Chapter 13 Notes
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Chapter 13 | 1 Learning Objectives 1. Describe the theory of plate tectonics and the evidence supporting the concept of continental drift. 2. Explain the various types of plate movements that occur as a result of continental drift. 3. Discuss how plate convergence promotes folding of rock, resulting in the formation of anticlines, synclines, and other structural features. 4. Explain the general development of the Appalachian Mountains and why segments of the Ridge and Valley Province may be structurally more complex than they a ppear. 5. Compare and contrast the various kinds of faults that occur on Earth. 6. Describe why earthquakes occur, the various features associated with them, and how their location can be precisely determined. 7. Compare and contrast the various types of volcanoes and why they occur. 8. Discuss the Pacific Ring of Fire and explain the geography of the Cascade Volcanic Arc. Plate Tectonics : the theory that the Earth’s crust is divided into a number of plates that move because they float on the asthenosphere. The Lithospheric Plates th In the early 20 century, a German geophysicist named Alfred Wegener noticed that many of the continents look as if they could have fit together at one time as one single landmass. • After further investigation, Wegener discovered that portio ns of some continents share the same kind of rocks and fossils with other continents. • As a result, Wegener proposed in 1915 that a supercontinent existed about 300 million y ears ago that he named Pangaea (meaning whole earth). Comment [KL1]: The hypothetical supercontinent, composed of all the present continents, that existed between 300 and 200 million years ago. • He argued that the continents had drifted part in a process he called continental drift. Comment [KL2]: The theory that the continents move relative to one another in association with plate tectonics. Chapter 13 | 2 Wegener’s ideas were completely contradictory to the current beliefs in the geologic community and were largely ignored until the 1950s, when studies began of the o cean basins around the world. At the same time, geologists began to investigate the cause of earthquakes more closely. • The combined studies proved that the Earth’s crust consists of a series of interconnected plates that indeed move relative to one another, which formed the foundation for the theory of plate tectonics that emerged in the 1960s. • The theory was revolutionary because it explained a great deal of volcanic and earthquake activity on Earth. • It also validated Wegener’s basic premise of continental drift because it provided the mechanism that would make the migration possible. The seven major plates (boldface type) cover 94% of Earth. About a dozen smaller plates cover the remaining 6% of Earth. Notice the way that the plates move (arrows) relative to one another . [The boundary between the Indian and Australian plates is uncertain; many sources refer instead to the “Indo-Australian” plate.] Plate Movement The next obvious question is why continental drift occurs. At a fundamental level, it happens because tectonic plates float on the asthenosphere and are moved by convection loops driven by geothermal activity. • Convection within Earth occurs because the radioactive decay of elements, such as uranium, deep within the Earth creates very high temperatures between 1000 -2000°C (1800-3600°F). These high temperatures cause plumes of magma to rise slowly by convection within the mantle and into the asthenosphere. As these magma plumes reach the ba se of the crust, the spread horizontally and cool, moving segments of the crust in the process. Rock created as the magma cools sinks back into the mantle, where it melts again. How do we know plates are moving? Evidence of plate movement appears in the ag e of the sea floor. • Convection in the mantle brings magma up through fractures in the crust, where it extrudes onto the seafloor, cools, and forms new oceanic crust. • The figure below shows isochron es (lines of equal age) for the ocean floors around the world and clearly indicates areas where oceanic plates have been spreading apart. Chapter 13 | 3 In this map, colors are associated with seafloors and the continents are gray. The youngest seafloor rocks (in red) occur in the zones of seafloor spreading, with progressively older rocks (yellows, greens, and blues, respectively) away from these regions. An additional piece of evidence supporting the theory of continental drift is that magnetic stripes run parallel to the Mid-Atlantic Ridge across the seafloor in a similar pattern to the isochrones shown in the figure above. • These stripes reflect the fact that the Earth’s magnetic field has periodically reversed through time, with magnetic north becoming south and vice versa. • The ocean floor contains a record of magnetic reversals