Physical Geology Lecture 1 2024 PDF

Physical Geology Year 1 Lecture 1 Introducing Geology By Prof. Mohamed Tawfik Date : 1/ 10 /2024 8/10/2024  Outlines Who needs geology Earth systems An o...

Physical Geology Year 1 Lecture 1 Introducing Geology By Prof. Mohamed Tawfik Date : 1/ 10 /2024 8/10/2024  Outlines Who needs geology Earth systems An overview of physical geology Geologic time  Who Needs Geology? Everyone on this planet. The clothes you wear, the food you eat, your smart phone, and your car exist because of what geologists have discovered about the Earth. The Earth can also be a killer. You might have survived an earthquake, flood, or other natural disaster thanks to action taken based on what scientists have learned about these hazards. Supplying Things We Need We depend on the Earth for energy resources and the raw materials we need for survival, comfort, and pleasure. Every manufactured object relies on Earth’s resources— even something as simple as a pencil. Earth processes, at work for millions of years, have localized material into concentrations that humans can mine or extract. By learning how the Earth works and how different kinds of substances are distributed and why, we can intelligently search for metals, sources of energy, and gems. Even maintaining a supply of sand and gravel for construction purposes depends on an understanding of geology. Supplying Things We Need The average American uses: Nonmetals (rock, clay, salt etc…) ~8,666 kg/person/year. Metals ~147 kg/person/year. Energy 3,626 (958 gallons) liters petroleum. 1,908 kg coal. 2,775 cubic meters natural gas. 0.06 kg of uranium. Supplying Things We Need Modern society currently depends on abundant and cheap energy sources. Nearly all our vehicles and machinery are powered by petroleum, coal, or nuclear power and depend on energy sources concentrated unevenly in the Earth. The U.S. economy, in particular, is geared to petroleum and natural gas as cheap sources of energy. It is important to remember, however, that these resources took hundreds of millions of years to form, and they are being rapidly depleted. In recent years, the United States has been able to reduce its reliance on imported oil by developing technology to access oil that was previously too difficult or too expensive to extract. Supplying Things We Need Finding more of this diminishing resource will require more money and increasingly sophisticated knowledge of geology. Although many people are not aware of it, we face similar problems with diminishing resources of other materials, notably metals such as iron, aluminum, copper, and tin, each of which has been concentrated in a particular environment by the action of the Earth’s geologic processes. Protecting the Environment Our demands for more energy and metals have, in the past, led us to extract them with little regard for effects on the environment and therefore, on ourselves. Mining of coal, if done carelessly, for example, can release acids into water supplies. Understanding geology can help us lessen or prevent damage to the environment—just as it can be used to find the resources in the first place. Protecting the Environment Protecting the Environment The environment is further threatened because these are nonrenewable resources. Petroleum and metal deposits do not grow back after being harvested. As demands for these commodities increase, so does the pressure to disregard the ecological damage caused by the extraction of the remaining deposits. As the supply of resources decreases, we are forced to exploit them from harder-to-reach locations. The Deepwater Horizon oil spill in the Gulf of Mexico in 2010 was due in part to the very deep water in which drilling was taking place Protecting the Environment Protecting the Environment Geology has a central role in these issues. Oil companies employ geologists to discover new oil fields, while the public and government depend on other geologists to assess the potential environmental impact of petroleum’s removal from the ground, the transportation of petroleum, and disposal of any toxic wastes from petroleum products. Protecting the Environment Understanding geology can help us lessen or prevent damage to the environment—just as it can be used to find the resources in the first place. Dwindling resources can encourage disregard for ecological damage caused by extraction activities. The Alaskan Pipeline Protecting the Environment Protecting the Environment In the 1960s, geologists discovered oil beneath the coast of the Arctic Ocean on Alaska’s North Slope at Prudhoe Bay. It is now the largest oil field in North America. In the late 1970s before Alaskan oil began to flow, the United States was importing almost half its petroleum, at a loss of billions of dollars per year to the national economy. At its peak, over 2 million barrels of oil a day flowed from the Arctic oil fields. Despite its important role in the American economy, some considered the Alaska pipeline and