Download Plate Tectonics Unit - Spring

PLATE TECTONICS
 Tectonic Plate = huge pieces of Earth’s crust that
cover its surface and fit together at their edges.
 Plate Boundaries = the places where tectonic plates
fit together (at the edges).
 There are three types of boundaries that are named
for how the plates interact with one another.
PLATE BOUNDARIES
 DIVERGENT = DIVIDE
 place where two tectonic plates are moving apart
(or away from each other)
 This process is known as seafloor spreading
 they produce volcanoes, earthquakes, and lots of heat
 almost always found on the ocean floor; are rarely found on a
continent
 In the ocean, on the ocean floor, this forms mid-ocean ridges.
PLATE BOUNDARIES
Mid-Ocean Ridge =
 long & wide mountain chains formed by divergent
volcanism
 are very tall, but they start on the ocean floor, so they
are mostly under water
 they can get so tall they come above the surface of the
ocean’s waters
PLATE BOUNDARIES
 On a continent, this forms rift valleys.
Rift Valley
 long, narrow depression that forms when crust
begins to separate on a continent
 will eventually (over millions of years) form a new
ocean floor
PLATE TECTONICS
 How were divergent boundaries discovered?
 In the 1960s, new technologies began to be used to study the
ocean floor. What they were used to discover new features on
the ocean floor
 This would eventually lead us to the discovery of seafloor spreading
1. Ocean floor topography was mapped using SONAR = Sound Navigation
and Ranging = uses sound waves to determine depth.
2. SONAR revealed underwater mountain chains (the mid-ocean ridges)
and deep depressions called deep sea trenches on the ocean floor; they
run along the Earth like seams on a baseball.
3. It was also discovered that the thickness of the ocean-floor sediments
increases with distance from the mid-ocean ridges
PLATE TECTONICS
 Another new technology was the magnetometer. It measures
and records magnetic fields/patterns.
 This would also lead us to the discovery of seafloor spreading
 Magnetic patterns on the seafloor showed:
1. Earth’s magnetic field reverses every few thousand years.
1. Iron rich minerals align with the Earth’s magnetic field as
new rock is formed.
PLATE TECTONICS
 A symmetrical pattern of the magnetic fields is
created as the crust moves out and away from the
mid-ocean ridge.
 These magnetic field patterns were used to figure
out the age of the ocean floor in different areas.
 Ocean crust is young right at the middle of the mid ocean ridge, and older farther away.
PLATE TECTONICS
 How does seafloor spreading work?
 New ocean crust is formed at mid-ocean ridges when magma
rises and fills in the gaps when plates separate; then the
magma solidifies into rock and new seafloor is created.
 Over time, that crust is pushed to the sides, and the oldest
ocean crust is destroyed at deep-sea trenches.
PLATE TECTONICS
 CONVERGENT = COLLIDE
 place where two tectonic plates are moving towards
each other
 Subduction – when 2 plates collide, the denser plate
descends (or sinks down) below the other, less -dense
plate
 This does not occur at all convergent
boundaries
PLATE TECTONICS
1) Oceanic – Oceanic
 two ocean plates (both have high density) collide
 resulting in subduction of the denser plate
 forms a deep-sea/ocean trench on the side that
subducts, and volcanic islands on the other plate
PLATE TECTONICS
2) Oceanic – Continental
 one ocean plate and one continental plate collide (the
continental plate has low density and the ocean plate has
high density)
 resulting in subduction of the ocean plate (it’s more dense!)
 forms a deep-sea trench on the ocean side, and volcanic
mountain ranges on the continental side
PLATE TECTONICS
3) Continental – Continental
 two continental plates (both have low density) collide
 forms tall, folded mountain ranges
PLATE TECTONICS
 TRANSFORM = SLIDE
 place where two
tectonic plates slide
past each other
 forms faults with
shallow earthquakes
PLATE TECTONICS
 Why do plates move?
 RECALL these parts of the Earth:
 Lithosphere -the solid, rigid outermost part of the Earth (crust
+ uppermost mantle).