because, as the new seafloor is progressively created, its magnetic orientation is aligned with the direction at that time. Types of Plate Movements The range of potential plate movements includes (1) moving away from each other, (2) sliding past each other, and (3) colliding head on. Passive Margins Comment [KL3]: A place where the continental crust and The simplest kind of tectonic interaction occurs at passive margins. the oceanic crust are on the same tectonic plate and thus don’t move relative to each other. • These regions frequently occur in places where the continental crust and the bordering oceanic crust are actually on the same tectonic plate. • One example is the eastern seaboard of N. America, which has a passive margin with the oceanic crust in the western part of the Atlantic Ocean basin. Although th e continental crust that forms the N. American landmass is a separate rock body from the adjoining oceanic crust, they are both part of the N. American plate. Transform Plate Margins Comment [KL4]: A plate boundary where opposing plates Transform boundaries are places where plates slide horizontally past each other. move horizontally relative to each other. Chapter 13 | 4 At these margins, the plane of motion is along a nearly vertical break (or fault) that extends through much of the lithosphere. • Examples are the San Andreas Fault in California and the Dead Sea Fault along the border of Israel and Jordan. Plate Divergence In some places, lithospheric plates move away from each other in a process called plate divergence. This type of movement occurs in regions where rising magma plumes within Earth move upward and outward between plate fractures, spreading Ear th’s plates apart in a process called rifting (or Comment [KL5]: The spreading apart of the Earth’s crust divergence). by magma rising between fractures in the Earth’s plates. • A great place to see rifting is on the seafloor where magma produces a ridge -like feature called a mid-oceanic ridge, which lies parallel to the rift zone. Comment [KL6]: A ridge-like feature that develops along a rift zone in the ocean due to magma upwelling. Plates can also diverge within continents, causing a gradual split in the landmass to occur. One of the best-known examples is in eastern Africa where the East African Rift is located. Continental rifting in this area has produced a distinct valley landscape bordered by steep canyon walls, as well as several large lakes. In the northern part of the rift zone, plate divergence is occurring in three places that merge at a single place known as a triple junction. • Two of the rifts are oceanic, with one opening the Red Sea and the other causing enlargement of the Gulf of Aden on the southern side of Saudi Arabia. Chapter 13 | 5 • The last rift, which is on land, extends south from the juncture of the Red Sea and the Gulf of Aden. The eastern part of the African continent is slowl y splitting in two, resulting in several large lakes such as Lake Victoria. Plate Convergence Plate convergence occurs in three general settings: (1) oce anic crust to continental crust , (2) oceanic crust to oceanic crust, and (3) continental crust to cont inental crust. (1) Oceanic Crust to Continental Crust Oceanic crust is generally denser than continental crust because oceanic crust consists of basaltic rock, whereas continental crust tends to be granitic. In convergence zones like this, the oceanic crust s inks beneath the lighter continental crust in a process called subduction. Comment [KL7]: The process by which one lithospheric • Subduction is initiated when oceanic plates diverge due to seafloor spreading. As the spreading plate is forced beneath another. forces slowly move away horizontally from the spreading zone and toward continenta l margins. • When the oceanic crust converges with the continental crust, the denser oceanic crust is forced beneath the continental crust and down into the upper mantle. As the slab is forced deeper, the internal temperature gradually increases in a distin ct geothermal gradient. • When the oceanic crust hits the upper mantle, it melts and is recycled into magma as part of the rock cycle. This process is not continuous, but instead is episodic. The oceanic plate doesn’t flow smoothly beneath the continental crust. It moves only when the buildup of stress behind it – caused by seafloor spreading – is more than the friction caused by sliding underneath the continental plate. • The stress can cause earthquakes and significant deformation of the rocks above the subduction zone. This can also result in the construction of mountain ranges and the formation of volcanoes where magma works its way to the surface. Chapter 13 | 6 (2) Oceanic Crust to Oceanic Crust When two bodies of oceanic crust collide with one another, one p late will typically be subducted beneath another (even though they are similar densities), forming an