the use of oil tankers to be unacceptable threats to the area’s ecology. Protecting the Environment The 1,287-kilometer-long pipeline crosses regions of ice-saturated, frozen ground and major earthquake- prone mountain ranges that geologists regard as serious hazards to the structure Building anything on frozen ground creates problems. The pipeline presented enormous engineering problems. If the pipeline were placed on the ground, the hot oil flowing through it could melt the frozen ground. On a slope, mud could easily slide and rupture the pipeline. Careful (and costly) engineering minimized these hazards. Much of the pipeline is elevated above the ground. Radiators conduct heat out of the structure. In some places, refrigeration equipment in the ground protects against melting. Protecting the Environment Records indicate that a strong earthquake can be expected every few years in the earthquake belts crossed by the pipeline. An earthquake could rupture a pipeline—especially a conventional pipe as in the original design. When the Alaska pipeline was built, however, in several places sections were specially jointed and placed on slider beams to allow the pipe to shift as much as 6 meters without rupturing. In 2002, a major earthquake (magnitude 7.9—the same strength as the May 2008 earthquake in China, described in chapter 16, that killed more than 87,000 people) caused the pipeline to shift several meters, resulting in minor damage to the structure, but the pipe did not rupture. Protecting the Environment Protecting the Environment In January 1981, 5,000 barrels of oil were lost when a valve ruptured. In 2001, a man fired a rifle bullet into the pipeline, causing it to rupture and spill 7,000 barrels of oil into a forested area. In March 2006, a British Petroleum Company (BP) worker discovered a 201,000 gallon spill from that company’s feeder pipes to the Trans-Alaska Pipeline. This was the largest oil spill on the North Slope to date. Subsequent inspection by BP of its feeder pipes revealed much more corrosion than expected. As a result, it made a very costly scaling back of its oil production to replace pipes and make major repairs. Protecting the Environment In August 2016, Native American protests in North Dakota halted construction of a section of the Dakota Access pipeline, which is intended to span over 1,000 miles between North Dakota and Illinois. The protests were sparked by concerns about negative impacts on the environment and damage to sites of cultural importance. The alternative to pipelines is transporting oil by rail, which can be hazardous. On December 30, 2013, a train carrying crude oil collided with another train in North Dakota. The collision caused a large explosion and fire, leading to a partial evacuation of the nearby town of Casselton. Earlier in 2013, a train carrying crude oil derailed in Quebec, Canada, killing more than 40 people in the town of Lac-Megantic. Protecting the Environment Oil can also be transported by sea. When the tanker Exxon Valdez ran aground in 1989, more than 240,000 barrels of crude oil were spilled into the waters of Alaska’s Prince William Sound. The spill, with its devastating effects on wildlife and the fishing industry, dramatically highlighted the conflicts between maintaining the energy demands of the American economy and conservation of the environment. Statistical studies of tanker accidents worldwide revealed the frequency with which large oil spills could be expected. The Exxon Valdez spill should not have been a surprise. Avoiding Geologic Hazards Earthquakes, volcanic eruptions, landslides, floods and tsunamis are the most dangerous geologic hazards. (A) Damage in Haiti (B) Damage in Chili following earthquake of 2010 following earthquake of 2010 Source: Tech. Sgt. James L. Source: Walter D. Mooney, Harper Jr., U.S. Air Force U.S. Geological Survey Avoiding Geologic Hazards geologists understand that the outer part of the Earth is broken into large slabs known as tectonic plates that are moving relative to each other. Most of the Earth’s geologic activity, such as earthquakes and volcanic eruptions, occurs along boundaries between tectonic plates. Both Chile and Haiti are located on plate boundaries, and both have experienced large earthquakes in the past. In fact, the largest earthquake ever recorded happened in Chile in 1960. Avoiding Geologic Hazards The impact of earthquakes can be reduced, or mitigated, by engineering buildings to withstand shaking. Chile has strict building codes, which probably saved many lives. Haiti, however, is one of the poorest countries in the Western Hemisphere and does not have the stringent building codes of Chile and other wealthy nations. Because of this, thousands of buildings collapsed and hundreds of thousands lost their lives. Avoiding Geologic Hazards Japan is seen as a world leader in earthquake engineering, but nothing could prepare the country for the events of March 11, 2011. At 2:46 P.M., a devastating