Plate Tectonics:

Earth's Plates and Continental

Drift
• Some questions we will answer today:

– How is the earth always changing?

– What forces inside the earth create and change
landforms on the surface?
– What is the theory of plate tectonics and how does it
work?
– What two theories help make up the theory of plate
tectonics?
– What is continental drift and sea floor spreading?
– What happens when the plates crash together, pull
apart, and slide against each other?
 The Earth’s Layers
• The Earth is made of many different and distinct layers. The
deeper layers are composed of heavier materials; they are
hotter, denser and under much greater pressure than the
outer layers.

• Natural forces interact with and affect the earth’s crust,

creating the landforms, or natural features, found on the
surface of the earth.
Before we start to look at the forces that contribute
to landforms,lets look at the different layers of
the earth that play a vital role in the formation of
our continents, mountains, volcanoes, etc.
 Earth diagram to label
Crust

Mantle
Outer Core

Inner Core

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 - it is composed of silicon,
oxygen, magnesium, iron, aluminum, and calcium. Convection (heat) currents
carry heat from the hot inner mantle to the cooler outer mantle.
outer core - the molten iron-nickel layer that surrounds the inner core.
inner core - the solid iron-nickel center of the Earth that is very hot and under
great pressure.
DID YOU KNOW?
 Land and Water
• Photographs of the earth taken from space
show clearly that it is a truly a ”watery
planet.”
• More than 70 percent of the earth’s
surface is covered by water, mainly the
salt water of oceans and seas.
• Most people know that Earth is moving around
the Sun and that it is constantly spinning.

• But did YOU know that the continents and

oceans are moving across the surface of the
planet?

• Volcanoes and earthquakes as well as mountain

ranges and islands all are results of this
movement.

• Why is this?
Plate Tectonics
• Most of these changes in the earth’s
surface takes place so slowly that they are
not immediately noticeable to the human
eye.

• The idea that the earth’s landmasses have

broken apart, rejoined, and moved to other
parts of the globe forms part of the
–plate tectonics theory.
 Plate Tectonic Theory
About forty years ago, scientists exploring the seafloor found that it is
full of tall mountains and deep trenches, a single seafloor mountain chain circles
Earth and contains some of Earth’s tallest mountains.
Along this mountain chain is a deep crack in the top layers of earth.
Here the seafloor is pulling apart and the two parts are moving in opposite
directions, carrying along the continents and oceans that rest on top of them.
These pieces of Earth’s top layer are called tectonic plates. They are
moving very slowly, but constantly. (Most plates are moving about as fast as your
fingernails are growing -- not very fast!) Currently Earth’s surface layers are
divided into nine very large plates and several smaller ones.
According to the theory of plate tectonics, the
earth’s outer shell is not one solid piece of rock.
Instead the earth’s crust is broken into a number
of moving plates. The plates vary in size and
thickness.
 Tectonic Plates
• Scientists have identified about 15 major
tectonic plates.

• Scientists identify plate boundaries

primarily by studying data from
earthquakes.

• The locations of volcanoes can also help

identify the locations of plate boundaries.
 Tectonic Plates, continued
The boundaries of tectonic plates do not always match the outlines of
continents.
• The North American Plate stretches from the mid-
Atlantic Ocean to the northern top of Japan. The
Cocos Plate covers a small area in the Pacific
Ocean just west of Central America.
• These plates are not anchored in place but slide
over a hot and bendable layer of the mantle.
 Chapter 10
How Continents Move
• plate tectonics the theory that explains how large
pieces of the lithosphere, called plates, move
and change shape

• lithosphere the solid, outer layer of Earth that

consists of the crust and the rigid upper part of
the mantle

• asthenosphere the solid, plastic layer of the

mantle beneath the lithosphere; made of mantle
rock that flows very slowly, which allows tectonic
plates to move on top of it
 How Continents Move,
continued
• The lithosphere forms the thin
outer shell of Earth and is
broken into several blocks or
tectonic plates.
• The tectonic plates ride on
the asthenosphere in much
the same way that blocks of
wood float on water.

• Tectonic plates can include continental crust, oceanic crust, or

both.

• Continents and oceans are carried along on the moving tectonic

plates in the same way that passengers
are carried by a bus.
– How is the earth always changing?
– What is the theory of plate tectonics and
how does it work?
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
Less than 100 years ago, many scientists thought
the continents always had been the same shape
and in the same place.

A few scientists noted that the eastern coastline of

South America and the western coastline of
Africa looked as if they could fit together.

Some also noted that, with a little imagination, all

the continents could be joined together like giant
puzzle pieces to create one large continent
surrounded by one huge ocean .
So, if my contintents fit together,
why does the earth look like it
does today?
 Continental Drift Theory
• When the tectonic plates under the continents
and oceans move, they carry the continents and
oceans with them.

