Download Chapter 6 Plate Tectonics

CHAPTER 6 PLATE
TECTONICS
6.1 EARTH HAS SEVERAL LAYERS
Objectives
•Learn about the different properties of Earth’s
layers
•Learn about the plates that make up Earth’s
outermost layers.
What are the layers of the Earth

Crust
Thin layer of rock
 Surrounds Earth like a shell surrounds an egg
 Two types of crust


Continental crust


Makes up the continents and some major islands
Oceanic crust

Makes up the ocean floor
Between 6-70 km thick (about 3.7 – 43.5
miles)
 Furthest we have ever been in 12.3 Km (7.6
miles)

Two Kinds of Crust

Continental Crust
Thicker of the two
 Less Dense
 Composed mostly of Granite
 Avg. of 30 – 40 Km thick (19 – 24.9 Miles)


Oceanic Crust
Thinner of the two
 More Dense
 Composed mostly of Basalt
 Avg. of 6 – 11 Km thick (3.7 – 6.8 Miles

Mantle





Thickest Layer of Earth
Made mostly of Hot Molten Rock
Very, Very top is cool ridged rock
About 2900 Km (1802 miles)
870 – 4400 C (1598 – 7952 F)
Outer Core

Layer of liquid metal
 Iron



& Nickel
2300 Km (1429 Miles)
4400 – 6100 C (7952 – 11,012 F)
There is more pressure than the mantle
but less pressure than the inner core
Inner Core

Solid ball of metal
 Iron




& Nickel
2400 Km in diameter (1491 Miles)
7000 – 8000 C (12,632 – 14,432 F)
Extreme pressure keeps it solid
Densest layer of the Earth
Lithosphere


Made up of the crust and very top rigid
part of the mantle
This is what the tectonic plates are
made of
 Most
plates have both continental and
oceanic crust


On continents it is believed to be an
average of 150 Km (93 miles)
In ocean it is believe 50-100 Km (31 –
62 miles)
Asthenosphere



The hot soft layer of molten rock in the
middle mantle on which the lithosphere
rests
Flows slowly like hot tar
Estimated to be 85- several hundred
Km thick (50 – several hundred miles)
6.2 Continents change position over
time

1.
2.
3.
Objectives
Learn how the continental drift hypothesis was
developed
Learn about evidence for plate movement from the
sea floor
Learn how scientists developed the theory of plate
tectonics
Continental Drift




Hypothesis proposed by Alfred Wegener in 1915
The idea that Earth’s continents were once joined in
a single landmass and gradually drifted apart.
This was not believed until the mid-1900’s
Wegener found many things that he claimed proved
his hypothesis
Evidence for Continental Drift

Fossils
 Ancient
reptile found in both South America and
Western Africa - Mesosaurus
Evidence for Continental Drift

Geology
 Rocks
in Brazil matched perfectly to rocks in Africa
 Limestone in Appalachian Mts. (N.A.) same as Scottish
Highlands
Evidence for Continental Drift

Climate
 The
same glacial till of the same age is found in
southern Africa, South America, India and Australia
 Large coal deposits were formed from tropical swamps
in both N. America and Europe at the same time
Super Continent

Wegener named his super continent
 Pangaea
 From
the Greek word meaning “all lands”
 Was a continent that reached from pole to pole
 Began
to split apart 200 million years ago
New Evidence


In the 1960’s scientists really started studying the
sea floor
Found underwater mountain ranges
 Called
mid-ocean ridges
 Found in every ocean
 Seemed to circle the Earth like the seams of a baseball

Sea-floor Spreading
 Where
the ridges form
 Cracks in the crust where molten rock rises, cools, and
forms new crust
New Evidence

Age of the Sea Floor
 Rock
near the cracks are younger then rocks further
away

Ocean Trenches
 Where
the oceanic crust sinks back into the
Asthenosphere

Scientists put this new evidence together with
Wegener's hypotheses to create the Theory of Plate
Tectonics
What causes Plate Movement

Convection Current
 Motion
that transfers heat energy in a material
 As molten rock heats up it becomes less dense, as it rises
it starts to cool becoming more dense and sinking again
Convection Currents Create

Ridge Push
 Happens
at mid-ocean ridges
 Pushes plates apart
 Creates new sea floor

Slab Pull
 Happens
at Ocean Trenches
 The denser plate sinks
 Melts back into the Asthenosphere
Theory of Plate Tectonics