oceanic trench. This subduction will cause stress to the overriding seafloor that may cause it to crumple upward into mountains below the ocean surface. • More often, the subduction will result in the formation of volcanoes (due to melting of the crust in the upper mantle) that grow upward in the water. If the volcanoes grow enough, they will breach the surface to form distinctive island chains. (3) Continental Crust to Continental Crust When two continental crusts collide, one plate does not ride over or below the other. Instead, they smash together like two cars in a head -on collision. The crust, like the cars, crumples, causing folding of the formerly horizontal bedrock through compression. You can see evidence of past compression and folding by looking at the rock structure. In general, a Comment [KL8]: The internal arrangement of rock layers. close correlation exists between the amount of compression and the nature of the fold. • A monocline is a one-sided slope where beds of horizontal rock are inclined in a single direction Comment [KL9]: A geologic landform in which rock beds over a large area. The kind of fold occurs when the amount of compression is relatively low. are inclined in a single direction over a large distance. More extensive compression causes anticlines and synclines to form. • An anticline is a portion of the fold where the rock layers arc upward to form a concave arch Comment [KL10]: A convex fold in rock in which rock along the fold axis. One-half of such a fold is called a limb. layers are bent upward into an arch. • A syncline is the portion of the fold where the rock layers dip downward to form a convex Comment [KL11]: A concave fold in rock in which rock trough. layers are bent downward to form a trough. • If the collision is especially intense, the rocks can be folded so much that an overturned fold (or Comment [KL12]: A structural feature in which the fold limb is tilted beyond vertical, which results in both limbs recumbent fold) forms. inclined in the same direction, but not at the same angle. • Still more compression results in an overthrust fold, which occurs when one part of the rock Comment [KL13]: A structural feature where one part of mass is shoved up and over the other. the rock mass is shoved up and over the other. Geomorphology and the Evolution of the Appalachian Mountains Large-scale compression of rocks is frequently associated with the formation of mountains. The study of mountain formation, as well as the shape of the Earth’s surface in general, falls within the subd iscipline of physical geography called geomorphology. Comment [KL14]: The branch of physical geography that investigates the form and evolution of the Earth’s surface. • Geomorphology is the study of the formation, shape, spatial distribution, and evolution of landforms on Earth – thus, the name geo (Earth) morphology (shape). • In contrast to a landscape, which is the overall appearance of a place in terms of its vegetation topography, or human modifications, a landform is a distinct geographic feature, such as a Comment [KL15]: A natural feature, such as a hill or mountain, river valley, coastline, or sand dune, for example. valley, on the surface of Earth. • The study of geomorphology is rooted deeply in geology because it requires a thorough understanding of how sediments are eroded, transported, and deposited. As far as the growth of mountains is concerned, they typically form during a distinct interval of time called an orogeny. There are many excellent examples of how continental collision and compression Comment [KL16]: A period of mountain building, such as the Allegheny Orogeny. cause orogenies in which rocks are intensely folded. Chapter 13 | 7 • The Appalachian Mountains are an excellent example, extending approximately 2500 km (~1600 mi) along the eastern United States. Severa l phases of mountain building occurred in this region, including the Taconic Orogeny (~450 million years ago), Acadian Orogeny (~375 million years ago), and Allegheny Orogeny (~290 to 248 million years ago). Hogback ridge: a ridge underlain by gently tipped rock strata with a long, gradual slope on one side and a relatively steep scarp or cliff on another. • The ridges on each side of the valley are remnants of the limbs of the anticline. Such a ridge is called a hogback ridge because one side of the ridge is steeper than the other. Anticlinal valley: a topographic valley that occurs along the axis of a structural anticline. Synclinal valley: a topographic valley that occurs along the axis of a structural syncline. A synclinal valley lies immediately to the ri ght of the hogback ridge-and-valley network at point A and is what you would expect since it lies below the adjacent ridge. (a) The landscape shortly after folding. Anticlines form the high ground ground, whereas valleys occur in synclines. (b) As time progresses, erosion modifies the landscape. Anticlines A and B are eroded by streams, forming valleys in upward-arching structures. Hogback ridges exist on both sides of the anticlinal valley at A, where limbs of the former anticline form the high ground. To the right at point B, extensive erosion has inverted the topography so that the ridge is underlain by a syncline. Chapter 13 | 8 Key Concepts to Remember About Plate Tectonics 1. The Earth’s crust is made up of interconnected plates. 