Download Plate Tectonics

What a strange trip it’s been…

Earth’s lithosphere is broken into plates (15)
that move on the asthenosphere

Some move towards, some away, some slide
past one another

Plate tectonics is the theory that describes the
formation, movement, and interaction of plates

CONTINENTAL MOVEMENT

Theory developed from observations about the
shapes of continents, fossil, and climate
evidence

Africa and South America look like they fit
together at one point in time

Coincidence!?!?!?
German scientist Alfred Wegener proposed
his hypothesis of continental drift
 He stated that continents move, or drift from
one location to another over time


Continents fit together like a puzzle

Fossils of Mesosaurus were found in Africa
and South America 270 million years old

Distinctive rock formations lined up on each
continent

They must have been together!!!
 Broke
up 200 million years ago
 Wegener knew that they must have
been together in the past

His hypothesis didn’t explain HOW the
continents moved

He knew the continents moved in the past and
are moving in the present, but had no
mechanism behind it

Wegener defended it, but did not succeed

In the 1950s and 1960s, studies showed that
Wegener’s observations were correct

Continents are embedded in lithospheric
plates

As the plates move, they carry the continents
with them

The theory explains why earthquakes and
volcanoes are likely to occur in particular
locations

Explains how new crust is created along the
ocean floor at the Mid-Ocean Ridge

Data indicates that Earthquakes and volcanic
activity do not occur randomly throughout the
world

Instead, they occur primarily in concentrated
belts around the world

The belts mark the location of the plate
boundaries, or where one plate meets another
RED CIRCLES = EARTHQUAKES AND
VOLCANOES
• The continental plate boundaries match up
perfectly with earthquake and volcanic
http://www.emscepisodes
csem.org/Earthquake/Map/jsmap.php?start
_date=2011-07-20&end_date=2011-09-27

Boundaries are places where two plates are
pushing toward, pulling away, or sliding past
one another

Strain builds up along plate boundaries, then
BOOM: Earthquake

Molten rock moves upward to the Earth’s
surface causing volcanic activity

Proposed seafloor spreading in the 1960s after
Wegener, supporting his hypothesis

The ocean floor is literally spreading out,
carrying the continents with it along ridges

Due to convection cells within the mantle

Proved this by magnetic polarity models
Some igneous rocks have magnetic minerals
in them
 In the past, these minerals would line up with
wherever the magnetic pole was
 Shifted between normal and reverse polarity
over time


Make a distinct pattern on the ocean floor that
Hess observed
Black = normal polarity
Grey = reverse polarity
 Alternating
bands
of normal and
reverse polarity

Geologists studied the magnetic record on
both sides of the mid-ocean ridge

A mid-ocean ridge is a long chain of volcanic
mountains on the ocean floor with a deep
central valley

When geologists figured out the ages of the
rocks on either side of the mid-ocean ridge,
they found that:
1.) The rocks at the center of a mid-ocean ridge
are the youngest
2.) The rocks furthest from the center of a midocean ridge are the oldest
New rock is formed by hot magma rising
up between the spreading plates
 Oozes up, and piles on both sides of the
ridge
 Convection currents keep it spreading
apart away from itself
 As new rock forms, the older rock
spreads away from the center on either
side

Red = youngest rock
Green = oldest rock
1.) List two pieces of evidence that
Wegener used to support his hypothesis
of continental drift
2.) True/False: Earthquakes and
volcanoes occur along plate boundaries
3.) What happens to the age of the rock
at the Mid-Ocean Ridge as it spreads
out from the center?
 Scientists
classify boundaries
between two plates according to plate
movement
 Three
main types:
1.) Divergent
2.) Convergent
3.) Transform

A boundary between two lithospheric
plates that are moving apart

Diverge = spread apart, separate

Divergent boundaries have rift valleys
associated with them

Most are on the ocean floor

Best example:

Rift valleys are at the center of the divergent
plate boundary
Mid-Ocean Ridge

In Iceland, the mid ocean ridge is exposed
 Features
associated:
1.) Volcanos
2.) Earthquakes
3.) Magma
4.) Black smokers
5.) LIFE! LIFE!

A boundary between two plates that are
moving toward each other

Convergence = toward, come together

Two classes of convergence:
1.) Subduction
2.) Collision
 When
an oceanic plate plunges
beneath another plate, it is said to be
subducting beneath the overriding
plate
 The
boundary between is called a
subduction boundary
Denser plate subducts or goes under the other
Long, deep sea trenches are at the
deepest parts of the ocean floor
 Example: Marianas trench


Over 9 km deep!!!
Subducted
rock melts,
rises, and
forms
volcanoes
on the
surface of
the other
plate
Subducted
rock melts,
rises, and
forms
volcanoes
on the
surface of
the land
 If
two plates are
carrying continents,
they will collide together
creating mountains
 Most
famous location:
Himalayan Mountains in
India
CONTINENTAL MOVEMENT
Continental-Continental: mountains
 Oceanic-Continental: subduction zones,
volcanoes along coast, deep trenches
 Oceanic-Oceanic: subduction zones,
volcanic island arcs, deep trenches

ALL ARE CONVERGING TOGETHER
WITH DIFFERENT RESULTS
Boundary between two plates that are
sliding past each other
 Located at mid-ocean ridges, and most
famously, California at the San Andreas
Fault

Pacific plate = Northwest
 North American plate = Southeast

Shallow Earthquakes
 Displacement of land


Mantle convection

Magma that rises at the mid-ocean ridge
is hot, less dense, and expands

This causes the rock to rise creating a
slope

More dense rock slides away from the
center, where new crust is created
At a subduction boundary, one plate is denser
and heavier than the other
 As it sinks, it pulls the rest of the plate with it
 The force that the sinking plate exerts on the
rest is called slab pull


Similar to keys in your coat pocket on a table

What are the three types of plate
boundaries?

What does subduction mean? Which
plate subducts under, more or less
dense?

What are the three crustal types of
convergent boundaries?