States that Earth’s lithosphere is broken into huge
plates that move over the surface of the Earth
Driving force of this movement is the convection
current in the Asthenosphere
Most major earthquakes, volcanoes, and mountain
ranges are where two plates meet.
6.3 Plates Move Apart



About different plate boundaries
What happens when plates move apart
How the direction and speed of plates
can be measured
Types of plate boundaries

Divergent boundary
 Where
two plates move apart from each
other

Convergent boundary
 Where

plates push into each other
Transform boundary
 Where
plates scrape past each other
Sea Floor Spreading

Spreading Center


Another name for a divergent boundary in
the sea floor
What can form here

Rift Valley


Gap between mid-ocean ridges
Mid-ocean ridges
Longest chains of mountains in the world
 Mid-Atlantic Ridge – Runs from Iceland to
Antarctica



11,000 Km ( 6,214 mi) long
Rift Valley – 24 Km (15 mi) wide, 9 Km (6 mi)
deep
Magnetic Reversal





Recorded in the sea floor
Magnetic minerals in igneous rock lines
up according to the direction of the
poles
Last one was about 760,000 yrs ago
Flipped 170 times over the last 100
million years
The prediction for it happening again is
1,500 – 2,000 yrs from now
Divergent boundaries on Continents


Crust is to thick to pull apart like in the
ocean
Two plates still pull apart
 Faults
are created at the edges of these
plates
 Causes
 Middle
lots of earthquakes to occur
sinks down
 Eventually
it will sink below sea level and fill
with water

Red sea
Divergent boundaries on Continents

The Great Rift Valley in Africa
 Thousands

Iceland
 Being

of Km long and 1800 m deep
split into two islands
Process takes millions of years
Hot Spots



Area where hot molten rock rises to the
surface in plumes (columns) from the
mantle
Usually found in the middle of tectonic
plates but can be near boundaries
Hawaiian Islands
 Can
tell the direction and speed of plate
movement

Yellowstone National Park
6.4 Plates Converge or Scrape past
each other
What you will
learn



What Happens when two continental
plates converge
What happens when an oceanic plate
converges with another plate
What happens when one plate scrapes
past another plate
Tectonic Plates push together at
convergent boundaries


Crust is either folded or destroyed
Subduction
 When

one plate sinks beneath another
Major geologic events occur at all
convergent boundaries
Continental-Continental Collision




When two plates carrying continental
crust push together
Because both crusts are the same
density neither sinks
Edges crumple and fold creating
mountains
Some of the worlds larges mountain
chains are formed along these
boundaries
 Alps
and Himalayas are examples
As long as
their plates
keep moving
these
mountains
keep getting
bigger!
Oceanic-Oceanic Subduction


This is where one plate with oceanic
crust sinks beneath another plate with
oceanic crust
The older plate sinks
 It
is more dense and colder than the
younger plate

When the plate reaches the
asthenosphere it melts into magma
What main features form at O-O
Subduction

Deep-Ocean Trenches
 Like
deep canyons that form in the ocean
floor as a plate sinks
 Most are found in the Pacific Ocean
 The Mariana Trench is where the Pacific
Plate is sinking under the Philippine Plate
 Deepest
place in the ocean
 Nearly 11,000 meters (36,000 ft) down
 Oldest
crust is found at these trenches
 Earthquakes can occur here
What main features form at O-O
Subduction

Island Arcs
 Chain
of volcanic islands
 These are violent volcanoes
 Examples
 Philippine
Islands
 Aleutian Islands of Alaska
 Islands of Japan
 Earthquakes
often happen here
Oceanic-Continental Subduction


This is when oceanic crust sinks under
continental crust because it is colder and
denser
Deep-Ocean Trenches also form here
 These
are younger trenches than at O-O
subduction
 Earthquakes also happen here
Oceanic-Continental Subduction
Continued

Coastal Mountains
 When
the oceanic crust sinks it can cause
the continental crust to buckle to form
mountains
 Run parallel to the deep-ocean trench
 Many of these mountains will also be
volcanic
 Cascade Mountains in Oregon and
Washington
 Mount
St. Helens – Active Volcano
Transform Boundaries




The only boundary where crust is
neither created or destroyed
Plates scrape past each other at these
boundaries
Site of most major earthquakes
Most occur on the sea floor but some
are on land
Transform Boundaries Continued

San Andreas Fault in California
 Runs
from the Gulf of California through
San Francisco
 Pacific Plate and part of the North
American Plate are moving in opposite
directions
 If it keeps moving at its current rate
 Los
Angeles will be a suburb of San Francisco
in about 10 million years