2. A variety of evidence, including fossil locations and measurement of motion, strongly supports the theory of continental drift. 3. Plate margins can be passive, transform, converging, or diverging. 4. At diverging plate margins, magma plumes from the asthenosphere causing rifting, either on continents or on the seafloor. 5. At converging plate margins, plates collide or subduct in a variety of settings, causing geomorphic features such as alpine chains and volcanoes. Earthquakes: shaking of the Earth’s surface due to the instantaneous release of accumula ted stress along a fault plane or from underground movements within a volcano. An earthquake occurs when the the sudden release of accumulated tectonic stress is released in an instantaneous movement of the Earth’s crust. Although this type of movement is associated with plate boundaries, it can also occur due to rapid movement of magma within a volcano. • The strongest earthquakes are usually most associated with plate boundaries because stress builds as the plates grind together. • This stress creates a fra cture between adjoining plates called a fault. At some locations, faults Comment [KL17]: A crack in the Earth’s crust that results extend deeply into the Earth’s crust, whereas at other places they are very shallow. in the displacement of one lithospheric plate or rock body relative to another. • Faults and earthquakes can also occur in the middle of plates but it is much less common. • Although most large earthquakes occur on major plate boundaries, smaller earthquakes can occur wherever a small fault occurs in a rock body. The important thing to remember is that the plates don’t gradually slide by each other. They are locked by friction for long periods of time, causing the stress to build. • The stress builds to a critical point when a rupture occurs along the fault plane and the stress is released analogous to the stress released when a stretch rubber band is cut. The rock layers on either side of the fault immediately adjust. • The place within the lithosphere where the fault breaks is called the focus and usually occurs a few kilometers deep within the ground. • The point on the surface directly above the focus is known as the epicenter. Locating the Epicenter When an earthquake occurs, the first goal of geologists is to determine the location of the epicenter. This is accomplished through a process of triangulation, during which the distance to the epicenter is compared among seismographs stationed at three separate locations. • This methodology is based off the fact that earthquakes produce two kinds of seismic waves, or body waves, that radiate through the Earth’s interior, P waves (or primary waves) and S waves (or secondary waves). • These two types of waves move in different ways and travel at different speeds. Chapter 13 | 9 • P waves are compressional waves that cause the Earth’s crust to expand and contract rapidly in a horizontal manner as the waves radiate out from th e epicenter, and typically move at about 1.5-8 km/s (~1-5mi/s) through the crust. • S waves move about 60-70% slower and move in a vertical fashion similar to the wave pulse in a rope when you whip it up and down. They cause the ground to vibrate in a rolling fashion that can be quite noticeable and scary. • The magnitude of these waves is measured at an observation station with a seismograph. Measuring Earthquake Magnitude The strength of an earthquake is measured on the Richter scale, which is related to the amplitude of the Comment [KL18]: The logarithmic scale used to measure seismic waves moving through the Earth’s crust as determined by the seismograph. the strength of an earthquake. These cubes show the logarithmic nature of the Richter scale, with the size of each cube representing power. For example, a magnitude 2 earthquake Is 10 times stronger than a magnitude 1 earthquake, and a magnitude 3 earthquake is 100 times stronger than a magnitude 1 earthquake. Chapter 13 | 10 Types of Faults Earthquakes also cause deformation of the rocks both within the crust and on Earth’s surface. This deformation is usually a ssociated with different types of faults that occur when opposing rock bodies move relative to one another in association with plate tectonics. (a) Normal Fault Comment [KL19]: A steeply inclined fault in which the hanging rock block moves relatively downward. One type of fault is a normal fault, which is a vertical fault in which one