Plate Tectonic Notes
The Continental Drift Hypothesis
A. Pangaea
1. The German scientist Alfred Wegener studied whether Earth’s continents move.
2. Wegener proposed that all continents were once part of a single
supercontinent called Pangaea.
a. Over time, Pangaea started breaking apart, and the continents
started moving to where they are now.
b. The hypothesis that suggests that continents are in constant
motion on Earth’s surface is continental drift.
3. Wegener observed the similarities of coastlines between
continents that were separated by oceans.
4. The continents that once formed Pangaea have coasts that fit
together like pieces of a(n) puzzle.
B. Evidence That Continents Move
1. Fossils of ancient plants and animals provide evidence for continental drift.
2. Remains of the same plants and animals are present on different
continents that are now separated by oceans.
3. Fossils of plants and animals that lived in wet, warm climates are in
areas that now have cold climates.
4. Deposits of coal in Antarctica are evidence of continental drift. The
fossilized plants in these deposits show that Antarctica was once near
the equator.
5. Wegener proposed that certain continents—including South
America and Australia—were closer to the South Pole 250
million years ago.
a. Wegener suggested that these continents were covered by a large ice sheet.
b. Today, all these continents except one are near the equator, where
the climate is warm enough to melt ice sheets.
6. Glacial grooves on continents that currently have warm climates
show that these continents once had cold climates.
7. Wegener observed that there were mountains and rocks on
different continents that shared common origins.
8. Evidence of continental drift also includes rocks on different
continents that have similar or identical chemistry, geologic
structure, and age.
9. If you pushed North America and Europe together again, their
mountains would look like one long belt with the same rock types.
C. What was missing?
1. Scientists questioned continental drift because it is such a(n) slow process. They
were unable to measure how fast the continents moved.
2. Wegener could not explain what forces causes the continents to move.
3. Additional scientific evidence to prove Wegener’s hypothesis existed on the seafloor
between the drifting continents.
4. Evidence to prove continental drift was discussed decades after Wegener’s death.
Discussion Question
Are similar fossils more likely to exist on Asia and South America or on Africa and South
America?
According to the shapes of their shorelines, it is likely that Africa and South America were
next to each other in Pangaea. Asia and South America did not touch. Therefore, it is more
likely that Africa and South America might have similar fossils.
Development of a Theory
A. Mapping the Ocean Floor
1. Scientists mapped the depth of the ocean floor using a device
called a(n) echo sounder.
2. In the middle of the oceans are large mountain ranges called mid-ocean ridges.
a. Existence of these mountain ranges was confirmed through
echo-sounder research.
b. These underwater mountain ranges are much longer than
mountain ranges on land.
B. Seafloor Spreading
1. Seafloor spreading occurs when new oceanic crust forms at a midocean ridge and old crust moves away from the ridge.
a. Molten rock, or magma, rises from the mantle through cracks
in the crust. It erupts as lava from volcanic vents along the
mid-ocean ridge.
b. The molten rock cools and becomes basalt, the rock that forms the oceanic crust.
c. New oceanic crust forms along a mid-ocean ridge, and older
crust moves away from the ridge.
2. The topography of the seafloor includes the abyssal plain and rugged mountains.
a. The rugged mountains that make up the mid-ocean ridge can
form in different ways. One way is through large amounts of lava
erupting from the center of the ridge, cooling, and building up
around the ridge. Another way is through upward-moving magma
pushing on the crust above it, causing it to crack and form jagged,
angular mountains on the seafloor.
b. Eventually, sediment forms on top of the oldest oceanic crust,
making a smooth seafloor called the abyssal plain.
3. Seafloor spreading helps explain continental drift because it shows
that continents move with the oceanic crust as it spreads away from
mid-ocean ridges.
C. Development of a Theory
1. Evidence to support seafloor spreading first came from studying the
magnetism of rocks on the seafloor.
