EARTH’S STRUCTURE. Drifting Continents. Piecing It All Together
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Piecing It All Together
German scientist Alfred Wegener noticed that the coastlines of some continents seem to fit together like a jigsaw puzzle. Look at the world map below and see if you can match the continents that seem to line up. What might this mean?
Wegener used three types of evidence to support his idea:
Mesosaurus and Lystrosaurus were both freshwater reptiles. These fossils have been found in places that are now separated by oceans.
Glossopteris fossils have been found in Africa, South America, Australia, India, and Antarctica.
The live plant
Pangaea and Continental Drift
Many types of evidence suggest that Earth’s landmasses were once joined together.
Scientists using sonar to map the ocean floor in the 1940s discovered long chains of mountains that rise up from the ocean floor. These are called mid-ocean ridges , and they extend into all of Earth’s oceans.
Mid-ocean ridges rise from the sea floor like stitches on the seams of a baseball.
Mid-ocean ridge erupting
In the central valley of mid-ocean ridges, geologists discovered pillow lava. These rocks form only when molten material hardens quickly after erupting under water.
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.
Some mid-ocean ridges have a valley that runs along their center. Evidence shows that molten material erupts through this valley and then hardens to form the ocean floor.
Scientists tested the age of rock samples taken from the ocean floor. The youngestrocks were always found at the center of the ridges. The farther away the rock sample was taken, the older the rock was.
The deepest part of
the ocean is along the Mariana Trench. Several trenches in the Pacific Ocean are shown in yellow.
Oceanic crust created
along a mid-ocean ridge is
destroyed at a deep-ocean trench.
During the process of subduction, oceanic crust sinks down beneath the trench into the mantle. Sea-floor spreading and subduction work together. These processes support the idea of continental drift.
The three main layers of Earth are the crust , the mantle , and the core . The crust and the upper mantle are grouped together into a single layer called the lithosphere . The lithosphere is broken into large pieces called plates that are separated by cracks. Below the lithosphere, the material is hotter and under increasing pressure. As a result, the part of the mantle just beneath the lithosphere is less rigid than the rock above, although it is still solid. This softer layer is called the asthenosphere .
Heat from the Earth’s core and the mantle itself causes convection in the mantle. Convection occurs when the heat from Earth’s core causes the mantle material to become hotter. The hotter rock is less dense and rises toward the crust. As it moves further from the core, it cools off, becomes more dense , and sinks back through the mantle. Over and over, the cycle of rising and sinking takes place, and is known as convection currents .
Earth’s plates move because they are the top part of the large convection currents in Earth’s mantle.
Plates move apart from each other, or diverge, at a divergent boundary. Most divergent boundaries occur in the oceans at the mid-ocean ridge . Where plates diverge on land, a deep valley called a rift valley forms.
Plates come together, or converge, at a convergent boundary. What happens when two plates collide depends on the density of the plates. Volcanoes often form where two plates collide.
Plates slip past each other, moving in opposite directions, at a transform boundary. Earthquakes often occur when the plates suddenly slip along the boundary that they form.
Earth's Changing Crust
As plates move, they produce mountains, volcanoes, and valleys as well as mid-ocean ridges and deep-ocean trenches. Use the terms from the list to label the diagram.
Earthquakes Around the World
Earthquakes are closely linked to plate tectonics. The map shows where past earthquakes have occurred in relation to plate boundaries.
The map shows
areas where serious earthquakes are likely to occur, based on the location of past earthquakes across the United States.
The Ring of Fire
The Ring of Fire is a belt of volcanoes that circles the Pacific Ocean. As with most of Earth’s volcanoes, these volcanoes form along boundaries
of tectonic plates.