slab of the rock is displaced up and the other slab down. Where such faults occur, the opposing blocks pull away from one another by gravity, which causes one of the fault blocks to slip up relative to the fault plane as an upthrown block (called a horst), while the other slips down as a downthrown block (called a graben). The exposed side Comment [KL20]: An upthrown block of rock that lies between two steeply inclined fault blocks. of the upthrown rock forms a cliff -like feature known as a fault escarpment (or scarp for short). (b) Reverse Fault Comment [KL21]: A downthrown block of rock that lies between two steeply inclined fault blocks. Another kind of fault is a reverse fault, which looks very similar to the normal fault but has a different Comment [KL22]: A step-like feature on the Earth’s cause and nature of movement. Whereas the normal fault entails movement of blocks away from each surface created by fault slippage. other, a reverse fault results when blocks move toward each other, causing one to ride up steeply over Comment [KL23]: A steeply inclined fault in which the the other. hanging rock block moves relatively upward. (c) Strike-Slip Fault Comment [KL24]: A structural fault along which two The strike-slip fault entails purely horizontal movement of the two plates past each other. Strike -slip lithospheric plates or rock blocks move horizontally in opposite directions and parallel to the fault line. faults and transform faults are closely related because they share the same kinds of horizontal movement. The primary difference between transform and strike -slip faults is that transform faults are associated with large, tectonic plate boundaries, whereas strike -slip faults occur where small rock blocks move horizontally relative to one another. (d) Overthrust Fault Overthrust faults occur when one rock body is thrust up and over another, usually in association with folding. These faults differ from normal and reverse faults because their fault planes are usually at a shallower angle in comparison. Chapter 13 | 11 Volcanoes: a mountain or large hill containing a conduit that extends down into the upper mantle, through which magma, ash, and gases are periodically ejected onto the surface of Earth or into the atmosphere. In most cases volcanoes are inactive for some time and erupt only when the pressure of material rising from the mantle becomes excessive. The l ength of inactivity varies dramatically between volcanoes. Some volcanoes lie dormant for hundreds or thousands of years before they erupt, whereas others are in a near-constant state of eruption. Explosive Volcanoes An explosive volcano is one that erupt s very quickly and with great force. The easiest kind to understand is a cinder-cone volcano, which usually forms very quickly after a single eruption. These volcanoes are small relative to other types of volcanoes, have steep sides (~30 °), and consist of solidified magma fragments, rock debris, and ash that are ejected from a central vent. Mount Capulin has erupted only once, about 62,000 years ago, is approximately 300 m (1000 ft) high, and is a typical cinder -cone volcano. In contrast to cinder-cone volcanoes, composite volcanoes are volcanoes that build up and grow over the course of several eruptions. They are typically inactive for long periods of time between eruptions, but when they do erupt, they tend to do so quite violently. • Such an eruption occurs because the magma within composite volcanoes is rich in silicas and therefore highly viscous (stick and slow flowing). As a result, gases are gradually trapped in the magma during the inactive phase and build up pressure inside volcano until it explodes. • The eruption sends volcanic ash high into the atmosphere and thick layers of volcanic debris will accumulate on the slopes of the volcano, causing it to enlarge. This may consist of alternating layers of lava (magma flowing on the surface) and fragmented rock debris called pyroclastic material (or tephra), such as volcanic ash, cinders, and boulders. • Also called stratovolcanoes because of the layers of volcanic debris, these volcanoes typically have moderately steep cones w ith a semi-horizontal top containing a crater. They are much larger than cinder cones, perhaps over 3000 m (10,000 ft) high. Chapter 13 | 12 Sometimes the eruption of a composite volcano is so explosive that it literally blows the top off the mountain, creating a large crater. After this massive kind of eruption, the crater may partially fill with a lava dome. While these feature sometimes occur alone and are classified as a particular type of volcano Comment [KL25]: A steep-sided volcanic landform by many geologists, the majority of the recently actively lava dom es occur in association with composite consisting of highly viscous lava that does not flow far from its point of origin before it solidifies. volcanoes. Volcanic Arcs at