magnitude 9.0 earthquake hit the east coast of Japan. The earthquake was the largest known to have hit Japan. Soon after the earthquake struck, tsunami waves as high as 38.9 meters (128 feet) inundated the coast. Entire towns were destroyed by waves that in some cases traveled up to 10 kilometers (6 miles) inland. The death toll from this disaster was almost 16,000, and almost half a million people were left homeless. Things could have been much worse. Due to the high building standards in Japan, the damage from the earthquake itself was not severe. Avoiding Geologic Hazards Japan has an earthquake early warning system, and after the earthquake struck, a warning went out to millions of people. In Tokyo, the warning arrived one minute before the earthquake was felt. This early warning is believed to have saved many lives. Japan also has a tsunami warning system, and coastal communities have clearly marked escape routes and regular drills for their citizens. Concrete seawalls were built to protect the coast. Unfortunately, the walls were not high enough to hold back a wave of such great height, and some areas designated as safe areas were not on high enough ground. Avoiding Geologic Hazards Still, without the safety precautions in place, many more thousands of people could have lost their lives. In some communities, lives were saved by the actions of their ancestors. Ancient stone markers along the coastline, some more than 600 years old, warn people of the dangers of tsunami. In the hamlet of Aneyoshi, one of these stone markers reads, “Remember the calamity of the great tsunami. Do not build any homes below this point.” The residents of Aneyoshi heeded the warning, locating their homes on higher ground, and the community escaped unscathed. Other Geologic Hazards Volcanic eruptions, like earthquakes and tsunami, are products of Earth’s sudden release of energy. Unlike earthquakes and tsunami, however, volcanic eruptions can last for extended periods of time. Volcanic hazards include lava flows, falling debris, and ash clouds (see box 1.2). The most deadly volcanic hazards are pyroclastic flows and volcanic mudflows. As described in chapter 4, a pyroclastic flow is a hot, turbulent mixture of expanding gases and volcanic ash that flows rapidly down the side of a volcano. Pyroclastic flows often reach speeds of over 100 kilometers per hour and are extremely destructive. A mudflow is a slurry of water and rock debris that flows down a stream channel Other Geologic Hazards Mount Pinatubo’s eruption in 1991 was the second largest volcanic eruption of the twentieth century. Geologists successfully predicted the climactic eruption in time for Philippine officials to evacuate people living near the mountain. Tens of thousands of lives were saved from pyroclastic flows and mudflows. Mount Pinatubo eruption from 1991 Source: Robert LaPointe, U.S. Air Force Other Geologic Hazards By contrast, one of the worst volcanic disasters of the 20th century took place after a relatively small eruption of Nevado del Ruiz in Colombia in 1985. Hot volcanic debris blasted out of the volcano and caused part of the ice and snow capping the peak to melt. The water and loose debris turned into a mudflow. The mudflow overwhelmed the town of Armero at the base of the volcano, killing 23,000 people. Colombian geologists had previously predicted such a mudflow could occur, and they published maps showing the location and extent of expected mudflows. The actual mudflow that wiped out the town matched that shown on the geologists’ map almost exactly. Unfortunately, government officials ignored the map and the geologists’ report; otherwise, the tragedy could have been averted. Other Geologic Hazards Armero, Colombia mudflow from 1985 Understanding Our Surroundings It is a uniquely human trait to want to understand the world around us. Most of us get satisfaction from understanding our cultural and family histories, or learning how things such as car engines or computers work. Music and art help link our feelings to that which we have discovered through our life. The natural sciences involve understanding the physical and biological universe in which we live. Most scientists get great satisfaction from their work because, besides gaining greater knowledge from what has been discovered by scientists before them, they can find new truths about the world around them. Even after a basic geology course, you can use what you learn to explain and be able to appreciate what you see around you, especially when you travel. Understanding Our Surroundings If, for instance, you were traveling through the Canadian Rockie s, you might see the scene in this chapter’s opening photo and wonder how the landscape came to be. You might wonder: (1) why there are layers in the rock exposed in the cliffs; (2) why the peaks are so jagged; (3) why there is a glacier in a valley carved into the mountain; (4) why this is part of a mountain belt that extends northward and southward