Key terms:
Asthenosphere - Part of the Earth's mantle that lies below the lithosphere, at depths between about
100 and 700 kilometres,
Conservative plate margin - A plate margin where two tectonic plates are moving past one another
with no addition or destruction or plate material
Constructive plate margin - A plate margin where new crust is generated as the plates pull away from
each other. These are found at mid-oceanic ridges.
Destructive plate boundary - A plate margin where crust is destroyed as two plates converge. These
are usually associated with island arcs or young fold mountains
Palaeomagnetism - A record of the history of the Earth's magnetic field, preserved in magnetic
minerals in volcanic rocks.
Plate tectonic theory - The theory that states that the Earth's crust is made up of several rigid plates
moving
Plume - A hot column of magma which rises from deep within the Earth.
Rift valley - A long, deep valley found in the centre of a spreading ridge. It is formed between parallel
faults where a block of the crust has sunk down.
Sea-floor spreading - The theory that the ocean floor is moving away from the mid oceanic ridge and
across the deep ocean basin, to disappear beneath continents and island arcs.
Tectonic plate - One of a series of rigid sections of the Earth's crust. They float on the upper mantle
and move
Earth's structure and internal energy sources
The core
The core is made up of dense rocks containing iron and nickel alloys. It is subdivided into a solid inner
core and a very hot molten outer one, with a temperature of over 5,000 °C. This heat is produced
mainly as the result of two processes: primordial heat left over from the Earth's formation and
radiogenic heat produced by the decay of radioactive isotopes, particularly Uranium-238, Thorium-232
and Potassium-40.
The mantle
The mantle, the thickest layer, is made of molten and semi-molten rock rich in iron and magnesium
(suchas peridotite). At a depth of between approximately 700 km and 100 km, the upper part of the
mantle becomes hotter and more fluid. This is called the asthenosphere. Above the asthenosphere
the mantle becomes solid.
The crust
The outermost layer is called the crust. There are two types of crust; a relatively thin layer of dense
basalt is found under the oceans and a thicker layer of less dense granitic rock makes up the
continents.
Different types of crust:
continental crust oceanic crust
, thickness 30-70km 6-10km
age Over 1,500 million years Less than 200 million years
density 2.6 (lighter) 3.0 (heavier)
composition Mainly granite, silicon, aluminium and Mainly basalt, silicon,magnesium,
oxygen oxygen
Plate tectonic theory:
Timeline
● Plate tectonics theory grew out of a theory that was first developed in the early 20th
century by the meteorologist Alfred Wegener.
● In 1912 he noticed that the coastlines of the east coast of South America and the
west coast of Africa appeared to fit together like jigsaw pieces.
○ He believed that the continents gradually began to drift apart around 300
million years ago. This was his theory that became known as continental drift.
● In 1929 Arthur Holmes,came up with the theory of convection currents and he said
that as a substance is heated its density decreases and it rises until it cools
sufficiently to sink again. According to Holmes it was this heating and cooling cycle
within the Earth's mantle that caused the continents to move. This theory gained very
little attention at the time.
● PALOMAGENTISM When basaltic lava cools on the sea floor, individual minerals
separate - especially iron - and these minerals then align themselves on the sea floor
in the direction of the magnetic pole. New technologies allowed these rocks to be
dated and their pattern of movement mapped between their origin and sampling. The
maps suggested the migration of seafloor rocks.
● Earth’s magnetic field reverses periodically and it is possible to see an identical
pattern between rock formations on either side of the mid Atlantic ridge.
● The discovery in 1960 of the asthenosphere, a weak, deformable layer beneath the
rigid lithosphere, on which the latter moves.
● SEA FLOOR SPREADING - 1962 Harry Hess - dated rocks of Atlantic sea bed from
the Mid-Atlantic Ridge outwards to the coast of North America. He discovered that
the newest rocks were at the centre near Iceland, and the oldest at the coast. This
suggested the earth’s surface was splitting and expanding in certain places.
