Earth’s Internal Properties
Continental Drift & Plate Tectonics
Science 2201
Chapter 17
October 17, 1989
May 18, 1980
Earthquakes and volcanoes: evidence
that our planet is not at rest.
• global distribution of earthquakes and
volcanic centers…above crustal plate
boundaries
• The surface of the earth is constantly
changing and no feature on earth is
permanent.
Why is the Earth so restless?
• “Convection Conveyer-belt”
• The heat that drives the motion of the
mantle comes from two sources:
---Radioactive decay of materials in the
mantle
---Heat left over from the formation of the
earth
James Hutton 1726-1797
• "catastrophism,"
earthly changes were
sudden & caused by a
series of catastrophes
• This belief prevailed
until the 1700s
• "uniformitarianism
• 1785 James Hutton
• The present is the key
to the past
Uniformitarian Principle
• the geologic forces and processes -gradual as well as catastrophic -- acting
on the Earth today are the same as those
that have acted in the geologic past.
Alfred Wegener 1880-1930
Theory of Continental Drift
• (Wegener, 1915)
• continents plowed through crust of ocean
basins, which would explain why the
outlines of many coastlines(like South
America and Africa) look like they fit
together like a puzzle
Continental Drift:
a theory ahead of its time
• impossible for a large mass of solid rock to
plow through the ocean floor without
breaking up
• mid 20th Century, new evidence from
ocean floor exploration rekindled interest
in Wegener's theory, ultimately leading to
theory of plate tectonics.
Plate Tectonics: A modern version of
Wegener’s old idea
• the outer rigid lithosphere consists of about
20 rigid segments called plates
• Most plates contain both continental and
oceanic crust—a major departure from the
continental drift theory
• The word tectonics comes from the Greek
root "to build."
Folding and Faulting
• Compressive forces from moving plates
can cause rock to bend & wrinkle into
wavelike folds
• The crack or break in the rock is called a
fault and the movement of the rock is
“faulting”
Earthquakes
• Great forces of stress occurs as plates of
rock pushed in opposite directions
• sudden movement as the blocks slide and
then come to rest in a new position that
eases the pressure
The entire San Andreas fault system is more than
800 miles long and extends to depths of at least 10
miles within the Earth. In detail, the fault is a
complex zone of crushed and broken rock from a
few hundred feet to a mile wide.
Seismograph
• Richter Scale based on the amount of
seismographic movement
• An earthquake with a number of 2 is just
strong enough to be felt, 5 can cause much
damage, 7 or more is classified as a major
earthquake
Plate Boundaries
• 3 Main Types
–Divergent
–Convergent
–Transform
Divergent Plate Boundaries
• plates are being pulled apart
• sea floor spreading at Mid-Atlantic Ridge
• Eurasia and North America are separating
at a rate of about 5 cm per year
• East African Rift Valley
Convergent Plate Boundaries
• two plates are coming together
• 3 associated surface feature
– deep ocean trench and chain of volcanic
islands
– folded mountains
– ocean plate subduction beneath continent
Transform Plate Boundaries
• one plate scrapes past the other
• San Andreas fault in California
• No mountain building or volcanism is
associated with transform boundaries.
Mountains
•
•
great masses of rock
pushed high into the
air by forces inside
the earth
may be folded,
tilted, shaped into
domes, or built up
from volcanic
material
Forming Mountains
• Folded mountains are formed when layers
of rock are pushed into a series of
wavelike folds by tremendous sideward
forces
• Appalachian Mountains & Rocky
Mountains
Forming Mountains
• Fault-block mountains formed when folded
layers of rock crack, producing a fault
• Sierra Nevadas
Forming Mountains
• Domed mountains formed when magma
flows up between two layers of rock
• Black Hills of shown rising from the Great
Plains of South Dakota
Forming Mountains
• Volcanic mountains formed by the build up
of lava and other materials thrown up when
a volcano erupts
• Hawaiian Islands
Volcanoes
• A mountain formed around a crack in the
earth’s crust
• Cracks in crust allow magma to rise up
• Some erupt quietly, others violently
• Magma  lava
• Obsidian
• Pumice
3 main types of volcanoes
• Shield volcanoes link –Kilauea in Hawaii
3 main types of volcanoes
• Cinder cone link—Paricutin in Mexico
3 main types of volcanoes
• Composite or Stratovolcano link
– Ranier in U.S.
Hot Springs
• Formed when underground water is heated
by rock, and then flows to surface before
cooling
Geysers
• A hot spring that throws its water into the
air from time to time
• Almost all geysers are on 3 places:
Yellowstone National Park, Iceland, and
New Zealand
• How geysers work:
– (understanding boiling)