continental drift and plate tectonics 2018-09-04آ continental drift wegenerâ€™s continental...
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and Plate Tectonics
Continental Drift Wegener’s continental drift hypothesis
stated that the continents had once been
joined to form a single supercontinent.
• Wegener proposed that the
began to break apart 200
million years ago and form
the present landmasses.
1. Matching Fossils
- Fossil evidence for
found on different
landmasses. Mesosaurus Fossils
2. Rock Types and Structures
- Rock evidence for continental drift exists in the form of
several mountain belts that end at one coastline, only to
reappear on a landmass across the ocean.
3. Ancient Climates
glacial till and glacial striations.
4. Puzzle Fit
continents fit together
A New Theory Emerges
• Wegener could not provide an explanation of
exactly what made the continents move. News
technology lead to findings which then lead to
a new theory called plate tectonics.
Evidence for Plate Tectonics
1. Paleomagnetism - natural remnant
magnetism in rock bodies; this permanent
magnetization acquired by rock can be
used to determine the location of the
magnetic poles at the time the rock became
• Normal polarity—when rocks show the same magnetism as the present magnetism field
• Reverse polarity—when rocks show the opposite magnetism as the present magnetism field
in Lava Flows
Testing Plate Tectonics
The discovery of strips
polarity, which lie as
across the ocean
ridges, is among
Evidence for Plate Tectonics 2. Earthquake Patterns
• Scientists found a close link between deep-focus earthquakes and ocean trenches.
• The absence of deep-focus earthquakes along the oceanic ridge system was shown to be consistent with the new theory.
Evidence for Plate Tectonics 3. Ocean Drilling
• The data on the ages of seafloor sediment confirmed what the seafloor spreading hypothesis predicted.
Evidence for Sea Floor Spreading
•The youngest oceanic crust is at the ridge crest, and the
oldest oceanic crust is at the continental margins
Evidence for Plate Tectonics 4. Hot Spot- concentration of heat in the mantle capable of
producing magma, which rises to Earth’s surface; The Pacific plate moves over a hot spot, producing the Hawaiian Islands.
• Hot spot evidence supports that the plates move over the Earth’s surface.
Causes of Plate Motion Scientists generally agree that convection
occurring in the mantle is the basic driving force
for plate movement.
1. Convective flow is the motion of matter resulting from changes in temperature.
2. The unequal distribution of heat within Earth causes the thermal convection in the mantle that ultimately drives plate motion.
What moves the plates? Convection Currents: the driving force
behind plate tectonics.
Ridge push - mechanism for plate motion in plate tectonics. Because mid-ocean ridges lie at a higher elevation
than the rest of the ocean floor, gravity causes the ridge to push
on the lithosphere that lies farther from the ridge.
Slab Pull - tectonic plate motion due to higher densities. Plate motion is partly driven by the weight of cold, dense plates
sinking into the mantle at trenches.
Isostasy - The up and down movement of the crust
- the concept of
floating crust in
Think about a series of wooden
blocks of different heights
floating in water.
The Earth’s outer
core is made
of a liquid
metallic iron that
• Sphere with a radius of 1216 km • Behaves like a solid • Composed of an iron-nickel alloy
•The Earth’s inner core is made up of solid iron, nickel, similar to metallic meteorites. It is solid because of all the pressure of Earth pushing down to one central point.
• Liquid layer
• 2270 km thick
• Convective flow of metallic iron within generates Earth’s magnetic field
The Earth’s outer core is made of a liquid metallic iron that generates Earth’s magnetic field. It is liquid because of the immense heat of the inner core.
• 660–2900 km
• More rigid layer
• Rocks are very hot and capable of gradual
The mantle makes up 82% of Earth’s volume and is a slightly less dense version of the inner core.
• Beneath the lithosphere
• To a depth of about 660 kilometers
• Soft, weak layer that is easily deformed
• Crust and uppermost mantle (about 100 km
• Cool, rigid, solid
According to the plate tectonics
theory, the uppermost mantle, along
with the overlying crust, behaves as a
strong, rigid layer. This layer is known
as the lithosphere
A plate is one of numerous rigid
sections of the lithosphere that
move as a unit over the material
of the asthenosphere.
• Continental crust
- Upper crust composed of granitic rocks
- Lower crust is more akin to basalt
- Average density is about 2.7 g/cm3
- Up to 4 billion years old
- Basaltic rock
- Density about 3.0 g/cm3
- Younger (180 million years or less) than the continental crust
• Velocity of seismic waves increases abruptly below 50 km of depth
How do we know? By studying earthquake waves.
Earth’s Interior Showing P and S Seismic Wave Paths
Tectonic Plate Boundaries
Divergent boundaries - the place where
two plates move apart.
Convergent boundaries form where two
plates move together.
Transform fault boundaries are margins
where two plates grind past each other
without the production or destruction of the
Divergent Boundaries Oceanic Ridges and Seafloor Spreading
• Oceanic ridges are continuous elevated zones
on the floor of all major ocean basins. The rifts at
the crest of ridges represent divergent plate
• Rift valleys are deep faulted
structures found along the
axes of divergent plate
boundaries. They can develop
on the seafloor or on land.
• Seafloor spreading
produces new oceanic
Continental Rifts • When spreading centers develop within a
continent, the landmass may split into two
or more smaller segments, forming a rift.
Convergent Boundaries A subduction zone occurs when one oceanic plate is
forced down into the mantle beneath a second plate.
• Denser oceanic slab sinks into the asthenosphere.
• Pockets of magma develop and rise.
• Examples include the Andes, Cascades, and the Sierra Nevada.
• Continental volcanic arcs form in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent.
• Two oceanic slabs converge and one descends beneath the other.
Oceanic-Oceanic • This kind of boundary
often forms volcanoes on the ocean floor.
• Volcanic island arcs form as volcanoes emerge from the sea.
• Examples include the Aleutian, Mariana, and Tonga islands.
• When subducting plates contain continental
material, two continents collide.
• This kind of boundary can produce new
mountain ranges, such as the Himalayas.
Transform fault boundary At a transform fault boundary, plates grind past
each other without destroying the lithosphere.
Transform faults • Most join two segments of a mid-ocean ridge.
• At the time of formation, they roughly parallel the direction of plate movement.
• They aid the movement of oceanic crustal material.
Collision of India and Asia
Boundary Interactions Animations Himalaya Formation
Himalaya Formation #2