Earth Science Chapter 17: Plate Tectonics Section 1 Drifting Continents 17.1 Essential Questions What are the lines of evidence that led Wegener to suggest that
Earths continents have moved? How does evidence of ancient climates support continental drift? Why was continental drift not accepted when it was first proposed? 17.1 - Main Idea
The shape and geology of the continents suggests that they were once joined together. Early Observations
With the exception of events such as earthquakes, volcanic eruptions, and landslides, most of Earths surface appears to remain relatively unchanged during the course of a human lifetime. On the geologic time scale, however, Earths surface has changed dramatically.
Early Observations In the late 1500s, Abraham Ortelius, a Dutch cartographer (map maker), noticed the apparent fit of continents on either side of the Atlantic Ocean. He proposed that North America and South America had been separated from Europe and Africa by earthquakes and floods.
Early Observations The first time that the idea of moving continents was proposed as a scientific hypothesis was in the early 1900s. In 1912, German meteorologist Alfred Wegener presented his ideas about continental movement to the scientific
community. Continental Drift Wegener developed a hypothesis that he called continental drift. He proposed that Earths continents had once been joined in a single landmass, a supercontinent called Pangaea (Meaning
All Lands), that broke apart about 200 mya (million years ago) and sent the continents adrift. Continental Drift Evidence from Rock Formations Wegener observed that many layers of rocks in the Appalachian Mountains in the
United States were identical to layers of rocks in similar mountains in Greenland and Europe. These similar groups of rocks, older than 200 million years, supported Wegeners idea that the continents had once been joined.
Continental Drift Evidence from Fossils Wegener gathered evidence of the existence of Pangaea from fossils. Similar fossils of animals and plants that once lived on or near land had been found on widely separated continents.
Evidence from Fossils Continental Drift Climatic Evidence Fossils of the plant Glossopteris had been found on many parts of Earth, including South America, Antarctica, and India. Wegener reasoned that the area separating these
fossils was too large to have had a single climate. Wegener argued that because Glossopteris grew in temperate climates, the places where the fossils had been found had been closer to the equator. This led him to conclude that the rocks containing these fossil ferns had once been joined. Continental Drift Climatic Evidence
Coal forms from the compaction and decomposition of accumulations of ancient swamp plants. Wegener used the existence of coal beds in Antarctica to conclude that Antarctica must have been much closer to the equator sometime in the geologic past.
Continental Drift Climatic Evidence Glacial deposits nearly 300 million years old on several continents led Wegener to propose that these landmasses might have once been joined
and covered with ice. The extent of the ice is shown in white. A Rejected Notion Although Wegener had compiled an impressive collection of data, the hypothesis of continental drift was not accepted by the
scientific community. Two unanswered questions What forces could cause the movement? How could continents move through solids? Main reasons that continental drift was rejected. A Rejected Notion
It was not until the early 1960s, when new technology revealed more evidence about how continents move, that scientists began to reconsider Wegeners ideas. Plate Tectonics Video http://www.youtube.com/watch?v=JmC-vjQGSNM
17.2 Seafloor Spreading 17.2 Essential Questions What evidence led to the discovery of seafloor spreading? What is the significance of
magnetic patterns on the seafloor? How is the process of seafloor spreading explained? Main Idea Oceanic crust
forms at ocean ridges and becomes part of the seafloor. Mapping the Ocean Floor Until the mid-1900s, many scientists thought that the ocean floors were essentially flat and
that oceanic crust was unchanging and was much older than continental crust. Advances in technology during the 1940s and 1950s (Sonar & Magnetmeter) showed that all of these widely accepted ideas were incorrect. Harry Hess Proposed Seafloor Spreading Mapping the Ocean Floor
One technological advance that was used to study the ocean floor was the magnetometer, a device that can detect small changes in magnetic fields. Towed behind a ship, it can record the magnetic field generated by ocean floor rocks. Developments in sonar technology enabled scientists to measure water depth and map the
topography of the ocean floor. Ocean-Floor Topography Using the maps made from data collected by sonar and magnetometers, scientists discovered that vast, underwater mountain chains called ocean ridges run along the ocean floors around Earth much
like seams on a baseball. Maps generated with sonar data revealed that underwater mountain chains had counterparts called deep-sea trenches. Ocean-Floor Topography The deepest trench, the Mariana Trench, is more than 11 km (6.3 miles) deep.
Mount Everest, the worlds tallest mountain, stands at 9 km (5.9 miles) above sea level, and could fit inside the Mariana Trench with six Empire State buildings stacked on top. Ocean Rocks & Sediments The ages of the rocks that make up the seafloor
vary across the ocean floor, and these variations are predictable. The age of oceanic crust increases with distance from a ridge. Ocean-floor sediments are typically a few hundred meters thick. Large areas of continents, on the other hand, are blanketed with sedimentary rocks that are
as much as 20 km thick. Ocean Rocks & Sediments Observations of ocean-floor sediments revealed that, like the age of ocean crust, the thickness of ocean-floor sediments increases with distance from an ocean ridge.
Magnetism Earth has a magnetic field generated by the flow of molten iron in the outer core. This field is what causes a compass needle to point to the North. A magnetic reversal happens when the flow in the outer core changes, and Earths magnetic field changes direction.
Magnetism A magnetic field that has the same orientation as Earths present field is said to have normal polarity. A magnetic field that is opposite to the present field has reversed polarity. Magnetic Polarity Time Scale Paleomagnetism is the study of the history of
Earths magnetic field. When lava solidifies, iron-bearing minerals such as magnetite crystallize. As they crystallize, these minerals behave like tiny compasses and align with Earths magnetic field. Magnetic Polarity Time Scale
Periods of normal polarity alternate with periods of reversed polarity. Long-term changes in Earths magnetic field, called epochs, are named as shown here.
