Plate Tectonics
Chapter 13 Earth Science Book
read pages 232-247
restate & answer Topic Questions 1-15
write and practice essays #1-3
Structure of the Earth

Mantle
The Earth is
made up of 3
main layers:
Outer core
Inner core
Core
 Mantle
 Crust

Crust
The Crust

This is where we live!

The Earth’s crust is made
of:
Continental Crust
Oceanic Crust
- thick (10-70km)
- buoyant (less dense
than oceanic crust)
- mostly old
- thin (~7 km)
- dense (sinks under
continental crust)
- young
Plate Types
 Oceanic
plates: basalt
 Dark
(black) and dense rock
type composed of silicates,
iron and magnesium
plates –
granite and andesite
 Continental
 Light
colored (pink, white
and gray) and low density
rock type composed almost
entirely of silicates.
Composition vs. Motion
We can look at the interior of the Earth based
on the composition of the rocks or based on the movement
Based on Composition



Crust – solid, relatively low density silicate rock
Mantle – Semi fluid, denser, mafic (iron and
magnesium bearing) rocks
Core – Liquid then solid iron and nickel with traces of
heavier elements
Based on Motion
 It
turns out that the upper section of
the mantle is adhered (stuck to the
underside side of the crust to form
what we call tectonic plates
What are tectonic plates made of?

Plates are
made of rigid
lithosphere.
The lithosphere is
made up of the
crust and the upper
part of the mantle.
What lies beneath the tectonic plates?

Below the
lithosphere
(which makes
up the tectonic
plates) is the
asthenosphere.
Topic 1:
Page 233 Earth Textbook
What Is Plate Tectonics?
The study of the formation
and movement of plates on Earth
* Earth’s surface consists of many plates
* 12 major plates and several minor plates
Topic 1:
Page 233 Earth Textbook
Moving Plates Cover Globe
* some plates move toward each other
(Indian toward the Eurasian)
* some plates move away from each other
(South American away from African)
* some plates slide under another
(Nazca under South American)
* some plates slide past each other
(Pacific past North American)
Topic 2:
Page 234 Earth Textbook
Lithosphere
outer solid layer of the Earth’s crust
*broken into plates about 100km thick
*mostly made of basalt (dark, mafic)
*continents are granite (light, felsic)
embedded on basalt
Topic 3:
Page 234 Earth Textbook
Asthenosphere
partially melted layer within mantle
*lithosphere rests upon asthenosphere
*magma in the asthenosphere flows
Topic 3:
Page 235 Earth Textbook
Convection Currents
magma slowly flows in asthenosphere
*heats up, expands, less dense, rises
*cools, contracts, more dense, sinks
Topic 3:
Page 235 Earth Textbook
Why Do Plates Move?
magma flows - convection currents
*heats up, expands, less dense, rises
(pushes plates apart - diverge)
*cools, contracts, more dense, sinks
(pulls plates together - converge)
Topic 4:
Page 236 Earth Textbook
Continental Drift Evidence
* proposed by Alfred Wegener in 1915
1. continents appear to fit together like a
puzzle (similar coastlines)
2. fossils on different continents related
3. distinctive rocks and mountains related
The fit between the edges of continents suggested that
they might have drifted.
jigsaw-puzzle fit
Alfred Wegener (1880-1930)
• German meteorologist and polar explorer
• in 1912, proposed the theory of
continental drift
Pangaea
• landmass
Panthalassa
• surrounding ocean
(http://physics.fortlewis.edu/Astronomy/astronomy%20today/CHAISSON/AT307/IMAGES/AT07FG21.JPG)
Pangea




What is Pangaea?
Pangaea was a super continent at one time.
Scientists use the similarity of rock types and fossil types that date
to the same age to support their theory that the continents were
connected to form a super continent.
The map below give just one example of areas on different
continents that show the same fossils and rock types.
Pangea
Pangea
Pangea
The break up
of Pangea
Look See
Watch Pangea Split
Current continents
Future continents?
Topic 5:
Page 236 Earth Textbook
Plate Tectonics Evidence
1. Earthquakes and volcanoes do not
occur randomly on the Earth
Topic 6:
Page 237 Earth Textbook
Plate Tectonics Evidence
2. Earth’s magnetic poles reverse
(flip) every few million years
*as rocks on ocean floor form, they
store Earth’s magnetic record
*newest rock at center, oldest rock
farther from spreading center
*at spreading centers plates move apart
leaving N-S-N-S-N bands
Facts Combine to Confirm the Theory of Plate Tectonics.
Earth has a magnetic field
iron
fillings
http://www.scifun.ed.ac.uk/card/images/left/earth-magfield.jpg
Continued

