Plate Tectonics & Boundaries

Introduction to Plate
Tectonics

Plate tectonics is the scientific theory that describes the large-scale

motion of the Earth's surface. This fundamental concept in geology

explains the formation and movement of the Earth's continents and

oceans over geological time.

by Dilenia Gomez

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The Earth's Internal Structure

The Earth is composed of distinct layers, each with its own unique properties. The deeper layers are hotter, denser, and under

greater pressure than the outer layers.

These natural forces within the Earth's interior interact with and shape the surface features we see on the planet.

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The Earth's Internal Structure

Crust

The rigid, rocky
outer surface of
the Earth,
composed mostly
of basalt and
granite. The crust
is thinner under
the oceans.

Mantle

A rocky layer located
under the crust -
composed of silicon,
oxygen, magnesium, iron,
aluminum, and calcium.
Convection currents carry
heat from the hot inner
mantle to the cooler outer
mantle.

OuterCore

The molten iron-
nickel layer that
surrounds the inner
core.

InnerCore

The solid iron-
nickel center of the
Earth that is very
hot and under great
pressure.

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Multiple Choice

As you travel from the inner core to the crust, how would temperature, pressure and density of the material change?

• decrease

• stay the same

• increase

The Tectonic Plates

Composition

The Earth's crust and uppermost

portion of the mantle are broken into

large, rigid plates.

Sizes

The plates range in size and

thickness from small, like the Juan

de Fuca plate, to large, like the Pacific

plate.

Movement

The plates are constantly moving,

driven by convection currents in the

underlying mantle.

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Fill in the Blanks

To really understand how the earth became to look

as it does today, and the theory of plate tectonics,

you also need to become familiar with two other

ideas:

Continental Drift

and

Seafloor Spreading

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Pangea: The Supercontinent

Pangea was a massive supercontinent that existed 300-175 million years

ago. It formed from the collision of earlier landmasses and eventually

broke apart into the continents we know today.

At its peak, Pangea covered almost a third of the Earth's surface and was

surrounded by a single global ocean called Panthalassa. The breakup of

Pangea was a gradual process that formed the modern continents and

major oceans.

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Evidence of Pangea

1

Coastline Fit

The coastlines of continents on

either side of the Atlantic Ocean

fit together remarkably well, as

if they were once connected.

2

Fossil Distribution

Identical or similar plant and

animal fossils have been found

across continents now

separated by oceans,

suggesting they were once

connected.

3

Geological Features

Mountain ranges and

sedimentary rock layers align

across continents, indicating

they were once joined together.

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Multiple Choice

Which of the following is NOT evidence to support the theory of plate tectonics?

• Fossils

• mountains

• Coastline Fit

• Sand erosion

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Convection Currents
in the Earth's
Mantle

Convection currents within the Earth's mantle drive the

constant movement of tectonic plates. Rising hot

material and sinking cooler material create a

continuous circulation, exerting forces that push and

pull the plates over geological time. This plate

movement shapes the Earth's surface through

processes like mountain building and seafloor

spreading.

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Multiple Choice

Which of the following causes Earth's tectonic plates to move?

• Energy from the sun

• Magnetic Pole Reversal

• Convection currents in the mantle

• Faults in mountain ranges

Multiple Choice

Which of the following changes would end Earth's tectonic activity?

• Earth's crust becomes a cooler solid

• Earth's mantles becomes a warmer semiliquid

• Earth's mantle becomes cooler and solid.

• 
  Earth's outer core becomes warmer

Continental Crust

Continental crust refers to the outermost solid

layer of the Earth, which is less dense than the

underlying mantle. It consists primarily of igneous

and metamorphic rocks, giving it a distinct

chemical composition compared to the oceanic

crust.

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Oceanic Crust

Oceanic crust refers to the outermost solid layer of

the Earth that lies beneath the ocean basins. It is

thinner and denser than continental crust, consisting

primarily of basalt and gabbro. The oceanic crust is

constantly being created at mid-ocean ridges through

the process of seafloor spreading, and is eventually

destroyed at subduction zones where it is pushed

back into the Earth's mantle.

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Plate Boundaries and
Movement

1

Divergent

Plates move apart, creating new crust at mid-ocean ridges.

2

Convergent

Plates collide, leading to mountain building, volcanic

activity, and subduction.

3

Transform

Plates slide past each other horizontally along fault lines.

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Divergent Plate Boundaries

Divergent plate boundaries occur where two tectonic plates move away

from each other. This allows molten magma to rise up and fill the gap,

creating new oceanic crust. The most well-known is the Mid-Atlantic

Ridge, where plates are slowly moving apart.

As the plates move, volcanic and seismic activity is common along the

boundary. New ocean floor is formed as the magma cools and solidifies,

leading to the gradual widening of ocean basins.

Divergent boundaries are also sites of shallow earthquakes, as the crust

is being pulled apart. The movement of plates and upwelling of magma

contribute to the dynamic geology and landscapes found at these

locations.

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Convergent Plate
Boundaries

Convergent boundaries occur where tectonic plates collide and move

towards each other. This can result in mountain building, volcanic

activity, and subduction.

At a convergent boundary, one plate is typically pushed or pulled under

the other, creating a deep ocean trench. As the subducting plate is forced

down, it melts and the molten material rises to the surface, forming

volcanoes. This is why many of the world's major mountain ranges and

volcanic arcs are found along convergent plate boundaries.

The collision of plates at convergent boundaries also generates intense

seismic activity, leading to frequent and powerful earthquakes. The

movement and uplift of the crust creates dramatic landscapes, such as

the Andes Mountains in South America and the Himalayan mountain

range in Asia.

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Transform Plate Boundaries

At transform boundaries, tectonic plates slide horizontally past each

other. This creates transform faults where the plates grind together. The

crust is displaced sideways without being created or destroyed.

Transform boundaries are sites of high seismic activity and shallow

earthquakes as the plates move in opposite directions. Examples include

the San Andreas Fault and Anatolian Fault.

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Multiple Choice

What type of plate boundary is this?

• Convergent

• Divergent

• Plate

• Transform

Multiple Choice

What type of plate boundary is this?

• Convergent

• Divergent

• Plate

• Transform

Plate Tectonics and Seafloor
Spreading

1

Mid-Ocean Ridges

New oceanic crust is continuously formed at these

divergent plate boundaries.

2

Seafloor Spreading

The oceanic plates move away from the ridges, carrying the

seafloor with them.

3

Subduction Zones

Older oceanic crust is eventually pushed back into the

mantle at convergent plate boundaries.

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The Theory of Continental
Drift

Puzzle Pieces

The continents fit together like

puzzle pieces, suggesting they

were once joined.

Fossil Evidence

Matching fossils of plants and

animals are found on different

continents.

Mountain Ranges

Matching mountain ranges extend

across multiple continents.

Climate Shifts

Dramatic changes in climate have

occurred as continents have

drifted.

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Plate Tectonics and Marine
Ecosystems

Seafloor Spreading

New oceanic crust is continuously formed at mid-ocean

ridges.

Hydrothermal Vents

Mineral-rich hot water erupts from the seafloor, supporting

unique chemosynthetic ecosystems.

Biodiversity

Plate tectonics shapes the seafloor and influences the

distribution of marine life worldwide.

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