for thousands of kilometers; (5) why there are mountain ranges here and not in the central part of the continent. After completing a course in physical geology, you should be able to answer these questions as well as understand how other kinds of landscapes formed. Earth Systems To understand geology, we must also understand how the solid Earth interacts with water, air, and living organisms. For this reason, it is useful to think of the Earth as being part of a system. A system is an arbitrarily isolated portion of the universe that can be analyzed to see how its components interrelate. For example, the solar system is a part of the much larger universe. The solar system includes the Sun, planets, the moons orbiting planets, and asteroids Earth Systems Atmosphere – the gases that envelop the Earth Hydrosphere – water on or near the Earth’s surface, such as the oceans, rivers, lakes and glaciers Biosphere – all living or once-living materials Geosphere – the solid rocky Earth Earth Systems The Earth system is a small part of the larger solar system, but it is, of course, very important to us. The Earth system has its components, which can be thought of as its subsystems. We refer to these as Earth systems (plural). These systems, or “spheres,” are the atmosphere, the hydrosphere, the biosphere, and the geosphere. The atmosphere, the gases that envelop the Earth. The hydrosphere is the water on or near Earth’s surface. The hydrosphere includes the oceans, rivers, lakes, and glaciers of the world. It also includes groundwater, which is water that lies beneath the ground surface. Earth Systems Earth is unique among the planets in that two-thirds of its surface is covered by oceans. The biosphere is all of the living or once-living material on Earth. The geosphere, or solid Earth system, is the rock and other inorganic Earth material that make up the bulk of the planet. The 2011 Japanese tsunami involved the interaction of the geosphere and the hydrosphere. The earthquake took place in the geosphere. Energy was transferred into giant waves in the hydrosphere. The hydrosphere and geosphere again interacted when waves inundated the shores. Can you think of other ways in which the four spheres interacted, either during or as a result of the tsunami? Earth Systems All four of the Earth systems interact with each other to produce soil, which is a mixture of decomposed and disintegrated rock and organic matter. The organic matter is from decayed plants—from the biosphere. The geosphere contributes the rock that has broken down while exposed to air (the atmosphere) and water (the hydrosphere). Air and water also occupy pore space between the solid particles in soil. Powering the Earth There are two sources of energy for the Earth: Internal – heat moving from hot interior of the Earth to the cooler exterior through convection Drives volcanoes, earthquakes and mountain building. External – energy from the Sun Drives atmospheric and oceanic circulation. Controls weathering of rocks at Earth’s surface. Internal Processes: How the Earth’s Internal Heat Engine Works The Earth’s internal heat engine works because hot, buoyant material deep within the Earth moves slowly upward toward the cool surface and cold, denser material moves downward—a process called convection. Visualize a vat of hot wax, heated from below. As the wax immediately above the fire gets hotter, it expands, becomes less dense (that is, a given volume of the material will weigh less), and rises. Wax at the top of the vat loses heat to the air, cools, contracts, becomes denser, and sinks. A similar process takes place in the Earth’s interior. Internal Processes: How the Earth’s Internal Heat Engine Works Rock that is deep within the Earth and is very hot rises slowly toward the surface, while rock that has cooled near the surface is denser and sinks downward. Instinctively, we don’t want to believe that rock can flow like hot wax. However, experiments have shown that under the right conditions, deeply buried rock that is hot and under high pressure can deform, like taffy or putty. But the deformation takes place very slowly. If we were somehow able to strike a rapid blow to the deeply buried rock with a hammer, it would fracture, just as rock at Earth’s surface would. Earth’s Interior: Compositional Layers Based on chemical composition Crust – outermost very thin rocky shell. Oceanic Crust – thin and dense. Continental crust – thick and less dense. Mantle - hot solid that flows slowly over time; (most voluminous of Earth’s layers). Core – innermost zone of the Earth; made of iron and nickel. Outer Core – liquid iron and nickel. Inner Core – solid iron and nickel. Source: NASA Access the text alternative for slide images. Earth’s Interior: Mechanical Layers Lithosphere - Rigid/brittle outer shell of Earth. Composed of both crust and uppermost mantle. Makes up Earth’s tectonic plates. Asthenosphere - Plastic zone on which the lithosphere floats. Lower Mantle - solid. Outer Core - liquid. Inner Core - solid. Source: NASA Access the text alternative for slide images. Earth’s Interior The two major types of crust are oceanic crust and continental crust. Oceanic crust underlies the oceans and is relatively thin (on average, approximately 7 km thick). It is made of basalt, a volcanic rock that is somewhat denser than the rock page 11 that underlies the continents. Continental crust is much thicker than oceanic crust, averaging approximately 35 kilometers thick. Unlike oceanic crust, continental crust is made up of many different types of rock. Its average composition is equal to that of granite, a rock you may have seen in many kitchens because it is a popular material used to make countertops. Earth’s Interior The mantle is the thickest and most voluminous of these zones, making up more than 80% of the Earth’s volume. The mantle is composed of rock that contains more iron and magnesium than crustal rocks and thus is more dense. Although the mantle is solid rock, parts of it flow slowly, generally upward or downward, depending on whether it is hotter or colder than adjacent mantle. The core, the innermost and densest layer of the Earth, is believed to be made of metal—not rock like the mantle and crust—mostly iron and nickel. The core is divided into two mechanical layers, the solid inner core and the liquid outer core. It is the convection of liquid iron in the outer core that generates the Earth’s magnetic field. Earth’s Interior: Mechanical Layers Source: NASA Access the text alternative for slide images. Earth’s Interior The expanded section of previous figure shows two very important mechanical layers of the Earth. The crust and the uppermost part of the mantle are relatively rigid. Collectively, they make up the lithosphere. (To help you remember terms, the meanings of commonly used prefixes and suffixes are given in appendix G. For example, lith means “rock” in Greek. You will find lith to be part of many geologic terms.) The uppermost mantle underlying the lithosphere, called the asthenosphere, is soft and therefore flows more readily than the underlying mantle. It provides a “lubricating” layer over which the lithosphere moves (asthenos means “weak” in Greek). Earth’s Interior Where hot mantle material wells upward, it will uplift the lithosphere. Where the lithosphere is coldest and densest, it will sink down through the asthenosphere and into the deeper mantle, just as the wax does in figure 1.6. The effect of this internal heat engine on the crust is of great significance to geology. The forces generated inside the Earth, called tectonic forces, cause deformation of rock as well as vertical and horizontal movement of portions of the Earth’s crust. The existence of mountain ranges indicates that tectonic forces are stronger than gravitational forces. (Mount Everest, the world’s highest peak, is made of rock that formed beneath an ancient sea.) Mountain ranges are built over extended periods as portions of the Earth’s crust are squeezed, stretched, and raised Earth’s Interior Most tectonic forces are mechanical forces. Some of the energy from these forces is put to work deforming rock, bending and breaking it, and raising mountain ranges. The mechanical energy may be stored (an earthquake is a sudden release of stored mechanical energy) or converted to heat energy (rock may melt, resulting in volcanic eruptions). The working of the machinery of the Earth is elegantly demonstrated by plate tectonics. Continental Drift Plate tectonics was seriously proposed as a hypothesis in the early 1960s, though the idea was based on earlier work—notably, the hypothesis of continental drift. the theory here to explain the origin of some rocks and why volcanoes and earthquakes occur. Plate tectonics regards the lithosphere as broken into plates that are in motion. The plates, which are much like segments of the cracked shell on a boiled egg, move relative to one another along plate boundaries, sliding upon the underlying asthenosphere. Plate boundaries are classified into three types based on the type of motion occurring between the adjacent plates. The Theory of Plate Tectonics Describes lithosphere as being broken into plates that are in motion. Explains origin and distribution of volcanoes, fault zones, and mountain belts. Included new understanding of the sea-floor and explanation of driving force. Gained significant support in the late 1960s. The Theory of Plate Tectonics Divergent Boundaries The first type of plate boundary, a divergent boundary, involves two plates that are moving apart from each other. Most divergent boundaries coincide with the crests of submarine mountain ranges, called mid- oceanic ridges. The mid-Atlantic ridge, which runs for approximately 16,000 kilometers from northeast of Greenland to the South Atlantic, is a classic, well-developed example. Motion along a mid-oceanic ridge causes small to moderate earthquakes. Divergent Boundaries Although most divergent