• In the early 1900s, a German explorer and scientist

proposed the continental drift theory. He proposed
that there was once a single “supercontinent” called
Pangaea.
• Wegner’s theory was that about 180 million years ago,
Pangaea began to break up into separate continents. To
back this theory up, he preserved remains and evidence
from ancient animals and plants from South America,
Africa, India, and Australia that were almost identical.
 Continental Drift Evidence:
1. Jig-Saw Fit
• Continents fit together like Puzzle pieces
2. Fossil Evidence
• fossils of same age and species were found in
connecting bands on different continents
3. Rock Types and Structure
• Similar bands of rocks types and mountain belts
connect
4. Ancient Climates
5. PANGAEA!
 Seafloor Spreading

• The other theory supporting plate tectonics

emerged from the study of the ocean floor.

• Scientists were suprised to find that rocks taken

from the ocean floor were much younger than
those found on the continents. The youngest
rocks were those nearest the underwater ridge
system which is a series of mountains that extend
around the world, stretching more than 64
thousand kilometers (40 thousand miles).
• The theory of seafloor spreading suggests that
molten rock (lava) from the mantle rises under the
underwater ridge and breaks through a split at the
top of the ridge/crust.
– The split is called a rift valley.
• The rock then spreads out in both directions from
the ridge.
• As the seafloor moves away from the ridge, it
carries older rocks away.
• Seafloor spreading, along with the continental drift
theory, became part of the theory of plate tectonics.
Plate motions also can be looked at into the future, and we can have a
stab at what the geography of the planet will be like. Perhaps in 250
million years time there will be a new supercontinent.
– What two theories help make up the
theory of plate tectonics?

– What is continental drift and sea floor

spreading?
 So....
• When a geologist or a geographer looks at
a piece of land they often ask, ”What
forces shaped the mountains, plains, and
other landforms that are here?”
• What is their answer?
 Plate Tectonics

But this doesn’t actually tell me how the

mountains or volcanoes were formed or
how earthquakes happen, does it?
 YES!
• As mentioned earlier, those tectonic plates
are always moving. They are always
moving:
– pulling away from each other
– crashing head-on
– or sliding past each other.

Depending on which way these plates are moving will

decide what will happen on earth where you and I are
standing on.
 Plate Boundaries
• Three types:
– Divergent Boundary
– Convergent Boundary
– Transform Boundary
 They’re Pulling Apart!
• When plates pull away from
one another they form a
diverging plate boundary, or
spreading zone.
spreading_zone

Thingvellir, the spreading zone in Iceland between the North American (left
side) and Eurasian (right side) tectonic plates. January 2003.
 Divergent Boundary
• Plate moving apart
• Examples:
– Mid Atlantic Ocean Ridge
– East Pacific Rise
• Plates are being pushed apart… as they are
pushed apart, magma rises from the mantle and
fills the void.
• Mechanism for plate movement
 Ocean Ridges
• Oceanic ridges are continuous elevated zones on the
floor of all major ocean basins. The rifts at the crest of
ridges represent divergent plate boundaries.
 The Crash!
• What happens when plates crash into
each other depends on the types of
plates involved.
– Because continental crust is lighter
than oceanic crust, continental
plates ”float” higher.
– Therefore, when an oceanic plate
meets a continentnal plate, it
slides under the lighter plate and
down into the mantle. The slab of
oceanic rock melts when the
endges get to a depth which is hot
enough. A temperature hot
enough to melt si about a
thousand degrees!) This process
is called subduction. Molten
material produced in a subduction
zone can rise to the earth’s
surface and cause volcanic
building, mountains, and islands.
 When they Crash
• When two plates of the same type meet,
the result is a process called converging.

– Depending on what type of plates these are,

depends on what occurs.
 Converging... They crash!
And they’re both ocean plates!
• When both are oceanic plates, one slides
under the other. Often an island group
forms at this boundary.
 Converging...They Crash!
And they’re both Continental Plates
• When both are continental plates, the plates
push against each other, creating mountain
ranges.
 They Crash and are both
continental plates!
• Earth’s highest mountain range, the Himalayas, was formed
millions of years ago when the Indo-Australian Plate crashed into
the Eurasian Plate. Even today, the Indo-Australian Plate continues
to push against the Eurasian Plate at a rate of about 5 cm a year!
 They meet and slide past each
other!

• Sometimes, instead of pulling away from

each other or colliding with eac hother,
plates slip or grind past each other along
faults. This process is known as faulting.
• These areas are
likely to have a rift
valley, earthquake,
and volcanic
action.
For example: Here, the San
Andreas Fault lies on the boundary
between two tectonic plates, the
north American Plate and the
Pacific Plate.
The two plates are sliding past
each other at a rate of 5 to 6
centimeters each year. This fault
frequently plagues California with
hearthquakes.
 Transform Boundaries
• Plates grind past each other without destroying
or creating new lithosphere
• Like cars passing each other on a highway
• Creates tremendous earthquakes
• Example: San Andreas Fault
– What forces inside the earth create and
change landforms on the surface?

– What happens when the plates crash

together, pull apart, and slide against
each other?