Chapter 3 – Plate Tectonics – Study Guide
· Know all your vocabulary. Be able to match each definition to the word it defines.
· Describe Alfred Wegener’s hypothesis about the continents.
Answer: Wegener’s hypothesis was that all the continents were once joined together in a single landmass and have since drifted apart. Wegener was a German scientist, and his hypothesis was formulated in 1910.
· What name did Wegener give the supercontinent, or single landmass, that existed about 300 million years ago?
Answer: Pangaea
· What evidence did Wegener suggest supported his hypothesis?
Answer: He pieced together maps of Africa and South America, and noticed that mountain ranges on the continents line up. He also noticed that coal fields in Europe and North America also match up. Also, Wegener used fossils to support the hypothesis because they contained fernlike plants and freshwater reptiles found in places now separated by oceans. The plants couldn’t have grown today in some of the places the fossils were discovered, and the reptiles couldn’t have swum the distance across the oceans.
· More evidence confirming the hypothesis of the continental drift was published in 1957 by geologist Marie Tharp. What evidence did she present?
Answer: Data taken from ships showed how the height of the ocean floor varied, and that in certain places the floor of the ocean appeared to be stitched together like the seams of a baseball. The seams formed mountain ranges that ran along the middle of some ocean floors. These were called mid-ocean ridges.
· What device using sound waves to measure the distance to an object was used in the mid-1900s by scientists to map mid-ocean ridges?
Answer: sonar
· Describe the process of sea-floor spreading.
Answer: Sea-floor spreading begins at a mid-ocean ridge, which forms along a crack in the oceanic crust. Along the ridge, new molten material from inside Earth rises, erupts, cools, and hardens to form a solid strip of rock. More crust is added to the ocean floor. At the same time, older strips of rock move outward from either side of the ridge.
· Name 3 types of evidence that geologists have found for sea-floor spreading.
Answer:
1-Ocean-floor Material shows rocks shaped like pillows that only form when molten material hardens quickly after erupting under water.
2-Magnetic Stripes-as magma erupts, cools, and hardens, magnetic minerals inside the rock line up in the direction of Earth’s magnetic poles. The pattern of magnetic stripes on one side of a mid-ocean ridge is usually a mirror image of the pattern on the other side of the ridge.
3-Drilling Samples-Rock samples from the ocean floor show that the farther away from a ridge a rock sample was taken, the older the rock was. The youngest rocks were always found at the center of the ridges.
· What happens at deep-ocean trenches?
Answer: In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle at deep-ocean trenches.
· Explain subduction.
Answer: Crust closer to a mid-ocean ridge moves away from the ridge and toward a deep-ocean trench. The new oceanic crust is hot, but as it moves away from the mid-ocean ridge, it cools. As it cools, it becomes more dense. Eventually, as it moves, the cool, dense crust might collide with the edge of a continent. Gravity then pulls the older, denser oceanic crust down beneath the trench and back into the mantle. This is called subduction.
· Earth’s plates meet at boundaries. What are the three types of boundaries? Explain each.
Answer:
1. Divergent Boundary-plates that move apart, or diverge from each other;
2. Convergent Boundary-plates that come together, or converge;
3. Transform Boundary-plates that slip past each other, moving in opposite directions. The sides of the plates are rocky and jagged, so the two plates can grab each other and “lock” in place. Forces inside the crust can cause the two plates to unlock. Earthquakes often occur when this happens. Crust is neither created or destroyed at transform boundaries!!!!!
· What is the ‘theory of plate tectonics’?
Answer: It states that Earth’s plates are in slow, constant motion, driven by convection currents in the mantle.
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