boundaries present today are located within oceanic plates, a divergent boundary typically initiates within a continent. It begins when a split, or rift, in the continent is caused either by extensional (stretching) forces within the continent or by the upwelling of hot asthenosphere from the mantle Either way, the continental plate pulls apart and thins. Initially, a narrow valley is formed. Fissures extend into a magma chamber. Magma (molten rock) flows into the fissures and may erupt onto the floor of the rift. With continued separation, the valley deepens, the crust beneath the valley sinks, and a narrow sea floor is formed. Divergent Boundaries Underlying the new sea floor is rock that has been newly created by underwater eruptions and solidification of magma in fissures. Rock that forms when magma solidifies is igneous rock. The igneous rock that solidifies on the sea floor and in the fissures becomes oceanic crust. As the two sides of the split continent continue to move apart, new fissures develop, magma fills them, and more oceanic crust is formed. As the ocean basin widens, the central zone where new crust is created remains relatively high. This is the mid- oceanic ridge that will remain as the divergent boundary as the continents continue to move apart and the ocean basin widens. Divergent Boundaries Plates move apart. Magma rises, cools, and forms new lithosphere. Typically expressed as mid –oceanic ridges. Leads to the opening of an ocean. Ocean to continent convergent boundary Access the text alternative for slide images. Divergent Boundaries A mid-oceanic ridge is higher than the deep ocean floor because the rocks, being hotter at the ridge, are less dense. A rift valley, bounded by tensional cracks, runs along the crest of the ridge. The magma in the chamber below the ridge that squeezes into fissures comes from partial melting of the underlying asthenosphere. Continued pulling apart of the ridge crest develops new cracks, and the process of filling and cracking continues indefinitely. Thus, new oceanic crust is continuously created at a divergent boundary. Not all of the mantle material melts—a solid residue remains under the newly created crust. New crust and underlying solid mantle make up the lithosphere that moves away from the ridge crest, traveling like the top of a conveyor belt. Divergent Boundaries The rate of motion is generally 1 to 18 cm per year (approximately the growth rate of a fingernail), slow in human terms but quite fast by geologic standards. The top of a plate may be composed exclusively of oceanic crust or might include a continent or part of a continent. For example, if you live on the North American plate, you are riding westward relative to Europe because the plate’s divergent boundary is along the mid-oceanic ridge in the North Atlantic Ocean The western half of the North Atlantic sea floor and North America are moving together in a westerly direction away from the mid-Atlantic ridge plate boundary. Convergent Boundaries The second type of boundary is a convergent boundary, wherein plates move toward each other. By accommodating the addition of new sea floor at divergent boundaries, the destruction of old sea floor at convergent boundaries ensures the Earth does not grow in size. Examples of convergent boundaries include the Andes mountain range, where the Nazca plate is descending or subducting beneath the South American plate, and the Cascade Range of Washington, Oregon, and northern California, where the Juan de Fuca plate is subducting beneath the North American plate. Convergent boundaries, due to their geometry, are the sites of the largest earthquakes on Earth. Convergent Boundaries Convergent Boundaries It is useful to describe convergent boundaries by the character of the plates that are involved: ocean- continent, ocean-ocean, and continent-continent. The difference in density of oceanic and continental lithosphere explains the contrasting geological activities caused by their convergence. Convergent Boundaries Plates move toward each other. Ocean-Continent. Ocean-Ocean. Continent-Continent. Subduction, volcanoes, powerful earthquakes. Access the text alternative for slide images. Ocean-Continent Convergent Boundaries Access the text alternative for slide images. Ocean-Continent Convergent Boundaries If one plate is capped by oceanic crust and the other by continental crust, the lessdense, more-buoyant continental plate will override the denser, oceanic plate. The oceanic plate bends beneath the continental plate and sinks along what is known as a subduction zone, a zone where an oceanic plate descends into the mantle beneath an overriding plate. Deep oceanic trenches are found where oceanic lithosphere bends and begins its descent. These narrow, linear troughs are the deepest parts of the world’s oceans. In the region where the top of the subducting plate slides beneath the asthenosphere, melting takes place and magma is created. Ocean-Continent Convergent