3. EARTH’S STRUCTURE
CRUST:
Two types
- Oceanic: (sima rock, silica and
magnesium) 6-10km think, more
dense
- Continental: granite (sial rock,
silica and aluminium) 20-70km
thick, can never sink, can never
be destroyed or created, less
dense
CORE:
Two types:
- Inner: a solid ball containing iron
and nickel
- Outer: semi-molten contains iron
and nickel
The core reaches 5000°C+ and is 4x
denser than the crust
As the earth rotates the liquid core
spins creating the magnetic field
Mantle:
Mostly made of silicate rocks
Part near core is rigid molten rock
Layer above this (Asthenosphere) is semi-molten
Top of mantle is rigid
2900km in thickness
Asthenosphere:
Semi-molten almost plastic-like rock
moves very slowly carrying the lithosphere
Lithosphere:
Consists of the crust and the upper mantle and varies
between 80-90km in thickness
Divided into 7main plates and a number of smaller ones
The plates are moving due to convection currents in
the asthenosphere. The places where plates meet are
called boundaries or plate margins
4. CONVECTION CURRENTS
The Tectonic Plates MOVE due to CONVECTION CURRENTS in the MANTLE
1) Radioactive decay of elements in the mantle/core, e.g. uranium generates lots of heat
2) Lower parts of the asthenosphere heat up and become less dense and slowly rise
3) As they move towards the top of the asthenosphere they cool down and become more dense, then slowly
sink
4) These circular movement of semi-molten rock are called convection currents
5) Convection currents in the asthenosphere create drag on the base of the tectonic plates and this
causes them to move
Tectonic plate Asthenosphere
Rising hot material
Sinking cool material
5. PLATE TECTONIC THEORY
The Theory Of Plate Tectonics started
with Alfred Wegener
The location of continents today is very
different
from what it was millions of years ago
it is believed that all continents were
joined to one land mass,
Pangaea, which later spilt into Laurasia
in the north
and Gondwanaland to the south.
Alfred Wegener put forward a theory
regarding their movement in 1912
with his theory of Continental Drift.
His theory was based on observations
such as:
6. GEOLOGICAL AND CLIMATOLOGICAL:
Coal is found in UK but needs warm, wet, humid conditions to
form.
Rock sequences in Northern Scotland match those in Eastern
Canada
Striations in Brazil match those in West Africa
7. BIOLOGICAL:
Mesosaurus fossils (land reptile) are found in Southern Africa and
South America
Fossilised remains of a plant which existed only when coal was being
formed located in India and Antartica
9. PALAEOMAGNETISM AND SEA FLOOR
SPREADING
Involves the study of the history of changes in the Earth’s polarity
Every 400,000 years Earth’s magnetic field switches polarity causing the
magnetic north and south poles to swap, once cooled and solidified, magnetite
on the ocean floor records the Earth’s magnetic field at that time.
The switching of magnetic fields is measured by magnetometers and the results
showed mirror-imaged patterns of switches in the Earth’s magnetic field as it
moves away from the Mid-Atlantic ridge.
This suggests that the Earth is expanding, however this is not the case, as crust
is created it also has to be destroyed, this happens in the deep ocean trench
where the ocean floor is being subducted and destroyed.
In 1962, Harry Hess studied the age of the rocks on the Atlantic Ocean floor and
found the youngest in the middle and the oldest near the USA and the
Caribbean, and the youngest/newest rocks being formed in Iceland. He also
found the Atlantic Sea floor is spreading outwards at a rate of 5cm per year.
Fred Vines uses palaeomagnetism to support Hess’s theories of sea floor
spreading at the Atlantic mid-ocean ridge.
10. PLATE MARGINS
Plate Margins can be one of three things:
Towards each other: CONVERGENT (destructive).
Away from each other: DIVERGENT (constructive)
Alongside each other: TRANSFORM (passive or conservative)
11. CONSRTUCTIVE/DIVERGENT
Plates move away from each other, for example, N. American and Eurasian
plates,
creating mid-ocean ridges such as the Mid Atlantic Ridge. New crust appears
at the
ocean ridge.
FEATURES OF CONSTRUCTIVE MARGINS
MID-OCEAN RIDGES- LONG HIGH AND OFTEN COMPLEX STRUCTURES OF RIFTS AND SCARPS
VOLCANOES- OCCUR ALONG MID-ATLANTIC RIDGES, SOMETIMES RISING ABOVE THE SEA TO
FORM ISLANDS SUCH AS SURTSEY, ICELAND. THESE VOLCANOES HAVE BASALTIC LAVA WHICH
HAS LOW VISCOSITY AND CAN FLOW OVER GREAT DISTANCES WITH GENTLE SIDES.
VOLCANOES ARE ALSO FORMED AT RIFT VALLEYS, AFRICA; THESE VOLCANOES ARE
DIFFERENT TO ONES ASSOCIATED WITH MID -OCEAN RIDGES.
RIFT VALLEYS-ON CONTINENTAL AREAS DUE TO THE FRACTURING OF THE BRITTLE CRUST.
AREA OF THE CRUST DROPS DOWN BETWEEN PARALLEL FAULTS TO FORM THE FEATURE. THE
AFRICAN RIFT IS THOUGHT TO BE AN EMERGING PLATE BOUNDARY AS EAST AFRICA SPLITS
12. DESTRUCTIVE/CONVERGENT
With the two type of plate there are three types on convergence:
OCEANIC AND CONTINENTAL
Off western South America where the denser oceanic Nazca
Plate is subducting under the less dense continental South
American Plate
OCEANIC AND OCEANIC
In the western Pacific Ocean where the Pacific Plate is
subducting under the smaller Philippine Plate forming island
arcs. One example would be the Solomon Islands
CONTINENTAL AND CONTINENTAL
The Indo-Australian Plate meeting the Eurasian Plate in
southern Asia. Here the two plates have lower density than the
underlying layers, so there is little subducting and the plate is
forced upwards to form fold mountains, Himalayas
13. DESTRUCTIVE/CONVERGENT
Features of Destructive Margins
Ocean trenches- as the denser plate subducts the ocean floor is pulled down to form a
trench. Peru-Chile trench off western South America and the Mariana trench in the western
Pacific
Fold mountains- sediments accumulating on the continental shelf are focused upward and
are deformed by folding and faulting e.g. Andes. When continental plate meet, the edges are
forced up e.g. Himalayas
Volcanoes- heat generated by friction, cause the plate to melt in an area known as the
Benioff zone. This is lighter than the surrounding asthenosphere and rises towards the
surface as magma. This is viscous and forms composite and explosive volcanoes.
Island arcs- magma comes to the surface under water to form a line of volcanoes e.g. the
Mariana Islands formed in association with the Mariana trench.
14. CONSERVATIVE/PASSIVE
Plates slide past each other and there is no creation or destruction of
crust. There is also no volcanic activity.
However there are shallow focused earthquakes. The best known
example is the where the Pacific Plate and North American Plate are
sliding past each other at different rates, forming the San Andreas
Fault, California
15. HOTSPOTS
There are examples where volcanic activity is not linked to plate
margins e.g. the Hawaiian Islands
This is believed to be due to the presence of 'hot spots' - places
of localised heat under the earth's crust that burn through the
lithosphere to create volcanic activity on the surface
As the hot spot remains stationary the movement of the
overlying plate results in the formation of a chain of active and
extinct volcanoes
Over time they are eroded by the sea and weathering