● The discovery in the 1960s of magnetic strips in the oceanic crust of the sea bed;
these are palaemagnetic signals from past reversals of the Earth's magnetic field and
prove that new ocean crust is created by the process of sea-floor spreading at
mid-ocean ridges. (seafloor spreading and palaeomagnetism occur at constructive
margins, where new crust is being made)
Continental drift theory
Asthenosphere - Part of the Earth's mantle that lies below the lithosphere, at depths between about
100 and 700 kilometres,
Conservative plate margin - A plate margin where two tectonic plates are moving past one another
with no addition or destruction or plate material
Constructive plate margin - A plate margin where new crust is generated as the plates pull away from
each other. These are found at mid-oceanic ridges.
Destructive plate boundary - A plate margin where crust is destroyed as two plates converge. These
are usually associated with island arcs or young fold mountains
Palaeomagnetism - A record of the history of the Earth's magnetic field, preserved in magnetic
minerals in volcanic rocks.
Plate tectonic theory - The theory that states that the Earth's crust is made up of several rigid plates
moving
Plume - A hot column of magma which rises from deep within the Earth.
Rift valley - A long, deep valley found in the centre of a spreading ridge. It is formed between parallel
faults where a block of the crust has sunk down.
Sea-floor spreading - The theory that the ocean floor is moving away from the mid oceanic ridge and
across the deep ocean basin, to disappear beneath continents and island arcs.
Tectonic plate - One of a series of rigid sections of the Earth's crust. They float on the upper mantle
and move
Earth's structure and internal energy sources
The core
The core is made up of dense rocks containing iron and nickel alloys. It is subdivided into a solid inner
core and a very hot molten outer one, with a temperature of over 5,000 °C. This heat is produced
mainly as the result of two processes: primordial heat left over from the Earth's formation and
radiogenic heat produced by the decay of radioactive isotopes, particularly Uranium-238, Thorium-232
and Potassium-40.
The mantle
The mantle, the thickest layer, is made of molten and semi-molten rock rich in iron and magnesium
(suchas peridotite). At a depth of between approximately 700 km and 100 km, the upper part of the
mantle becomes hotter and more fluid. This is called the asthenosphere. Above the asthenosphere
the mantle becomes solid.
The crust
The outermost layer is called the crust. There are two types of crust; a relatively thin layer of dense
basalt is found under the oceans and a thicker layer of less dense granitic rock makes up the
continents.
Different types of crust:
continental crust oceanic crust
, thickness 30-70km 6-10km
age Over 1,500 million years Less than 200 million years
density 2.6 (lighter) 3.0 (heavier)
composition Mainly granite, silicon, aluminium and Mainly basalt, silicon,magnesium,
oxygen oxygen
Plate tectonic theory:
Timeline
● Plate tectonics theory grew out of a theory that was first developed in the early 20th
century by the meteorologist Alfred Wegener.
● In 1912 he noticed that the coastlines of the east coast of South America and the
west coast of Africa appeared to fit together like jigsaw pieces.
○ He believed that the continents gradually began to drift apart around 300
million years ago. This was his theory that became known as continental drift.
● In 1929 Arthur Holmes,came up with the theory of convection currents and he said
that as a substance is heated its density decreases and it rises until it cools
sufficiently to sink again. According to Holmes it was this heating and cooling cycle
within the Earth's mantle that caused the continents to move. This theory gained very
little attention at the time.
● PALOMAGENTISM When basaltic lava cools on the sea floor, individual minerals
separate - especially iron - and these minerals then align themselves on the sea floor
in the direction of the magnetic pole. New technologies allowed these rocks to be
dated and their pattern of movement mapped between their origin and sampling. The
maps suggested the migration of seafloor rocks.
● Earth’s magnetic field reverses periodically and it is possible to see an identical
pattern between rock formations on either side of the mid Atlantic ridge.
● The discovery in 1960 of the asthenosphere, a weak, deformable layer beneath the
rigid lithosphere, on which the latter moves.
● SEA FLOOR SPREADING - 1962 Harry Hess - dated rocks of Atlantic sea bed from
the Mid-Atlantic Ridge outwards to the coast of North America. He discovered that
the newest rocks were at the centre near Iceland, and the oldest at the coast. This
suggested the earth’s surface was splitting and expanding in certain places.
● The discovery in the 1960s of magnetic strips in the oceanic crust of the sea bed;
these are palaemagnetic signals from past reversals of the Earth's magnetic field and
prove that new ocean crust is created by the process of sea-floor spreading at
mid-ocean ridges. (seafloor spreading and palaeomagnetism occur at constructive
margins, where new crust is being made)
Continental drift theory
