Theory of Plate Tectonics

PLATE TECTONIC

LESSON 2.1

plate tectonics
continental drift
convergent boundary
subduction zone
divergent boundary
mid-ocean ridge
transform boundary
asthenosphere

The Plate tectonics Theory

Vocabulary Bank

states that Earth's crust is broken into rigid plates that move slowly
over Earth's surface.

The theory of plate tectonics states that Earth's surface is divided into rigid plates that move relative to one another. These plates are
"floating" on top of a hot and semi-plastic mantle.

Ecosystems can be created when biotic and
abiotic components are met.

Plate Tectonics

Population

The map below illustrates Earth's major plates and the boundaries that define them. The
Pacific Plate is the largest plate. The Juan de Fuca Plate is one of the smallest plates. It
is between the North American and Pacific Plates. You live on the North American Plate.

Development of the Theory

In 1912, Alfred Wegener, a German scientist, developed the
hypothesis that continents are in constant motion on the surface
of Earth, calledcontinental drift.

Wegener also used geological evidence and fossil evidence to support his
theory.

Fossil and rock evidencefrom ancient
continents supports the hypothesis of continental
drift.

Geological Evidence
Rocks that are made of similar substances and mountains that formed at
similar times are present on continents that are now far apart. Geologists
can look for similarities in these rocks and mountains, as well as the
locations of ancient glaciers, deserts, and coal swamps, from one
continent to the next.

Geologists have discovered the same types of fossils
on continents that are now separated by vast oceans.

For example, fossils of the freshwater reptile
Mesosaurus, shown above, have been found in
South America and Africa. These two landmasses
now are separated by the Atlantic Ocean.

Mesosaurus was a freshwater reptile that lived 300

to 270 million years ago

It was not until the early 1960s when new technology. Scientists now understand that continents move
because the plastic asthenosphere moves underneath the lithosphere. Which is the plastic laver of the
mantle

that occur within the molten rock in the mantle, act like a conveyor belt for the
plates.

Plate tectonic activity is related to convection in the mantle. Radioactive elements, such as uranium,
thorium, and potassium, heat Earth's interior. When materials such as solid rock are heated, they
expand and become less dense. Hot mantle material rises upward and comes in contact with Earth's
crust. Thermal energy is transferred from hot mantle material to the colder surface above. As the
mantle cools, it becomes denser and then sinks, forming a convection current. As the mantle moves, it
pushes and pulls tectonic plates over Earth's surface.

occurs in the mantle underneath earths tectonic plates.

Plate Motion

Convection Currents

Convection

Divergent Boundaries

A convergent boundaryis where two plates
move toward each other. Recall that oceanic

crust is denser than continental crust. When two
plates come together, the denser oceanic plate
usually is forced down into the mantle. The less

dense continental plate remains on Earth's

surface. The area where one plate slides under

another is called asubduction zone.

A divergent boundary is where two plates

move apart from each other. When plates move apart, a

rift forms between the two plates. A rift can form
within continents when continental crust moves in

opposite directions. When two oceanic plates separate,
hot rock from the mantle rises and pushes the seafloor
upward. This creates a long, tall mountain range in the
middle of the ocean called a mid-ocean ridge. Molten
rock erupts from the mid-ocean ridge. As the molten
rock cools, it forms new crust. This process, called
seafloor spreading, creates and builds ocean basins

over millions of years.

Convergent Boundaries

TECTONIC PLATE BOUNDARIES

A transform boundaryis where plates slide

horizontally past each other. The two sides of the
boundary move in opposite directions. This can

deform or break features such as fences, railways, or

roads that cross the boundary.

Transform Boundaries

that occur within the molten rock in the mantle, act like a conveyor belt for the plates.

INTERACTIONS OF EARTH'S
TECTONIC PLATES

A Plate Tectonics Pattern

Because tectonic plates move very slowly, most changes to Earth's surface take a
long time. But some changes, like earthquakes and volcanic eruptions, occur very
quickly and violently. The theory of plate tectonics provides an explanation for why
earthquakes and volcanoes occur in certain places.

Because plates are rigid, tectonic activity occurs where plates meet. When plates separate,
collide, or slide past each other along a plate boundary, stress builds. A rapid release of energy
can result in earthquakes. Earthquakes are the vibrations in the ground that result from
movement along breaks in Earth's lithosphere. These breaks are called faults.

Volcanoes

Earthquakes

Molten rock below Earth's surface is called magma. Because magma is hot, it is
also less dense than the surrounding rock around it. It moves upward causing cracks to
form in the solid rock. Volcanoes are landforms that form when magma erupts onto Earth's
surface as lava. Volcanoes form where plates separate along a mid-ocean ridge or a continental
rift or collide along a subduction zone.

Multiple Choice

Why did no one believe Wegener's thoery that the continents were once together?

• He could not explain HOW the continents moved

• He didn't have evidence

• His data was wrong

• He didn't use the right technology to gather data

Multiple Choice

Name the boundary

• convergent boundary

• divergent boundary

• transform boundary

• sergent boundary

Multiple Choice

A fault is formed by the movement of a __________ plate boundary

• Transform

• Convergent

• Divergent

Multiple Select

Fossils can also give evidence for a Pangaea theory

• True

• False

Multiple Choice

The upper part of the mantle that is semisolid where convection pushes the tectonic plates

• Mantle

• Asthenosphere

• Lithosphere

• Crust

Multiple Choice

Which layer of the Earth consists of tectonic plates?

• asthenosphere

• mantle

• mesosphere

• lithosphere