and down the coast. As a result, the beach is narrow after the storm. Rocky coastlines are more stable than sandy coastlines. Roads that are perpendicular to beaches provide a corridor for sand and debris, as roads should actually be oriented diagonally to the beach. Glaciers The temperature among glaciers is low enough to retain snow all year round, considering high latitude and altitude. Some polar climates are very dry, so glaciers do not develop due to insufficient snowfall. Tropical glaciers do exist, as long as they are high enough and cool enough. During the last glacial maximum period, all of Canada was covered in ice. Mountain glaciers are in mountainous areas where the snow doesn’t melt during the summer, while a continental glacier is a large, continental-scale feature flowing out from the centre. The ice in continental glaciers is thickest in the middle, so they form domes. To form a glacier, snow is initially accumulated through snowfall, and is eventually incorporated as ice by pressure. Water forms at pressure points of contact, refreezes, and then eventually the glacier becomes solid ice. The continuous accumulation of ice and snow will produce the glacier. The glacial ‘budget’ depends on snow accumulation, and ice is lost due to melting and evaporation (ablation). Accumulation takes place above the snowline, while ablation takes place below the snowline. The glacier is at equilibrium when the rate of accumulation matches the rate of ablation. However, it is not stuck frozen in time, as the ice is still moving down with gravity. The glacier is advancing when the rate of accumulation is higher than the rate of ablation. A glacier is decreasing/retreating when the rate of ablation is higher than the rate of accumulation. The snowline of the glacier separates the zone of ablation from the zone of accumulation. If the size of the zone of accumulation is decreasing, then the glacier is retreating. The movement of glacial ice depends on the response of ice to stress. The top of ice fractures are brittle and crevasses form, while the lower zone of the ice is ductile and flows. At terminus, the glide planes stack up like thrust faults. The water at the base allows for movement, and the buildup of this water can cause surging. Glacial Erosion: A V-shaped river valley forms before glaciation. The glacier then carves out a U-shaped value with tributary glaciers in the side valleys. After glaciers have melted, the valleys are U-shaped. Eventually, there are hanging valleys, and waterfalls, if there is enough drainage. Glacial Deposition: A till is composed of unsorted, unlayered debris, while moraines are deposits of till either on a glacier or left behind when the glacier melts. Erratics are boulders that are left behind. Carbon dating can be used to determine the age of glaciers, where moraines carry carbon from all sorts of material. (*end moraines) (*till) Moraines can also occur as they converge where valleys meet. The retreat of glaciers can leave various deposits, as moraine deposits form at terminus. Drumlins are composed of till, and their shape is characterized by the direction of the glacier, and they help people know where the ice is moving. Eskers are sediment ridges that are thought to form from meltwater streams within the glacier, as they tunnel in the glacier where sand and gravel are deposited. Long Term Changes: Ice ages happen throughout history, although they are rare. Triggers can include: - Continent over poles - Continents over hot spots and they rise in elevation - Slowing mid ocean ridge spreading, causing more space in the ocean, and fewer shallow seas on land - Mountain building, CO2 using in weathering and fresh rock Short Term Changes: Once an ice age has begun, periodic cycles of glacial advance and retreat take over. Milankovid (orbital) cycles control the input of solar heat. A general circulation mathematical model (GCM) can be used to predict future climate change. The model takes the temperature and moisture relationships within the atmosphere, hydrosphere and lithosphere. The following is generally considered: - Addition of greenhouse gasses such as CO2 and CH4 - Land clearing may have decreased the capability of plants to remove CO2 from the atmosphere It is predicted that the temperature will increase from 2 to 5 degrees celsius over the next 100 years. The method which involves going back into the past and determine how accurate the model was is called backcasting.

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