Plate Boundaries Composite volcanoes are most commonly associated with plate boundaries in places where subduction is occurring. Given that subduction tends to occur along the length of ce rtain plate boundaries, these zones are places where a chain of volcanoes is typically located at the surface. Such a chain is called a volcanic arc. A well-known tectonic feature associated with a number of volcanic arc is the Pacific Ring Comment [KL26]: A chain of volcanoes created by rising of Fire which follows more of the outline of the Pacific plate. magma derived from a subducting tectonic plate. Chapter 13 | 13 Fluid Volcanoes In addition to the volcanoes that erupt explosively, a number of promin ent volcanoes have very fluid eruptions with flowing rivers of lava. • The magma that results in fluid eruptions contains far less silica and is thus less viscous. • As a result, the lava associated with such an eruption flows across the surface until it cools to from basalt. • Such eruptions can occur for week s and months at a time and generally tak e place more frequently than explosive eruptions Although fluid eruptions are occasionally associated with composite volcanoes, they usually result in the formation of a shield volcano that has shallow sloping sides. • This type of volcano develops because successive eruptions of fluid magma cause the volcano to build upward gradually over a muc h broader area than composite volcanoes. • A number of prominent shield volcanoes occur on Earth in association with subduction zones and rift zones. Hotspots In some cases, a volcano is associated with a stationary zone in the asthenosphere where upwelling magma from a mantle plume is r eleased at the surface. Such a geological feature is called a hotspot. • Although hotspots occur more randomly than subduction and rift zones, they are associated with some of the most intensive volcanic activity on the pl anet. These eruptions can be of either fluid or explosive nature. An example of fluid eruptions associated with a hot spot can be seen in the state of Hawaii, which lies in the middle of the Pacific tectonic plate where the Hawaiian hotspot occurs. • This hotspot is releasing highly fluid magma that has built a variety of shield volcanoes on the island, such a Mauna Loa. • Mauna Loa is the largest volcano on Earth and is one of the five that collectively comprise the island. Chapter 13 | 14 • Evidence indicates that the Hawaiian hotspot has been active for a long time. The picture below shows a trail of islands and sub merged highlands called seamounts that extend to the northwest from the island of Hawaii. An example of an explosive volcanic eruption is the Yellowstone hotspot, which lies bene ath Yellowstone National Park in northwe st Wyoming. • Similar to the Hawaiian hotspot, the Yellowstone ho tspot is a fixed zone of upwelling magma. • In this case, the hotspot lies beneath the North American plate and the magma is hig hly viscous. • The Yellowstone hotspot has been in its present location for approximately the past 2 million years, with three major eruptions during this time. • Each of these eruptions was cataclysmic, result in the formation of a giant caldera. • Geothermal features such as geysers, mudpots, and fumeroles are very common within the park. • A geyser is a superheated fountain of water that suddenly sprays into the air on a periodic basis. This process occurs because boiling water beneath Earth i s constricted as it rises through a subterranean passageway. When the pressure builds sufficiently, the water bursts into the sky. • A mudpot consists of a bubbling mixture of a gaseous mud and water. These systems form where hot water is limited and hydrogen sulfide gas is present, creating sulfuric acid. This acid dissolves the surrounding rock into fine particles of silica and clay that mix with what little water there is to form the mudpot. • A fumerole is a steam vent that results because underlying groundwater is boiled away before reaching the surface. Key Concepts to Remember About Volcanoes 1. Volcanoes occur most frequently on plate boundaries. Most of the world ’s volcanoes are found along the Pacific Ring of Fire. 2. Volcanoes are most frequently associated with the process of subducti on because oceanic crust melts and then rises through cracks in overlying continental crust. 3. Volcanic eruptions can be broadly classified as explosive or fluid. Explosive eruptions occur when magma is viscous, formin g composite volcanoes. When the magma is not viscous, it flows freely, resulting in broad rivers of lava that collectively f orm shield volcanoes. Cinder -cone volcanoes evolve through the accumulation of solidified magma fragments, rock debris, and ash that are ejected from a central vent. 4. Some volcanoes form over hotspots, which are places where upwelling magma reaches the surface.
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