PLATE TECTONICS
Internal Structure of the Earth
A. Earth's Crust
1. Oceanic
6-12km thick. Dense heavy rock. Younger than continental crust
2. Continental
40-60kn thick. Light and less dense than oceanic
B. Plates
Float on the magma in the mantle beneath them because of convection (heat) currents in the
mantle. This movement causes earthquakes volcanoes and fold mountains along plate
boundaries
C. Diagram p.2 and 3
Plate Tectonics
Theory that the plates move (collide/seperate/slide past each other) and this movement causes
earthquakes, volcanoes and fold mountains.
A. Continental Drift
Theory proposed by Alfred Wegener in 1912.
Based on three facts:
(a) That identical fossils are found in different parts of the world
(b) Matching mountain ranges and rock types on different continents
(c) Matching coastline shape e.g. Africa and South America
B. Sea-floor spreading
Proposed by Harry Hess who saw volcanoes erupting from the sea floor and knew that new
land was being formed and that the sea floor was spreading.
Proofs:
(a) The sea floor is youngest at a mid-ocean ridge and older as you move further away
(b) Magnetisms locked into rock match at each side of a mid-ocean ridge proving they must
have been together at some stage
(c) Young volcanic islands are found near a mid-ocean ridge and older ones are found further
away.
Plate Boundaries
A. Destructive Plate Boundary
Two plates collide and land is destroyed
1. oceanic-oceanic plates collide
When one oceanic plates slides under another deep trenches are formed.
A line of volcanoes called an island arc marks the point where two oceanic plates meet.
These volcanoes are caused by sinking plates melting in the mantle and the pressure forcing
magma out of the crust.
Diagram p.8
2. oceanic-continental plates collide
Heavy oceanic crust slides under lighter continental crust and melts in the mantle. This is
called subduction.
The extra magma puts so much pressure on the continental crust above it that it bursts out in
some of the most explosive volcanoes in the world e.g. Mount St. Helens USA.
At the point where two plates meet a trench is formed. Earthquakes are common in these
areas.
Diagram p.7
3. continental-continental plates collide
When two continental plates collide rock buckles and forms fold mountains.
These mountains are often made of sedimentary rock.
Earthquakes can happen when continental plates collide.
E.g. of fold mountains are the Himalayas (Indian plate and Eurasian Plate) and the Alps.
Diagram p.9
B. Constructive Plate Boundary
When plates move apart magma comes up to fill the space between them and forms new land.
When this happens under the ocean it forms a mid-ocean ridge e.g. Mid-Atlantic ridge.
Diagram p.10
C. Passive
Land is neither created nor destroyed.
A fault line is formed and shallow earthquakes occur.
Most fault lines are under the sea but some are on land e.g. The San Andreas Fault in
California (Photo p.13).
Diagram p.12