CH. 7 PLATE TECTONICS
Tectonic Plates
-pieces of lithosphere that move around
on
the asthenosphere
-plates fit together like a puzzle
-plates are not divided along continent
lines
How do scientists Map the Earth’s
Interior?
-when quakes happen-seismic waves
travel thru earth at different speeds
-speed depends on density & composition
of material they go through
(faster thru solids & slower thru
liquids)
-seismographs measure vibrations
(seismic waves) & use distances from
epicenter & wave travel time to
calculate density & thickness of each
COMPOSITION OF THE EARTH
 Crust,
Mantle, Core
Layers are based on chemical composition:
crust is made of less dense compounds (O, Si, Al)
core is made of denser compounds (Fe, & some
N)
5 PHYSICAL LAYERS OF THE EARTH
& THEIR PROPERTIES
 Lithosphere:
made up of crust & rigid upper
mantle & is divided into tectonic plates
 Asthenosphere:
a plastic, soft layer of mantle
on which lithospheric plates move
 Mesosphere:
strong, lower part of mantle to
core
 Outer
core: liquid layer of core below mantle
 Inner
core: solid, dense center of Earth
MAJOR TECTONIC PLATES
MOVEMENT OF TECTONIC PLATES
CHAPTER 7.2 NOTES: RESTLESS
CONTINENTS
 1910-
Alfred Wegener developed the
Continental Drift hypothesis
 *Hypothesis
states that 245 million
years ago the continents once formed
a super continent (Pangaea) when
reptiles, winged insects, and great
tropical forests which formed coal
deposits, covered large parts of the
Earth’s surface.
Wegener published his evidence to support the
Continental Drift
1. Evidence from Landforms:
-piece together Africa & S. America &
mountain
ranges line up where the continents join.
-European coal fields match up with coal fields
in
N. America.
2. Evidence from Fossils:
-fossils of Mesosaurus & Lystrosaurus have been
found in places now separated by oceans.
-Glossopteris (fern) lived 250 mill. yrs. ago and
appears on separated landmasses of Africa,
S. Amer., Australia, India, & Antarctica.
3. Evidence from climate:
-island of Spitsbergen in Arctic Ocean is ice-
Wegener’s hypothesis was not accepted
b/c he couldn’t explain the force that
moves the continents.
Many geologists (from 1920’s to 1960’s)
believed Earth was slowly cooling &
shrinking & mountains formed when
the crust wrinkled like the skin of a
dried-up apple.
Wegener disputed this by stating that if
this were true, then mountains would
be found everywhere instead of the
edges of continents. Also, they knew
that by the strength of rock it wasn’t
possible for the crust to move.
Scientists theorize that
supercontinents occur in cycles
 500
mill. yrs. before Pangaea, a
supercontinent (Rodinia) existed
 245 mill. yrs. ago Pangaea existed
 135 mill. yrs. ago Pangaea split up into
2 huge continents (Laurasia &
Gonwana)
 65 mill. yrs. ago, split again to form
present day continents
In 1960, Harry Hess, supported
Wegener’s hypothesis
Hess proposed sea-floor spreading
(that at the mid-ocean ridge, sea
floors moved like conveyor belts
carrying the continents with them)
-Mid-ocean ridges--underwater
mountain chains that run thru center
of Atl. Ocean-cracks in ocean crust
allow molten material to spread out,
pushing older rock to both sides of
the ridge
Evidence of sea-floor spreading:
1. Evidence from molten material-scientists
dived to mid-ocean ridge in Alvin
(submersible) & found pillow rocks formed
when molten material hardens quickly
after erupting under water.
2. Evidence from magnetic pole reversal:
throughout history, north & south
magnetic poles have reversed themselves
several times. Last happened 780,000 years
ago. Scientists found that the ocean floor
rock with iron bits in them lined up in the
direction of Earth’s magnetic poles & lie in
a pattern of magnetized stripes which hold
a record of reversals.
Ch. 7.3 notes:
Theory of Plate Tectonics
 All
tectonic plates share boundaries
w/other tectonic plates where they
collide, separate, or slide past each
other.