Boundaries Magma is less dense than the overlying solid rock and therefore works its way upward; it either erupts at volcanoes on the Earth’s surface to solidify as extrusive igneous rock, or it solidifies within the crust to become intrusive igneous rock. Hot rock, under high pressure, near the subduction zone that does not melt may change in the solid state to a new rock—metamorphic rock. Near the edge of the continent, above the rising magma from the subduction zone, a major mountain belt, such as the Andes or Cascades, forms. Ocean-Continent Convergent Boundaries The mountain belt grows due to the volcanic activity at the surface, the emplacement of bodies of intrusive igneous rock at depth, and intense compression caused by plate convergence. Layered sedimentary rock that may have formed on an ocean floor especially shows the effect of intense squeezing (for instance, the “folded and faulted sedimentary rock. In this manner, rock that may have been below sea level might be squeezed upward to become part of a mountain range. Ocean-Ocean Convergent Boundaries Access the text alternative for slide images. Ocean-Ocean Convergent Boundaries If both converging plates are oceanic, the denser plate will subduct beneath the less-dense plate. A portion of a plate becomes colder and denser as it travels farther from the mid- oceanic ridge where it formed. Thus the older of the two oceanic plates will subduct beneath the younger. After subduction begins, molten rock is produced just as it is in an ocean-continent subduction zone; however, in this case, the rising magma forms volcanoes that grow from an ocean floor rather than on a continent. The resulting mountain belt is called a volcanic island arc. Examples include the Aleutian Islands in Alaska and the islands that make up Japan, the site of the great earthquake and tsunami of 2011, described earlier. Continent-Continent Convergent Boundaries Access the text alternative for slide images. Continent-Continent Convergent Boundaries If both converging plates are continental, a quite different geologic deformation process takes place at the plate boundary. Continental lithosphere is much less dense than the mantle below and, therefore, neither plate subducts. The buoyant nature of continental lithosphere causes the two colliding continental plates to buckle and deform with significant vertical uplift and thickening as well as horizontal shortening. A spectacular example of continent-continent collision is the Himalayan mountain belt. The tallest peaks on Earth are located here, and they continue to grow in height due to continued collision of the Indian subcontinent with the continental Eurasian plate. Continent-Continent Convergent Boundaries Continent-continent convergence is preceded by oceanic- continental convergence. An ocean basin between two continents closes because oceanic lithosphere is subducted beneath one of the continents. Unlike oceanic crust, continental crust is too buoyant to be subducted into mantle. When the continents collide, subduction ceases and they become wedged together. India collided with Asia around 40 million years ago, yet the forces that propelled them together are still in effect. The rocks continue to be deformed and squeezed into higher mountains. Continent-Continent Convergent Boundaries Transform Boundaries Plates slide past one another. Fault zones and earthquakes mark boundaries. San Andreas fault in California. Transform Boundaries The third type of boundary, a transform boundary, occurs where two plates slide horizontally past each other, rather than toward or away from each other. The San Andreas fault in California and the Alpine fault of New Zealand are two examples of this type of boundary. Earthquakes resulting from motion along transform faults vary in size depending on whether the fault cuts through oceanic or continental crust and on the length of the fault. The San Andreas transform fault has generated large earthquakes, but the more numerous and much shorter transform faults within ocean basins generate much smaller earthquakes. Transform Boundaries The significance of transform faults was first recognized in ocean basins. Here they occur as fractures perpendicular to mid-oceanic ridges, which are offset As shown in the previous figure, the motion on either side of a transform fault is a result of rock that is created at and moving away from each of the displaced oceanic ridges. Although most transform faults are found along mid-oceanic ridges, occasionally a transform fault cuts through a continental plate. Such is the case with the San Andreas fault, which is a boundary between the North American and the Pacific plates. Surficial Processes: The Earth’s External Heat Engine Tectonic forces can squeeze formerly low-lying continental crustal rock along a convergent boundary and raise the upper part well above sea level. Portions of the crust also can rise because of a process called isostatic adjustment, the vertical