Download 2.4 Plate Tectonics - Northside Middle School

2.4
Plate Tectonics and
Ocean Floor Geology
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Earth’s lithospheric crust:
- is divided into large and
small crustal plates
- makes up the ocean floor
and the continental land
masses, along with the
upper mantle.
The oceanic crust averages 8 km deep but is dense enough
to support the continental crust riding on it.
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The continental crust averages 35 km. deep both are relatively thin.
The crust covers a thick,
molten, moving mantle (68% of
Earth’s volume) and a heavy
core, possibly composed of
nickel and iron.
The Earth from the Moon.
The plates are moving in
different directions further
cracking the crust.
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By looking at seismic activity around the world, scientists can
identify the plate boundaries because of the volatility of these areas.
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These are the major plates: North American, South American, Pacific,
Eurasian, African, Nazca, Australian, and the Antarctic plate.
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3 TYPES OF PLATE BOUNDARIES
1. Divergent – meaning to move apart
ex: boundary to two oceanic plate edges
a. Both plates have the same high density rock.
b. The result is a vent where lava
extrudes and solidifies to form new
ocean floor.
c. These volcanoes with rift valleys
form a 40,000-long mountain ridge
found in many places other than the
middle of the Atlantic Ocean.
ex: Mid-ocean ridge (MOR)
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NOAA Photo Library
Mid-Ocean Ridge
About 15 percent of all
volcanoes occur at divergent
boundaries such as the midocean ridges.
As the rift expands, magma
flows in and may deposit
enough material to build an
island.
Iceland is a large section of the
MOR extending above the water.
NOAA
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Islands can also form in the middle of plates in places called hot
spots. These are high-temperature plumes of mantle material
rising from great depths to eventually melt through the crust to
form volcanoes, seamounts and island arcs.
NOAA
When the plate over-riding the hot spot is moving, a plume will
break through the plate in several places causing island chains.
The Hawaiian Islands are a chain of islands formed in this way and
smaller volcanic seamounts (future islands) are still submerged.
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Hawaii’s volcano, Mauna Kea,
is actually the highest
mountain on Earth.
It’s 31,000 ft. from the
Pacific floor - although
only 13,823 ft. of it are
above water.
Earth
Observatory
NASA
NASA
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Islands rising from the ocean floor are all volcanic seamounts.
When enough material is accumulated, they extend out of the water.
Natural islands on the continental shelf are generally from deposits
of sediment, sands and coral.
island
Other features on the ocean floor are similar to surface topography
but are larger underwater because erosion is less due to slower
moving water.
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2. Convergent - meaning to come together
Ex #1: Continental to Continental boundary
a. plates have the same low
density rocks
b. The result is that edges are
forced up into mountains and
seismic activity is common.
ex: Himalayas, Atlas Mountains
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Ex #2: Oceanic to Continental
boundary - also convergent,
where plate edges over ride
each other (called subduction)
a. Lighter continental plate edge may override the denser
oceanic plate.
b. Oceanic plate edges are subducted down into the asthenosphere
and are remelted.
c. About 80 percent of all volcanic activity occurs in these
subduction zones.
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d. The resulting deep valleys in the ocean lithosphere are called
trenches.
Trenches are so common around the boundary of the Pacific plate
that the area is known as the Ring of Fire because of its seismic
activity.
Mt. Everest, at 29,000 ft., could fit
into the deepest Pacific trench
(Mariana) and still be 7,000 ft.
below sea level.
NASA
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3. Transform Boundary - convergent
Here plates slide past each other and as
they hang, stick and jolt, earthquakes
result.
Mountains, like these along the San
Andreas Fault, form as sediments are
folded and faulted.
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In 1912, Alfred Wegener
proposed the idea that the
continents were once part of a
huge land mass, called
Pangaea (all lands), that later
broke apart.
He was convinced because:
1. the continent’s shape roughly fit together like a puzzle
2. the rocks and fossils on different continents matched
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Wegener’s theory of
continental drift was
ignored because of
thin data – ocean
exploration was still
in its infancy.
As a German meteorologist,
he lacked credentials, and he
couldn’t explain how huge
continents could move.
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1969 - The Glomar Challenger drilled a series of holes in the
Mid-Atlantic ridge. What they found revolutionized ocean geology!
Continental rocks date the Earth at about 5 billion years old. Since
the ocean floor is lower in the lithosphere, scientists expected to
find older rocks at those depths.
Continental geology’s Law of Superposition states that oldest
rocks are laid down first and should be found horizontally
lowest in a bed unless uplifted.
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Instead, they found no rock older than 3 billion years old and most
were much younger. How could the ocean floor be younger than
the continents riding on it?
The Deep Sea Drilling Project
showed that rocks became
older as they moved away
from the MOR.
Oceanic geology showed rock
layers are created vertically not according to the Law of
Superposition.
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Core samples revealed matching
magnetic striping in rocks on both sides
of MOR that were parallel distances from
continents.
NOAA
NOAA
In 1977, Project FAMOUS, using Alvin,
photographed magma squeezing out
of the MOR and creating new ocean
floor.
Today, satellites can track that the Atlantic Ocean is widening by
about 1 inch per year. Seismic activity marks the edges of these
plates.
The sea floor is spreading - moving the continents with it. The
theory of sea floor spreading explains continental drift. Wegener
was right.
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