2. Earth’s outer core causes Earth’s magnetic field.
a. The direction of Earth’s magnetic field reverses often.
b. When a magnetic field causes a magnet to point north, the
magnetic field has normal polarity.
c. A magnetic field reverses direction during a(n) magnetic reversal.
d. After a magnetic reversal, a magnet points south because Earth’s magnetic field
has reversed polarity.
3. Magnetic signatures form when iron-rich minerals in cooling lava align with the
direction of Earth’s magnetic field.
a. The direction of a magnetic field in minerals can be determined by using a device
called a(n) magnetometer.
b. Magnetometers show parallel magnetic stripes on either side of a mid-ocean
ridge.
c. These stripes alternate normal polarity and reversed polarity, showing that each
stripe was formed at the mid-ocean ridge and then moved away.
4. Sediment collected from the seafloor show that sediment farther away from a
mid-ocean ridge is older than the sediment that is closer to the ridge.
Discussion Question
How do magnetometer results show that continental drift is a slow process?
Magnetometer results show stripes of opposite magnetic signatures. Each change in
magnetic signature shows a magnetic reversal, which can take millions of years. Because
magnetic reversal is such a slow process, it indicates that seafloor spreading is slow. Because
seafloor spreading is slow, continental drift must also be slow.
The Theory of Plate Tectonics
A. The Plate Tectonics Theory
1. The theory of plate tectonics states that Earth’s surface is divided into
rigid plates of rock. Each plate moves over Earth’s mantle and
changes position with respect to other plates.
a. The word tectonic describes the forces that shape Earth’s
surface and rock structures that form as a result.
b. Of all Earth’s tectonic plates, the Pacific Plate is the largest.
2. The cold, rigid rock layer on the outermost part of Earth is called the
lithosphere. It consists of crust and the upper part of the mantle.
3. Below the lithosphere is the asthenosphere, which is so hot
that it flows like plastic.
4. Plates of lithosphere move because they rest on the flowing asthenosphere.
B. Plate Boundaries
1. The place where two plates meet is called a(n) plate boundary.
2. When two plates move away from each other, a(n) divergent plate boundary forms.
a. In the ocean, mid-ocean ridges are located at divergent plate boundaries.
b. If divergent plate boundaries separate parts of a continent, rift valleys form.
3. When two plates slide by each other, a(n) transform plate
boundary forms. This type of movement causes earthquakes.
4. When two plates collide, a(n) convergent plate boundary forms.
a. When plates collide, the plate that is denser slides under the lessdense plate in the process of subduction.
b. When an oceanic plate slides under a continental plate, a deep
ocean trench forms. Near the trench, a line of volcanoes forms.
c. When two oceanic plates collide, a trench and a(n) island arc form.
d. When two continental plates collide, neither plate is subducted,
and mountains form.
C. Evidence for Plate Tectonics
1. Scientists now use satellites to help measure how continents move.
2. The theory of plate tectonics explains why earthquakes and
volcanoes occur in certain locations.
D. Plate Motion
1. Earth’s mantle moves because warmer, less-dense materials rise, and cooler, denser
materials sink.
a. Materials move based on differences in their temperatures and densities in the
process of convection.
b. Inside Earth, radioactive elements provide some of the thermal energy that causes
convection.
c. Convection currents form in the mantle when thermal energy transfers from the
core to the mantle.
2. Three forces interact to cause tectonic plate motion.
a. Convection currents in the mantle produce a force that causes motion called
basal drag.
b. Plates are pushed away from each other at mid-ocean ridges by the force of ridge
push.
c. When a plate sinks below another plate, it pulls on the rest of the plate, exerting
a force called slab pull.
E. A Theory in Progress
1. Plate tectonics is the unifying theory of geology.
2. Plate tectonics theory is still being revised as scientists learn more about how
Earth’s tectonic plates move.
Discussion Question
How could you use a pot of water on a stove to model the convection currents in Earth’s
mantle?
Like the core heating the lower mantle, the burner heats the water at the bottom of the pot.
The heated water at the bottom of the pot becomes less dense and rises, as Earth’s heated
mantle does. As heated water rises, cooler water sinks because it is denser. This movement
models the sinking of cooler, denser parts of the mantle.