Download Plate Tectonics

Plate Tectonics:
Earth's Plates and Continental
Drift
ESS 9.5
ESS 9.6
ESS 9.7
• 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?
– 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.
Let’s look at the different layers of the earth
that play a vital role in the formation of
our continents, mountains, volcanoes, etc.
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.
Remember the crust and the
upper part of the mantle make
up the lithosphere
the rest of the mantle is the
asthenosphere
outer core - the molten ironnickel layer that surrounds the
inner core.
inner core - the solid iron-nickel
center of the Earth that is very
hot and under great pressure.
• 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
• 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
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.
• 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.
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.
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
• Wegner’s theory was that about 180
million years ago, Pangaea began to break
up into separate continents. To back this
theory up, he perserved remains and
evidence from ancient animals and
plants from South America, Africa, India,
and Australia that were almost identical.
Rock sequences and glaciation also was
evidence.
Seafloor Spreading
• The other theory 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.
• The theory of seafloor spreading
suggests that molten rock, the hot
substance (lava) from the mantle rises
under the underwater ridge and breaks
through a split at the top of the ridge
(the plate). The split is called a rift
valley. The rock then spreads out in both
directions from the ridge as if it were on
two huge conveyor belts. As the
seafloor moves away from the ridge, it
carries older rocks away.
• Evidence to support sea-floor spreading is
magnetic mapping. When the rock is
magma/lava it allows the atoms of the
metals to align with the magnetic poles of
the earth. These magnetized metals cool
and form rocks and then can be mapped
using magnetic equipment. This along
with radiometric dating proves sea-floor
spreading.
• 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?
• As mentioned earlier, tectonic
plates are always moving. They are
always moving:
–pulling away from each other
(Divergent plate boundary)
–crashing head-on (Convergent
plate boundary)
–or sliding past each other.
(Transform plate boundary)
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.
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.
– When an oceanic plate meets a continetnal
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. This process is called
subduction. Molten material produced in a
subduction zone can rise to the earth’s
surface and cause 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.
A 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!
Transform plate boundary
• Sometimes, instead of pulling away
from each other or colliding with
each other, 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.
What evidence supports
continental drift.
25%
25%
25%
25%
1.
2.
3.
4.
similar rock sequences.
Similar glaciation patterns.
Similar fossils of plant and animals.
All of the above.
1
2
3
4
5
6
7
8
9
10
11
12
21
22
23
24
25
26
27
28
29
30
31
32
13
14
15
16
17
18
19
20
What evidence supports sea floor
spreading.
25%
25%
25%
25%
1.
2.
3.
4.
Magnetic mapping.
Igneous intrusion.
Similar glaciation patterns.
All of the above.
1
2
3
4
5
6
7
8
9
10
11
12
21
22
23
24
25
26
27
28
29
30
31
32
13
14
15
16
17
18
19
20
When two plates slide past each
other what type of plate boundary
is it?.
25%
25%
25%
25%
1.
2.
3.
4.
Convergent .
Divergent.
Transform.
Subduction.
1
2
3
4
5
6
7
8
9
10
11
12
21
22
23
24
25
26
27
28
29
30
31
32
13
14
15
16
17
18
19
20
When two plates go away from
each other what type of plate
boundary is it?.
25%
25%
25%
25%
1.
2.
3.
4.
Convergent .
Divergent.
Transform.
Subduction.
1
2
3
4
5
6
7
8
9
10
11
12
21
22
23
24
25
26
27
28
29
30
31
32
13
14
15
16
17
18
19
20