# Plate Tectonics - St John Brebeuf

Mechanics

Nov 14, 2013 (4 years and 6 months ago)

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Plate

Tectonics

The Earth is a restless planet. The ground beneath your feet is
constantly moving.

Great slabs of rock called plates make up the Earth’s crust.

Early 20th century geologist Alfred
Wegener realized that the puzzle
-
like fit of the continents were
more than a coincidence, but he
couldn't correctly explain what
powered their movement
.

Geologists now know that the
Earth's outermost layer,
the
lithosphere,
is divided into
independently moving plates into
which the continents are
embedded. The plates "float" on a
layer called the
asthenosphere
.

Plate tectonics

is an important theory
that explains the movement of
the Earth’s lithosphere (surface).

Plate tectonic theory explains how the lighter, solid plates that form the Earth’s
crust (surface) float on denser material just below the crust.

Types of plate movement we will be looking at in further detail:

Divergent

Convergent

Transform

Looking at the Earth’s layers will help us understand how
the Earth’s plates move.

The
Lithosphere can be divided up into
layers

The lithosphere is solid
rock.

Beneath
it is a softer, hotter layer of solid rock called the
asthenosphere
that
extends down to around 220 kilometers depth.

T
emperatures of the asthenosphere are extremely hot, and as a
result, the
rock
is
weak ("
astheno
-
" means weak in
Greek
).

It will be effected by stress and
bends in a plastic
way.

Tectonic plates are able
to move because the
Earth's lithosphere has a
higher strength and
lower density than the
underlying
asthenosphere.

The
lateral relative
movement of the plates
typically varies from
zero to 100

mm
annually
.

Where plates meet, their
relative motion
determines the type of
boundary:

convergent

Divergent

transform
.

Earthquakes, volcanic
activity, mountain
-
building, and oceanic
trench formation occur
along these plate
boundaries.

Convergent Boundaries

2 plates are running into each
other

Where
plates serving landmasses collide, the crust crumples and buckles into
mountain ranges. India and Asia crashed about 55 million years ago, slowly giving
rise to the Himalaya, the highest mountain system on
Earth.

As
the mash
-
up continues, the mountains get higher.

C
onvergent
boundaries also occur where
an ocean plate and
continental plate meet. This process is called
subduction
.

Ocean plate density > Continental plate.

Thus dives beneath the continental plate, creating an ocean trench.

As
the overlying plate lifts up, it also forms mountain ranges.

In
addition, the diving plate melts and is often spewed out in volcanic
eruptions such as those that formed some of the mountains in the
Andes of South America.

Two ocean plates can collide as well. One plate will have a slight
density difference and
subduct

.

This creates an ocean trench and can create chains of composite
volcanoes. Example: Philippines.

Divergent Boundaries

Two plates are moving away from each other.

The process renews the ocean floor and widens the giant
basins
.

I
n
the oceans, magma from deep in the Earth's mantle rises
toward the surface and pushes apart two or more plates.
Mountains and volcanoes rise along the seam.

On land, giant troughs such as the Great Rift Valley in Africa
form where plates are tugged apart.

If the plates there continue to diverge, millions of years from
now eastern Africa will split from the continent to form a new
landmass. A mid
-
ocean ridge would then mark the boundary
between the plates.

Transform Boundaries

T
wo
plates grind past each other along what are called strike
-
slip faults.

Example: San Andreas Fault in California

These boundaries don't produce spectacular features like mountains or
oceans, but the halting motion often triggers large earthquakes, such as the
1906 one that devastated San Francisco.