Inside Earth
Continental Drift
Sea Floor Spreading
Plate Tectonics
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.
Crust
Mantle
Outer Core
Inner Core
Crust
crust - the rigid, rocky outer surface of the Earth,
The crust is composed of two rocks. The continental
crust is mostly granite. The oceanic crust is basalt.
Basalt is much denser than the granite. Because of
this the less dense continents ride on the denser
oceanic plates.
The Crust
The Earth's Crust is like
the skin of an apple. It is
very thin in comparison to
the other three layers.
The crust is only about 35 miles (8 kilometers)
thick under the oceans
(oceanic crust) and
about 25 miles (32
kilometers) thick under
the continents
(continental crust).
The Lithospheric Plates
The crust of the Earth
is broken into many
pieces called plates.
The plates "float" on
the soft, semi-rigid
asthenosphere.
The Lithosphere
The crust and the upper layer of the mantle
together make up a zone of rigid, brittle rock
called the Lithosphere.
The Mantle
The Mantle is the
largest layer of the Earth.
The middle mantle is
composed of very hot
dense rock that flows like
asphalt under a heavy
weight. The movement of
the middle mantle
(asthenosphere) is the
reason that the crustal
plates of the Earth move.
The Mantle Composition
mantle - a rocky layer located under the
crust - it is composed of silicon, oxygen,
magnesium, iron, aluminum, and calcium.
Convection Currents
The middle mantle
"flows" because of
convection currents.
Convection currents
are caused by the
very hot material at
the deepest part of
the mantle rising, then
cooling and sinking
again --repeating this
cycle over and over.
The Outer Core
The core of the Earth
is like a ball of very
hot metals.
The outer core is
so hot that the metals
in it are all in the
liquid state. The outer
core is composed of
the melted metals of
nickel and iron.
The Inner Core
The inner core of the
Earth has temperatures
and pressures so great
that the metals are
squeezed together and
are not able to move
about like a liquid, but
are forced to vibrate in
place like a solid.
DID YOU KNOW?
Land and Water
Photographs of the earth taken from space
show clearly that it is 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.
Continental Drift
and
Seafloor Spreading
Continental Drift Theory
In the early 1900s a German explorer and scientist
(Alfred Wegener) proposed the continental drift
theory.
He proposed that there was once a single “super
continent” called Pangaea.
He believed that the continent floated on the
oceanic crust (like an iceberg in the ocean).
Most scientist rejected his theory due to lack of
evidence.
Evidence of continental drift
Continents fit together like a jigsaw puzzle
Fossils match across oceans
Rock types and mountain ranges match
across oceans
Climate Evidence (Glacial Deposits)
“Puzzle Pieces”
Continents look like
they could be part of
a giant jigsaw puzzle
Distribution of Fossils
Plant and animal
fossils found on the
coastlines of different
continents
Sequence of Rocks
Same rock patterns found in South
America, India, Africa, Antarctica and
Australia
Climate
Tropical plant remains
(coal deposits) found
in Antarctica
Glacial deposits in
Africa, South
America, India, and
Australia during the
same time
Sea-Floor Spreading
Ocean floor moves like a conveyor belt
carrying continents with it.
New ocean floor forms along cracks in the
ocean crust as molten material erupts from
the mantle spreading out and pushing
older rocks to the sides of the crack. New
ocean floor is continually added by the
process of sea-floor spreading.
EVIDENCE OF SEA-FLOOR
SPREADING……
Sea-Floor Spreading
1. Evidence from
Molten Material –
Rocks shaped like
pillows (rock
pillows) show that
molten material
has erupted again
and again from
cracks along the
mid-ocean ridge
and cooled quickly
Mid-Ocean Ridge
The mid-ocean ridge
system is the most
extensive chain of
mountains on earth,
but more than 90% of
this mountain range
lies in the deep
ocean. The midocean ridge wraps
around the globe for
more than 65,000 km
like the seam of a
baseball.
Mid-Ocean Ridge
Mid-ocean ridges occur along the kind of plate
boundary where new ocean floor is created as
the plates spread apart. "divergent plate
boundary." The plates spread apart at rates of 1
cm to 20 cm per year. As oceanic plates move
apart, rock melts and wells up from tens of
kilometers deep. Some of the molten rock
ascends all the way up to the seafloor,
producing enormous volcanic eruptions of
basalt, and building the longest chain of
volcanoes in the world!
Sea-Floor Spreading
2. Evidence
from Magnetic
Stripes – Rocks
that make up
the ocean floor
lie in a pattern
of magnetized
stripes which
hold a record of
the reversals in
Earth’s magnetic
field
Sea-Floor Spreading
Sea-Floor Spreading
3. Evidence
from Drilling
Samples – Core
samples from the
ocean floor show
that older rocks
are found farther
from the ridge;
youngest rocks are
in the center of
the ridge
Sea-Floor Spreading
Sea-Floor
Spreading –
Harry Hess in
the 1960’s; the
process that
continually
adds new
material to the
ocean floor
while pushing
older rocks
away from the
ridge
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 tectonic theory.
Plate Tectonic Theory
Along the mid-ocean ridge 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.
These plates are not anchored in place but
slide over a hot and bendable layer of the
mantle.
As mentioned earlier, those tectonic plates are
always moving.
– pulling away from each other (Divergent)
– crashing head-on (Convergent)
– or sliding past each other. (Transform)
Divergent Boundaries
Boundary between two plates
that are moving apart or rifting

RIFTING causes SEAFLOOR
SPREADING
They’re Pulling Apart!
When plates pull away from
one another they form a
diverging plate boundary, or
spreading zone.
Thingvellir, the spreading zone in Iceland between the North American (left
side) and Eurasian (right side) tectonic plates. January 2003.
Features of Divergent
Boundaries
Mid-ocean ridges
rift valleys
fissure volcanoes
Convergent Boundaries
Boundaries between two
plates that are colliding
 
There are 3 types…
Ocean to Continent
Ocean plate
colliding with a less
dense continental
plate
Subduction Zone:
where the more
dense plate slides
under the less
dense plate
VOLCANOES
occur at subduction
zones
Continental/Oceanic Crush
Subduction –
Process by
which the ocean
floor sinks
beneath a deepocean trench
and back into
the mantle;
allows part of
the ocean floor
to sink back
into the mantle
Subduction zone
Deep-Ocean
Trench –
Occurs at
subduction
zones. Deep
underwater
canyons form
where oceanic
crust bends
downward
Subduction Zone
Andes Mountains,
South America
Ocean to Ocean
Ocean plate colliding with
another ocean plate
The more dense plate slides
under the less dense plate
creating a subduction zone
called a TRENCH
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.
Aleutian Islands, Alaska
Continent to Continent
A continental plate
colliding with another
continental plate
Have Collision Zones:
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!
Transform Fault Boundaries
Boundary between two plates
that are sliding past each other
EARTHQUAKES along faults
San
Andreas
Fault,
CA
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 wit
hearthquakes.
These areas are likely to
have a rift valley,
earthquake, and volcanic
action.
All graphics were taken from Google Images, enchanted
learning, boom zone, and other educational sites.
All written information was taken from Prentice Hall,
World Geography, PBS.org, and other educational
websites.
A good website for a deeper understanding is
www.observe.arc.nasa.gov/
Animations of sea-floor spreading at the mid-oceanic rifts
Image:
http://www2.nature.nps.gov/geology/usgsnps/animate/plt
ecan.html