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Test on Thursday (2/16)
Plate Tectonics Study Guide
About 260 million years ago, Earth’s continents moved together to form the supercontinent
Pangaea.
Supercontinent – a single landmass once formed by the continents (Pangaea)
In 1912 Alfred Wegener proposed the hypothesis of continental drift.
Continental drift – hypothesis that a single landmass broke up, and the pieces formed the
modern continents which drifted to their present locations.
There was other evidence that supported Wegener’s hypothesis…
 Landforms
Wegener thought that the crumpling of the crust in places may have produced mountain ranges
such as the Andes Mountains. Mountain ranges on one continent seemed to continue on other
continents across the ocean.
 Fossils
He believed that if the continents had been joined that fossils of the same plants and animals
could be found in areas that had once been connected. In addition, the age and types of rocks in
both of these areas were very similar.
 Glaciation
Geologists discovered layers of debris from ancient glaciers in southern Africa and South
America. Today these areas have climates that are too warm for glaciers to form. Other
evidence from fossils showed that tropical or subtropical swamps now have much colder
climates.
Although Wegener had evidence that supported his hypothesis, there were many people that
opposed his ideas because they believed that he could not identify a force that could move the
continents.
Mid-ocean ridges - a long, undersea mountain range with a steep, narrow valley at its center.
It forms at divergent boundaries. At the center of the mid-ocean ridge there is a rift valley. The
magma rises from the asthenosphere and creates a new oceanic sea floor (lithosphere) as
tectonic plates move apart.
Sea floor rocks closer to the ridge are younger and those further away from the ridge are older.
sea-floor spreading – the process where molten material (magma) rises from the mantle and
erupts along the mid-ocean ridge. During sea-floor spreading a new ocean lithosphere forms.
Scientists believe that sea-floor spreading provides a way for the continents to move over
Earth’s surface.
Paleomagnetism – the study of the magnetic property of rocks.
Magnetic stripes helped provide evidence for sea-floor spreading.
Iron bearing minerals in igneous rocks can show the direction of Earth’s magnetic field at the
time the rocks formed.
Plate tectonics – theory that explains how large pieces of the lithosphere move.
Lithosphere – the solid outer layer of Earth that consists of the crust and the rigid upper part of
the mantle. The lithosphere is broken into several blocks called tectonic plates.
Asthenosphere – a layer of “plastic rock” that is just below the lithosphere.
Earth’s crust is classified into two types – oceanic crust and continental crust. Tectonic plates
can include one or both of the crusts.
Oceanic crust
- Dense
- made of rock that is rich
in iron and magnesium
- you might find basalt on this crust
Continental crust
- low density
- Rock is rich in silica
- you might find granite on this crust
There are three types of plate boundaries – divergent, convergent and transform.
Divergent boundaries (mostly located on the ocean floor) – when two plates move away from
each other.
Magma from the asthenosphere rises to the surface as the plate moves apart. The magma cools
to form a new oceanic lithosphere. The new rock sits higher than the sea floor around it and
forms undersea mountain ranges also called mid-ocean ridges. In the center of the ridge is a
rift valley that shows where the plates separated.
Convergent boundaries – when two plates collide with one another. Here the ocean crust is
being destroyed.
Three types of collisions can occur with convergent boundaries.
1. Oceanic-Continental Convergence - Since oceanic plate is denser it sinks
(subducts) under the less dense continental lithosphere. The oceanic lithosphere heats
up, releases fluid which heats up the mantle and creates magma. The magma rises to
the surface and forms volcanic mountains.
Subduction zone - the region along a plate boundary where one plate moves under
another plate. (deep ocean trenches form here)
2. Continental-Continental Convergence - Neither plate sinks, instead they collide and
the edges crumple and thicken causing an uplift to form mountain ranges (Himalaya
Mountains).
3. Oceanic-Oceanic Convergence - One plate sinks under the other forming a deep
ocean trench. The plate that sinks releases fluid causing the mantle to melt and form
magma. The magma rises and forms an island arc which is a chain of volcanic
islands. The Pacific Ring of Fire is a good example of this.
Transform boundaries (usually occur along mid-ocean ridges) – the boundary at which two
plates slide past each other. The plate edges do not slide along smoothly and instead scrape
against each causing earthquakes. These boundaries do not produce magma.
Convection - the movement of heated materials (such as magma) through the earth’s mantle.
Supercontinent cycle - the explanation of why and how continents move.
** Take a look at the lab we have been working on in class where you have been labeling the
different plates, ridges, trenches etc.