Short-term changes are called events. Magnetic Symmetry Regions of normal and reverse polarity form a series of stripes across the ocean floor parallel
to the ocean ridges. The ages and widths of the stripes match from one side of the ridges to the other. Magnetic Symmetry By matching the magnetic patterns on the
seafloor with the known pattern of magnetic reversals on land, scientists were able to determine the age of the ocean floor from magnetic recording and to create isochron maps of the ocean floor. Magnetic Symmetry An isochron is an imaginary line on a map
that shows points that have the same agethat is, they formed at the same time. Visualizing Seafloor Spreading Data from topographic, sedimentary, and paleomagnetic research led scientists to propose seafloor
spreading. Seafloor Spreading Seafloor spreading is the theory that explains how new ocean crust is formed at ocean ridges and destroyed at deepsea trenches.
Seafloor Spreading During seafloor spreading, magma, which is hotter and less dense than surrounding mantle material, is forced toward the surface of the crust along an ocean ridge. As the two sides of the ridge spread apart, the rising magma fills the gap that is created. When the magma solidifies, a small amount
of new ocean floor is added to Earths surface. Seafloor Spreading As spreading along an ocean ridge continues, more magma is forced upward and solidifies. The cycle of spreading and the intrusion of magma continues the
formation of ocean floor, which slowly moves away from the ridge. Seafloor Spreading Videos http://www.youtube.com/watch?v=GyMLlLxbfa4 17.3 Plate Boundaries
17.3 Essential Questions How does the movement of Earths tectonic plates result in many geologic features? What are the three types of plate boundaries and the features associated with each?
What are the processes associated with subduction zones? 17.3 Main Idea Volcanoes, mountains, and deep-sea
trenches form Theory of Plate Tectonics Tectonic plates are huge pieces of crust and rigid upper mantle that fit together at their edges to cover Earths surface. Theory of Plate Tectonics
Plate tectonics is the theory that describes how tectonic plates move and shape Earths surface. They move in different directions and at different rates relative to one another, and they interact with one another at their boundaries. Divergent Boundaries Regions where two tectonic plates are moving apart
are called divergent boundaries. Most divergent boundaries are found along the seafloor in rift valleys. The formation of new ocean crust at most divergent boundaries accounts for the high heat flow, volcanism, and earthquakes associated with these boundaries.
Divergent Boundaries Some divergent boundaries form on continents. When continental crust begins to separate, the stretched crust
forms a long, narrow depression called a rift valley. Convergent Boundaries At convergent boundaries, two tectonic plates are moving toward each other. When two plates collide, the denser plate
eventually descends below the other, less-dense plate in a process called subduction. There are three types of convergent boundaries, classified according to the type of crust involved. The differences in density of the crustal material affect how they converge Convergent Boundaries
In the oceanic-oceanic convergent boundary, a subduction zone is formed when one oceanic plate, which is denser as a result of cooling, descends below another oceanic plate. The process of subduction creates an ocean trench. In an oceanic-oceanic convergent boundary, water carried into Earth by the subducting plate lowers the melting temperature of the overlying mantle, causing it to melt.
The molten material is less dense so it rises back to the surface, where it often erupts and forms an arc of volcanic islands that parallel the trench. Convergent Boundaries When an oceanic plate converges with a continental plate, the denser oceanic plate is subducted. Oceanic-continental
convergence produces a trench and volcanic arc. The result is a mountain range with many volcanoes. Convergent Boundaries Continental-continental boundaries form when two continental plates collide, long
after an oceanic plate has converged with a continental plate. This forms a vast mountain range, such as the Himalayas. Transform Boundaries A region where two plates slide
horizontally past each other is a transform boundary. Transform Boundaries Transform boundaries are characterized by long faults, sometimes hundreds of
kilometers in length, and by shallow earthquakes. Most transform boundaries offset sections of ocean ridges. Sometimes transform boundaries occur on continents. 17.4 Causes of
Plate Motions 17.4 Essential Questions How is the process of convection explained? How is convection in the mantle related to the movements of tectonic plates?
What are the processes of ridge push and slab pull? Main Idea Convection currents in the mantle cause
plate motions. Convection Many scientists now think that large-scale motion in the mantle Earths interior between the crust and the coreis the mechanism that drives the
movement of tectonic plates. Convection Currents Convection is the transfer of thermal energy by the movement of heated material from one place to another. The cooling of matter causes it to contract slightly and increase in density.
The cooled matter then sinks as a result of gravity. Warmed matter is then displaced and forced to rise. This up-and-down flow produces a pattern of motion called a convection current. Convection Currents Water cooled by the ice cube sinks to the
bottom where it is warmed by the burner and rises. The process continues as the ice cube cools the water again. Convection in the Mantle Convection currents develop in the mantle, moving the crust and outermost part of the mantle and transferring
thermal energy from Earths interior to its exterior. Plate Movement The rising material in a convection current spreads out as it reaches the upper mantle and causes both upward and sideways forces, which
lift and split the lithosphere at divergent plate boundaries. Plate Movement The downward part of a convection current occurs where a sinking force pulls tectonic plates downward at convergent
boundaries. Push and Pull Ridge push is the tectonic process associated with convection currents in Earths mantle that occurs when the weight of an elevated ridge pushes an oceanic plate toward a subduction zone.
Push and Pull Slab pull is the tectonic process associated with convection currents in Earths mantle that occurs as the weight of the subducting plate pulls the trailing lithosphere into a subduction zone.
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