See Page 24
Earth’s magnetic poles have switched
places several times.
“flip”
magnetic reversal
During the last 100 million years, the earth's magnetic
field has reversed about 170 times.
In basaltic magma, there are small particles of a magnetic
mineral called magnetite
When magma erupts at mid-ocean ridges, it cools to form rock.
Magnetite acts like a small compass needle.
• so we know the direction
of Earth’s magnetic field
when the rock became solid
• paleomagnetism
See Page 23
See Page 24
These magnetic reversals are caused by changes in Earth’s
magnetic fields. Bands of rock record periods of magnetic
reversals. As molten material cools, magnetic minerals line up
with the magnetic field. When it hardens, the minerals act like
tiny compass needles.
Topic 7:
Page 238 Earth Textbook
Plate Tectonics Evidence
3. heat flow at spreading centers
*spreading center – more heat flow
*heat decreases away from centers
Plate Movement

“Plates” of lithosphere are moved around
by the underlying hot mantle convection
cells
Plate Boundaries
 Convergent
– plates move toward
one another
– plates move away from
each other
 Divergent
– plate moves sideways
from each other
 Transform
Three types of plate boundary

Divergent

Convergent

Transform
(sliding)
Topic 8:
Page 240 Earth Textbook
Diverging Boundaries
Plates Move Apart
*magma rises, pushing plates apart
*mid-ocean ridges are visible
Divergent Boundaries

Spreading ridges

As plates move apart new material is erupted to
fill the gap
See Page 26
Age of Oceanic Crust
Courtesy of www.ngdc.noaa.gov
Iceland: An example of continental rifting

Iceland has a divergent
plate boundary running
through its middle
Topic 9:
Page 241 Earth Textbook
Sliding Boundaries
Plates Slide Past Each Other
*site of many earthquakes
*faults are visible
Transform Boundaries

Where plates slide past each other
Above: View of the San Andreas
transform fault
San Andreas Fault, CA
San Andreas Fault
• 1300 km long
http://nationalatlas.gov/articles/geology/features/images/sanandreas.jpg
Convergent Boundaries

There are three styles of convergent
plate boundaries
Continent-continent collision
 Continent-oceanic crust collision
 Ocean-ocean collision

Convergent Plates
Convergent Plates
Topic 10:
Page 242 Earth Textbook
Collision Boundaries
Converging – Plates Together
*continental into continental
*mountains are visible
Continent-Continent Collision

Forms mountains, e.g. European Alps,
Himalayas
Himalayas
Topic 11:
Page 243 Earth Textbook
Subduction Boundaries
Converging – Plates Together
*oceanic into & under continental
or oceanic into & under oceanic
*deep-sea trenches with volcanoes on
the overriding plate
Subduction




Oceanic lithosphere
subducts underneath the
continental lithosphere
Oceanic lithosphere heats
and dehydrates as it
subsides
The melt rises forming
volcanism
E.g. The Andes
Continent-Oceanic Crust Collision

Called SUBDUCTION
Andes Mountains,
South America
Topic 12:
Page 245 Earth Textbook
The Craton
Ancient Continental Core
*oldest most altered rock on continent
*exposed craton are called shields
(the Canadian Shield is exposed North American Craton)
Topic 13:
Page 246 Earth Textbook
Sources of Growth Material
*deep-sea sediments
*volcanic rock
*sediments deposited from rivers
Topic 14:
Page 246 Earth Textbook
Thin-Skinned Thrusting
large sheets of rock pushed
*continents get shuffled and stacked
*mountains are usually formed
Topic 15:
Page 246 Earth Textbook
Growth by Terranes
large block of plate that is moved
1. block is bound by faults
2. fossils do not match others in rock
3. magnetic polarity does not match
Plate Tectonics
Chapter 13 Earth Science Book
read pages 232-247
restate & answer Topic Questions 1-15
write and practice essays #1-3