 Asthenosphere - the partially melted layer of the mantle that
is just underneath the lithosphere.
 The composition of oceanic lithosphere is basalt (heavy,
dense), while the continental portion of the lithosphere is
granite (light, less-dense).
PLATE TE
PLATE TECTONICS
 So, Why Do the Plates Move?
 There is a flowing movement in the asthenosphere called
CONVECTION CURRENTS.
 Convection is a circulatory motion that occurs in fluids that
are not the same temperature throughout. Hot and Cool areas
create dif ferences in density.
 Convection currents occur when heated material expands &
rises (because it is less dense) but then sinks when it cools &
contracts (because it is more dense).
HEATING = DECREASE IN DENSIT Y → FLOATS/RISES
COOLING = INCREASE IN DENSIT Y → SINKS
Plate
Tectonics
PLATE TECTONICS
 This movement is VERY SLOW.
 The movement of the tectonic plates happens because of the
convection currents in the asthenosphere. The plates are
moved by the moving material below.
 The rising mantle material spreads out at the top and forces
the plates apart. (divergent, mid-ocean ridge)
 As the crust moves far away from the divergent boundary, it
cools and sinks back down into the mantle. (convergent,
subduction)
PLATE TECTONICS
Early Observations
 Late 1500’s, Abraham Ortelius, a Dutch cartographer noticed
and described the apparent fit of the continents .
 From 1500-1800, many scientists and writers made similar
observations.
PLATE TECTONICS
 In 1912, a scientist named Alfred Wegener was the first to
propose a hypothesis called Continental Drift to the scientific
community.
 Continental Drift says: The Earth’s continents were once
joined as a single landmass that broke apart and sent the
continents adrift.
 Wegener called the supercontinent Pangaea
(Greek word meaning All the Ear th)
PLATE TECTONICS
 Pangaea began to break
apart 200 MYA (million
years ago)
 Continents have been
slowing moving since
then, to the positions
they are in now.
 Wegener is important
because he was the first
to have more evidence
than just the fit of the
continents.
PLATE TECTONICS
 Rock Formations – Layers of rock in the Appalachian
Mountains (U.S.) are identical to layers of rock in
similar mountains in Greenland and Europe.

 Fossils – Identical land-dwelling plant and animal
fossils were found on widely separated continents.
 Glossopteris (plant)
 Cynognathus and Lystrasaurus
 Mesosaurus (freshwater reptile)
 Fossils were older than 200 MY
PLATE TECTONICS
Mesosaurus
PLATE TECTONICS
 Climate
 Glossopteris (fossil) – seed fern (a plant) that grows in temperate
climates. Fossils were found very wide-spread and in places that are
not temperate today.
 Coal Deposits – were found in Antarctica; coal forms from the
compaction and decomposition of ancient swamp plants (that grow in
tropical climates). Wegener said that Antarctica must have been
closer to the equator in the geologic past.
 Glacial Deposits – were found in Africa, India, Australia, and South
America (all places that today are much too warm for thick ice to
accumulate).
PLATE TECTONICS
PLATE TECTONICS
 During Wegener’s lifetime, his hypothesi s was not accepted. Although he
had lots of suppor ting data and evidence, scientists of the time rejected
the hypothesi s because of the following:
 Many people in the scientific community considered the continents and
ocean basins to be fixed features.
 Hypothesis did not explain what kind of force could move such large
masses over such great distances.
 Hypothesis did not explain how the continents were moving. How could
continents move through solid ocean floor? (They did know the ocean floor
is made of rock!)
 There were two problematic, unanswered questions:
What force could move the continents?
How could continents move through solid rock?
PLATE TECTONICS
 When Seafloor Spreading was discovered in 1960 by Harry
Hess, he and other scientists started to answer those
questions.
 SONAR was used to discover the features that form at the
boundaries.
 Magnetometers were used to discover new, young ocean crust
at mid-ocean ridges and old crust at deep -sea trenches.
 As scientists learned more, and collected more evidence, the
ideas of Continental Drift and Seafloor Spreading became the
THEORY OF PLATE TECTONICS.