Plate Tectonics Reviewer

Plate Tectonics

Plate tectonics is a fundamental scientific theory explaining how large sections of Earth's
outermost layer, called tectonic plates, move. These movements are responsible for major
geological phenomena like earthquakes, volcanic eruptions, and the formation of
mountain ranges.

Four Main Layers of the Earth (Based on Chemical Composition)

1. Crust: This is Earth's outermost and thinnest layer.

o Continental Crust: Thicker and less dense, primarily composed of granite. It
forms the continents.
o Oceanic Crust: Thinner and denser, made up of basalt. It forms the ocean
floors.
2. Mantle: The thickest layer, located beneath the crust. It consists of semi-solid rock
that moves slowly due to convection currents (heat-driven circulation).
3. Outer Core: A liquid layer composed mainly of iron and nickel. The movement of
this liquid metal generates Earth's magnetic field.
4. Inner Core: The Earth's innermost and hottest layer. Despite extreme temperatures, it
remains solid due to immense pressure. It's also primarily composed of iron and
nickel.

Layers Based on Physical Properties

Earth's layers can also be classified by their physical characteristics:

1. Lithosphere: Comprises the crust and the uppermost solid part of the mantle. It's
the rigid, brittle outer shell of Earth, broken into large pieces called tectonic plates.
2. Asthenosphere: Located beneath the lithosphere, this part of the upper mantle is
semi-fluid or "plastic-like" due to heat and pressure. It's where the convection currents
that drive plate movement occur.
3. Mesosphere (Lower Mantle): The more solid part of the mantle below the
asthenosphere. Although solid, it still flows very slowly.
4. Outer Core: (As mentioned above) Liquid iron and nickel.
5. Inner Core: (As mentioned above) Solid iron and nickel.
Continental Drift Theory

The Continental Drift Theory was proposed by Alfred Wegener in 1912. It suggests that
Earth's continents were not always in their current positions. Instead, they were once part of a
single supercontinent called Pangaea (meaning "all lands"). Over millions of years, Pangaea
gradually broke apart, and the continents drifted to their present locations.

Wegener's Evidence:

 Fit of the Continents: The coastlines of continents, especially the east coast of South
America and the west coast of Africa, appear to fit together like puzzle pieces.
 Fossil Evidence: Discovery of identical fossils (like Mesosaurus, Lystrosaurus, and
Glossopteris flora) on widely separated continents now separated by oceans.
 Rock and Mountain Formations: Matching rock types and mountain ranges (e.g.,
Appalachian Mountains in North America and Caledonian Mountains in Europe)
across different continents.
 Paleoclimate Evidence: Evidence of ancient climates, such as glacial deposits in
modern-day warm regions (e.g., India and Australia), and tropical coal deposits in
cold regions.

Despite strong evidence, Wegener couldn't explain the mechanism driving continental
movement, leading to initial skepticism. This mechanism was later explained by the theory of
Plate Tectonics.

Plate Boundaries and Their Types

Plate boundaries are the zones where two tectonic plates meet. Most geological activity
occurs along these boundaries.

1. Divergent Plate Boundary (Moving Apart):

o Movement: Two plates move away from each other.
o Cause: Hot magma rises from the mantle, pushing the plates apart.
o Results:
 Mid-Ocean Ridges: Underwater mountain ranges where new oceanic
crust is formed (e.g., Mid-Atlantic Ridge).
 Rift Valleys: Large valleys formed on land (e.g., East African Rift
Valley).
 Volcanism: Frequent volcanic eruptions.
 Earthquakes: Shallow earthquakes.
o Example: North American Plate and Eurasian Plate at the Mid-Atlantic
Ridge.
2. Convergent Plate Boundary (Moving Together):
o Movement: Two plates collide. There are three sub-types based on the plates
involved:
 Oceanic-Oceanic Convergence:
 Movement: One oceanic plate subducts (sinks) beneath
another oceanic plate. The denser plate subducts.
 Results:
  Oceanic Trenches: Deep underwater troughs (e.g.,
Mariana Trench).
 Volcanic Island Arcs: A chain of volcanic islands
formed above the subducting plate (e.g., Philippine
Islands, Japan).
 Strong Earthquakes: Frequent and deep earthquakes.
 Oceanic-Continental Convergence:
 Movement: The denser oceanic plate subducts beneath the
lighter continental plate.
 Results:
 Oceanic Trenches: Along the coast of the continent
(e.g., Peru-Chile Trench).
 Volcanic Mountain Ranges (Volcanic Arcs):
Mountain ranges with volcanoes along the continent's
edge (e.g., Andes Mountains in South America,
Cascade Range in North America).
 Strong Earthquakes: Frequent and deep earthquakes.
 Continental-Continental Convergence:
 Movement: Two continental plates collide. Since both are
relatively light and not very dense, neither subducts
significantly.
 Results:
 Fold Mountains: Formation of high mountain ranges
due to the crumpling and uplifting of the crust (e.g.,
Himalayan Mountains – collision of the Indian and
Eurasian plates).
 Strong Earthquakes: But infrequent volcanism.
3. Transform Plate Boundary (Sliding Past Each Other):
o Movement: Two plates slide horizontally past each other. No crust is created
or destroyed.
o Results:
 Fault Lines: Large fractures in the Earth's crust.
 Strong Earthquakes: This is the primary activity at this boundary,
due to immense stress build-up. Volcanism is rare.
o Example: San Andreas Fault in California, USA.

Seismic Waves

Seismic waves are energy waves generated and propagated through Earth's interior (and
along its surface) during an earthquake or explosion. The study of these waves, known as
Seismology, is crucial for understanding Earth's internal structure.

Main Types of Seismic Waves:

1. Body Waves: Waves that travel through Earth's interior.

o P-waves (Primary Waves):
 Fastest seismic wave.
 Compressional waves: They move by compressing and expanding the
material (like sound waves).
 Can travel through solids, liquids, and gases.
 They are the first to arrive at a seismic station.
o S-waves (Secondary Waves):
 Slower than P-waves.
 Shear waves: They move by shaking the material perpendicular to the
direction of wave propagation (like shaking a rope).
 Cannot travel through liquids or gases. This is key evidence that
Earth's Outer Core is liquid, as S-waves disappear in that region.
2. Surface Waves: Waves that travel only along Earth's surface. They are slower than
body waves but cause the most damage during an earthquake.
o Love Waves: Move horizontally, causing a "side-to-side" ground motion.
o Rayleigh Waves: Move in a rolling motion (like ocean waves), causing "up-
and-down" and "back-and-forth" ground motion.

The differences in speed and the ability of P-waves and S-waves to travel through different
materials have provided scientists with vital information about the composition and physical
properties of Earth's layers.