3 Types of Plate Boundaries
1.)CONVERGENT BOUNDARIES: when 2
plates collide (converge) their density
determines which one comes out on top.
Types of Convergent Boundaries:
A) continental/continental-plates crash
head
on & create mountain ranges
B) continental/oceanic--denser oceanic
plate
subducts under continental plate at a
subduction zone.
C) oceanic/oceanic--the denser plate is
subducted thru a trench & returned
2.) DIVERGENT BOUNDARIES: when 2
plates separate new sea floor forms.
-Most divergent boundaries form at
mid-ocean ridges.
-Divergent boundaries on land form
rift valleys.
3.) TRANSFORM BOUNDARIES: 2
plates slip past each other in opposite
directions & crust is neither created or
destroyed.
*Earthquakes occur. (ex.San Andres
What causes tectonic plates to move?
-plates float on the asthenosphere
-heat from the earth’s core is the
driving force of plate movement
-heat of core causes convection
currents (flow of currents heats rock,
it rises & cools, then sinks which
causes movement)
How fast do tectonic plates move?
The movement is measured in cm/year
(between 1-10 cm/year) *which is about as
fast as your fingernails grow.
To measure the speed plates move:
scientists use a system of GPS satellites to
record the exact distance between the
satellites & the ground. Over time
distances change & can record the time it
takes for GPS to move the distance to get
speed of plates.
Critical thinking:
On average, North America is moving 1.25 cm/year away
from the Mid-Atlantic ridge. Using this rate, determine
how far apart the continents of North America &
Africa will be after 200 million years?
(Hint: distance = rate x time)
*Remember that they are both moving away at that
rate!
(b/c both plates are moving away, you must multiply the
rate by 2)
1.25 cm/year x 2 = 2.5 cm /year
D=RxT
D = 2.5 cm/yr x 200,000,000 yrs
ANSWER: 500,000,000 CM
KM
OR
_______
Chapter 7.4 Notes
Deformation of the Earth’s Crust
Deformation: any stress (pushing or pulling)
on the crust which changes the
volume/shape of crust
Two types of deformation
tension: stress when forces stretch an object
compression: stress when an object is
squeezed
Folding: bending of rock layers b/c of
stress
Types of folding
anticlines: a fold in rock that bends
upward into an arch
syncline: a fold in rock that bends
downward in the middle
monocline: folded rock layers where
both ends of the fold are horizontal
Faulting: occurs along plate boundaries
where rocks break & slide past each
other
Three types of faults
Normal Faults: occur when tectonic forces
cause tension that pulls rocks apart &
hanging wall slips downward past the
footwall
Reverse Faults: happen when tectonic forces
cause compression that pushes rocks
together & hanging wall slides up & over
the footwall.
Stike-Slip Fault: form when opposing forces
cause rock to slip past each other sideways
with little up/down motion.
When tectonic plates undergo compression or
tension, they form mountains several ways:
Folded Mountains: form at convergent boundaries
where continents collide.
Ex. Appalacian Mts. formed when N. Amer. &
Africa collided.
Fault-Block Mountains: occur when tension causes
large blocks of crust to drop down.
Ex. Tetons in Wyoming
Volcanic Mountains: located at convergent
boundaries where oceanic plates sink at
subduction zones & under the sea to sometimes
cause islands.
Ex. majority of volcanic mts. are formed around
rim of Pacific Ocean Ring of Fire.
Types of movement in the crust:
1. Uplifting of Depressed Rock: -rising of regions
of the crust to higher elevations.
*Mountains form or areas rise or rebound when
weight on it is removed.
2. Subsidence of Cooler Rocks:
-sinking of regions of crust to lower elevations.
*when lithosphere is hot, it takes up more
space at mid-ocean ridge
*lithosphere away from ridge is cooler, denser,
& takes up less space, so oceanic lithosphere
crust subsides.
3.Tectonic letdown: lithosphere is stretched in
rift zones & stress causes faults to form &
blocks of crust subside