Download The theory of plate tectonics

The theory of plate tectonics
The outer layer of the Earth’s crust – the Lithosphere is not a continuous
structure but is split a little like the cracked shell on a hard boiled egg:
The result is a series of plates floating on the hot mantle beneath. Heat in
the mantle creates convection currents that cause the plates to move.
Plate margins exist where the plates meet.
The history of the Earth’s plates – see also
http://tanasi.gg.utk.edu/courses/101/public/BBC/default.html
The beginning of time
Map – plates and continents of the World – today.
Plate Margins – where plates meet
Earthquakes and volcanoes (evidence of unrest in the Earth), help locate the
edges of plates known as plate margins or boundaries.
Take for example the ‘Ring of Fire’ around the Pacific. Volcano’s can be
joined together using a ‘dot to dot’ technique. The pattern that follows
exactly matches known plate margins.
Different types of plate margin and how they contribute to the
formation and deformation of rocks.
As plates move; they can push together or move apart.
1.
ridge.
Moving apart – CONSTRUCTIVE, e.g. Atlantic mid-ocean
The floor of the Atlantic Ocean is widening as plates are moving apart. As
magma gradually rises and cools between plates new rock called igneous rock
is formed.
Formation and uses of Igneous Rocks
The word igneous means fire. Igneous rocks are formed directly from magma as it cools.
The type of igneous rock depends upon the speed at which magma cools.
Slow cooling – intrusive igneous rock
Igneous rocks such as granite are formed when magma finds weaknesses and intrudes into the crust.
As it does so it cools and solidifies slowly below the Earth’s surface. This process often occurs during
plate tectonic activity; either when plates push together or move apart. Granite only appears when
rock above has been worn away; it is younger than the rock that surrounds it. Igneous rocks such as
granite can take hundreds of years to form. Granite is a very hard rock used in building and road
making.
Fast cooling – extrusive igneous rock
Igneous rocks such as basalt or obsidian are formed when magma erupts from a volcano as lava and
cools on reaching the surface. Basalt cools and solidifies quickly over a number of days.
Appearance of Igneous Rocks

This is the intrusive igneous rock
GRANITE.

Granite take hundreds of years to form

It is made up of RANDOM,
INTERLOCKING crystals.

There are no bands, swirls or fossils
present
Intrusive igneous rocks

This is the extrusive igneous rock
OBSIDIAN

Obsidian takes days/weeks to form

It contains small/no crystals

There are no bands, swirls or fossils
present

intrusive igneous rocks such as
pumice contain lots of air pockets
2. Pushing together
Extrusive igneous rocks
- destructive e.g the Andes and Himalayas
In the above diagram the continental crust is pushing harder than the
lighter oceanic crust. This is similar to the plate margin responsible for
the Asian tsunami.
On the left of the diagram the oceanic crust or plate is being forced
under the continental plate. This is called subduction. Rock at the
bottom of the oceanic plate is re-melted. This is called deformation.
Meanwhile as the continental crust is forced upwards, magma also moves
upwards and cools forming new rock.
This is called uplift.
Mountains/igneous rock are formed in this way.
The Rock Cycle
Together deformation and mountain formation form part of the rock
cycle. During this cycle rock is constantly being created and destroyed.
Wegener and the theory of plate tectonics.
Utter, damned rot!" said the president of the prestigious American Philosophical Society.
"If we are to believe this hypothesis, we must forget everything we have learned in the
last 70 years and start all over again," said another American scientist.
Anyone who "valued his reputation for scientific sanity" would never dare support such a
theory, said a British geologist.
All this was said of Alfred Wegener’s theory
in 1915 that the crust was not one continuous land mass but was
divided into plates. He also proposed that the Earth’s continents
had once all been part of one large super continent which he
called Pangaea.
Wegener based this on several pieces of
evidence;
1. Wegener noticed that despite being separated by an ocean,
South America and South Africa look as though they could
fit together like pieces of a jigsaw puzzle.
2. Fossils of plants and animals from different sides of the
Atlantic were the same.
3. Rock strata in North America and the Scottish Highlands
appeared to match.
Wegener’s theory that continents making up Pangaea had
drifted wasn’t accepted until two Cambridge University
scientists found further evidence in the 1960’s from looking at
magnetic field patterns at the bottom of the Atlantic Ocean.
Wegener died at 50 on a rescue exhibition to Greenland.
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