The Theory of Tectonic
Plates
Chapter 7 Section 3
Notes are the underlined items
and you should write them down
to study.
Tectonic Plates—the lithospheric plates
The Theory of Plate Tectonics

The Earth’s lithosphere (crust and upper
mantle) is divided into tectonic plates.

These plates move on top of the
asthenosphere (upper mantle).

Plate tectonic theory explains how the
Earth’s crust is divided into plates & they
move around on the asthenosphere.
Why do the plates move?

We know that these plates move in
different ways:

A. ridge push

B. slab pull

C. convection
Causes of Plate Movement Described

A. Ridge push: plates are pushed away from the
mid-ocean ridge

B. Slab pull: a tectonic plate is sucked down &
under another plate

C. Convection: hot magma deep from within the
Earth rises, cools, then sinks.
– The number 1 reason for plate movement.
– This movement of magma causes plates to move.
Reasons for plate movement ( see p. 204 in
textbook)
Ridge push occurring
at the mid-ocean ridge
Subduction
zone
Subduction
zone
Tectonic Plate Boundaries

A boundary is a place where tectonic
plates touch.

There are 3 types of tectonic plate
boundaries:
– Convergent (together)
– Divergent (apart)
– Transform (sliding past one another)
The numbers on the map represent the rate at which plates are moving in
centimeters.
Convergent Boundary

When two plates collide, or move toward each
other, the boundary is called convergent.

There are 3 types of convergent boundaries,
depending on what type of crust comes
together:
– Continental-continental
– Continental-oceanic
– Oceanic-oceanic
Convergent Boundary: Continental-Continental

The two continental plates come together and either:
– Buckle, thicken, or push continental crust upward,
making mountains on land or under sea (ocean ridges)
Convergent Boundary: Continental-Oceanic

Dense oceanic crust sinks below the continental crust & sinks into
the asthenosphere—this is a subduction zone.
– Sub means below (submarine, subway).

Old ocean crust is pushed below the asthenosphere at the
subduction zones, getting melted & thus recycled.
Convergent Boundary: Oceanic-Oceanic

One oceanic plate sinks below the other, creating a
subduction zone, where the plate is melted & recycled
into molten magma.
Divergent Boundary

Two tectonic plates move away from each
other:
– Either ocean & ocean plates or continental &
continental plates.

Sea floor spreading occurs when a divergent
boundary occurs at a mid-ocean ridge & is
where new crust is made.

As the plates move apart, magma rises to fill
the gaps.

The magma is cooled and forms new ocean
Divergent Boundary
The volcanic country of
Iceland, which straddles
the Mid-Atlantic Ridge,
offers scientists a natural
laboratory for studying on
land the processes also
occurring along the
submerged parts of a
spreading ridge.
Iceland is splitting along
the spreading center
between the North
American and Eurasian
Plates, as North America
moves westward relative
to Eurasia.
Divergent Boundary
On-land exposure of the
Mid-Atlantic Ridge in Iceland.
Sea Floor Spreading
Transform Boundary

Two tectonic plates move
past one another.
– Can be an ocean & an ocean
plate or a continental & a
continental plate

Their edges are jagged
and irregular.

As they grind together and
jerk, they slide past each
other creating tsunamis &
earthquakes.
Tracking Tectonic Plate Movement

Plates move very slowly—only a few
centimeters a year.
– N. Am. Plate moves 4 cm each yr away
from African plate.

Calculate the number of years that it took
New York and the west coast of Africa to
reach their current locations, 676,000,000
cm apart, if the sea floor is spreading an
average of 4 cm a year.
Tracking Plate Movement

GPS, Global Positioning System, measures
the rate of movement.
– Radio signals beam continuously from
satellites to the Earth.
– By recording the time it takes for the ground
station to move a given distance, scientists
can measure the speed at which plates move.
Make a chart that identifies the type of
boundaries, the type of plates that can be at
each, and what results at these boundaries.
Boundary
Type of Plate
Results
Transform
Ocean/ocean or
continent/continent
Earthquakes &
tsunamis
Divergent
Convergent
What your chart should look like completed:
Boundary Type of Plate
Results
Transform
Ocean/ocean or
continent/continent
Earthquakes &
tsunamis
Divergent
Ocean/ocean
Makes new crust @
mid-ocean ridges
continent/continent
faults; earthquakes
Continent/continent
Makes mountains;
ocean ridges (under
water mtns)
Continent/ocean or
ocean/ocean
Makes subduction
zones which recycle
Earth’s crust
Convergent
Practice Naming Boundaries

Refer back to Figure 4 on page 194 to locate
examples of different types of tectonic plate boundaries.

Identify the types of boundaries found on p. 194:
– Locate at least one type of convergent boundary:
 Continental/continental collide, continental/oceanic collide, or
oceanic/oceanic collide
– Locate at least one divergent boundary: where ocean
ridges move apart
– Locate at least one transform boundary: where two plates slide
past each other