Transcript Chapter 3 Plate Tectonics

Chapter 3 Plate
Tectonics
3-1 Earth’s Drifting
Continents
• Theory-guess
• A land bridge once stretched across
the Atlantic Ocean and connected
South America and Africa
• Evidence for this includes fossils of
plants and animals that could not
have possibly crossed the ocean.
• Fossils-Preserved remains of
ancient organisms.
• The earth once had a single landmass that
broke up into large pieces, which have since
drifted apart.
• The name of this giant landmass is Pangaea
which means all earth.
• Wegner’s –Theory of continental drift
• One supporting piece of evidence is the fossil
Glossopteris( extinct now longer living plant)
located in rocks about 250 M years ago have
been found in South Africa, Australia, India
and Antarctica.
• Presence in Antarctica indicates that at one
time the climate for Antarctica was much
different than it is today.---end 57
Evidence from rocks
• Rocks from Africa and South America piece
together.
• An ancient folded mountain chain that stretches
across South Africa links up with an equally
ancient folder mountain chain in Argentina.
• Coal fields with distinctive layers in Brazil line up
with coal field with identical layers in Africa
• End 58
3-1 Section Review
• What is continental drift?
• The theory that the Earth had one
giant landmass that split to form
today’s continents.
• Who first developed a scientific
argument for continental drift?
• Alfred Wegener
• 2. How do scientists explain the
existence of fossils of the same
plants and animals on continents
thousands of kilometers apart?
• Scientists use the theory of
continental drift to explain this
phenomenon.
3-2 Earth’s Spreading
Ocean Floor
• How could the continents plow
through hard, solid ocean floor?
• In 1950’s and 60’s they discovered a
large system of underwater
mountains that have a deep crack,
called a rift valley running through
their center. These underwater
mountains are known as midocean
ridges
• End 60
• Midocean ridges form the single largest mountain
chain in the world. The chain is approx. 80,000
kilometers long-roughly 20 times the distance
from NYC to LAX and 3 KM high.
• A great deal of volcanic activity occurs at the
mid-ocean ridges. Lava erupts from the rift valley
that runs the length of the ridge.
• As the ocean floor moves away on either side of
the ridge lava wells up and hardens
• The hardened lava forms new ocean floor
• This process is called ocean-floor spreading.
• Ocean-floor spreading helps to explain how
continents drift.
• Although sections of the midocean ridges are
straight, the ridges as a whole curve because
straight sections are offset by thin cracks
called transform faults
• Rock samples from the ocean floor indicate
that rocks next to a midocean ridge are
younger than rocks farther away the youngest
rocks are in the center of the ridge
• End 61
Section 3-2 Pg 62
• Magnetic stripes in ocean-floor rocks
confirmed ocean-floor spreading
• The history of the earth’s magnetism is
recorded in rocks, when molten rock
hardens a permanent record of the earth’s
magnetism remains in the rocks
• As a result, scientist learned that the
earth’s magnetic poles reverse themselves
from time to time-during the past 3.5 M
years they have reversed 9 times
• The pattern of magnetic stripes is identical on
both sides of a mid-ocean ridge.
• As magma hardens into rock at a midocean ridge
half the rock moves in one direction and the other
half moves in the opposite direction.
• The oldest rocks on land are 4 billion years old
but the oldest rocks in the ocean are 200 million
years old.
• Because the Earth’s surface remains the same
size, the ocean floor is being destroyed as fast as
it is being formed by ocean-floor spreading.
• End 62
63
• Trenches-V shaped valleys that lie
along the bottom of the oceans, they
are the deepest parts of the oceans
close to continents or near strings of
islands such as Alaska’s Aleutian
Islands .
• Why is the location of these
trenches significant?
• The trenches create the Ring of Fire.
Subduction
• Older ocean floor moves away from the midocean
ridges as new floor is formed.
• Eventually the older floor moves down deep into
the Earth along trenches.
• Subduction-The process in which crust plunges
back into the Earth.
• When the rocks are pushed deep enough they are
melted by the heat of the earth.
• Some of the molten rock will rise up through the
crust and produce volcanoes,, but most will
become part of the mantle.
• As new rocks are formed along the ocean ridges,
older rocks are subducted into trenches, the
process balances itselt. End 63
Section 3-2 Review
• 1. What process helps to explain how
continents drift?
• The process of ocean-floor spreading
helps to explain how continents drift
• 2. Where are the youngest rocks on
the ocean floor found?
• The youngest rocks on the ocean
floor are found at the midocean
ridges
Section 3-2 Review
• 3. How can the magnetic orientation
of rocks be used to trace the way a
continent moved as it drifted?
• The magnetic orientation of rocks of
different ages can be compared to
get an idea of how the position of a
continent has changed over time.
The short, sad life of Kid
Crusty
The Life of Kid Crusty
Crusty Celebrates his
birthday
• Happy 200 millionth Birthday
Crusty is attacked by
subduction
Crusty meets his maker
and is born again
3-3 Earth’s Moving Plates
• Plate- irregularly shaped slabs that fit
together like paving stones to form the
surface layer of the earth.
• Theory of Plate Tectonics-links together
ideas of continental drift and ocean-floor
spreading, explains how the Earth has
evolved over time and helps to explain the
formation, movements, collusions, and
destruction of the Earth’s crust. 64
Lithospheric Plates
• Lithosphere- The topmost solid part of the earth
• There are seven major lithospheric plates and
many smaller plates, some too small to list on a
map
• They move at different speeds, and in different
directions, small plates that lack landmasses move
as much as several centimeters a year-large
plates that are weighted down with landmasses
may only move a few millimeters a year.
• In a few cases, the edges of the continents are
the boundaries of the plates, however most plate
boundaries are on the ocean floor. 65
Plate Boundaries
• There are 3 types of boundaries
• Divergent-Midocean ridges-aka
constructive boundaries• Convergent-trenches-plates come
together at the trenches-cause pressure
and friction• Strike-slip boundaries-Two plates grind
together and slip past each other
horizontally-earthquakes usually occur
here
Plate Motion