movement of sections of Earth’s crust to achieve balance. That is to say, lighter rock will “float” higher than denser rock on the underlying mantle. Isostatic adjustment is why an empty ship floats higher on the water than an identical one that is full of cargo. Surficial Processes: The Earth’s External Heat Engine Continental crust, which is less dense than oceanic crust, will tend to float higher over the underlying mantle than oceanic crust (which is why the oceanic crust is below sea level and the continents are above sea level). After a portion of the continental crust is pulled downward by tectonic forces, it is out of isostatic balance. It will then rise slowly due to isostatic adjustment when tectonic forces are relaxed. Surficial Processes: The Earth’s External Heat Engine When a portion of crust rises above sea level, rocks are exposed to the atmosphere. Earth’s external heat engine, driven by solar energy, comes into play. Circulation of the atmosphere and hydrosphere is mainly driven by solar energy. Our weather is largely a product of the solar heat engine. For instance, hot air rises near the equator and sinks in cooler zones to the north and south. Solar heating of air creates wind; ocean waves are, in turn, produced by wind. When moist air cools, it rains or snows. Rainfall on hillsides flows down slopes and into streams. Streams flow to lakes or seas. Glaciers grow where there is abundant snowfall at colder, high elevations and flow downhill because of gravity. Surficial Processes: The Earth’s External Heat Engine Where moving water, ice, or wind loosens and removes material, erosion is taking place. Streams flowing toward oceans remove some of the land over which they run. Crashing waves carve back a coastline. Glaciers grind and carry away underlying rock as they move. In each case, rock originally brought up by the Earth’s internal processes is worn down by surficial processes. As material is removed through erosion, isostasy works to move the landmass upward, just as part of the submerged portion of an iceberg floats upward as ice melts. Or, going back to our ship analogy, as cargo is unloaded, the ship sits higher in the water. Erosion, Deposition, and Uplift Access the text alternative for slide images. Surficial Processes: The Earth’s External Heat Engine Rocks formed at high temperature and under high pressure deep within the Earth and pushed upward by isostatic and tectonic forces are unstable in their new environment. Air and water tend to cause the once deep-seated rocks to break down and form new materials. The new materials, stable under conditions at the Earth’s surface, are said to be in equilibrium—that is, adjusted to the physical and chemical conditions of their environment so that they do not change or alter with time. For example, much of an igneous rock (such as granite) that formed at a high temperature tends to break down chemically to clay. Clay is in equilibrium—that is to say it is stable—at the Earth’s surface. Surficial Processes: The Earth’s External Heat Engine The product of the breakdown of rock is sediment, loose material. Sediment may be transported by an agent of erosion, such as running water in a stream. Sediment is deposited when the transporting agent loses its carrying power. For example, when a river slows down as it meets the sea, the sand being transported by the stream is deposited as a layer of sediment. In time, a layer of sediment deposited on the sea floor becomes buried under another layer. This process may continue, burying our original layer progressively deeper. Surficial Processes: The Earth’s External Heat Engine The pressure from overlying layers compresses the sediment, helping to consolidate the loose material. With the cementation of the loose particles, the sediment becomes lithified (cemented or otherwise consolidated) into a Sedimentary rock that becomes deeply buried in the Earth may later be transformed by heat and pressure into metamorphic rock. Surficial Processes: The Earth’s External Heat Engine Isostatic Adjustment/Uplift Volcanic and/or tectonic forces build crust up above sea level. Removal of material by erosion allows isostatic uplift of underlying rocks. Weathering and Erosion Rainfall and glaciers flow down slopes. Moving water, ice and wind loosen and erode geologic materials, creating sediment. Deposition Loose sediment is deposited when transport agent loses its carrying power. Earlier sediments get buried and harden into sedimentary rock. Geologic time Deep Time Most geologic processes occur gradually over millions of years. Changes typically imperceptible over the span of a human lifetime. Current best estimate for age of Earth is ~4.55 billion years. Geologic Time and the History of Life 541 million years: complex life forms first became abundant. 230 million years: reptiles became abundant. 66 million years: dinosaurs became extinct. 5 million years: Earliest hominids. Nothing hurries geology — Mark Twain Geologic time