Plate Tectonics Theory

1) At the centre of the earth is the CORE, which is split into an inner core and an outer core:

• The inner core is a solid ball containing lots of iron and nickel.
• The outer core is semi-molten and also contains a lot of iron and nickel. (The earth's core is over 4000 degrees celsius) 

2) Around the core is the MANTLE, which is mostly made of silicate rocks (rocks that have lots of the element silicone in them):

• The part of the mantle nearest the core is quite rigid.
• The layer above this, called the asthenosphere, is semi-molten (it can flow).
• The very top bit of the mantle, where it meets the crust is rigid.

3)The outer layer of the Earth is called the CRUST.

4) The rigid top part of the mantle and the crust together are called the LITHOSPHERE.

Lithosphere = rigid mantle and crust

Asthenosphere = Semi-molten mantle

5) There are two types of crust- CONTINENTAL and OCEANIC:

• Continental crust is thicker (30-70km thick) and less dense,
• Oceanic crust is thinner (6-10km thick) and more dense.

1) The lithosphere is divided into lots of slabs called tectonic plates.

2) The plates are moving due to convection currents in the asthenosphere.

3) The places where plates meet are called boundaries or plate margins.

4) The idea that the Earth's lithosphere is made up of plates that are moved around by convection currents is the theory of plate tectonics.

1) Radioactive decay of some elements in the mantle and core, e.g. uranium, generates a lot of heat.

2) When lower parts of the asthenosphere heat up they become less dense and slowly rise.

3) As they move towards the top of the asthenosphere they cool down, become mode dense, then slowly sink.

4) These circular movements of semi-molten rock are called CONVECTION CURRENTS.

5) Convection currents in the asthenosphere create drag on the base of the tectonic plates (which are solid and rigid)- and this causes them to move.

Crust is Created When Plates Move Apart

1) Rising convection currents diverge (move apart) at the base of the lithosphere.

2) The drag of the convection currents causes the plates above them to diverge too.

3) Magma rises up to fill the gap created, then cools to form new crust.

4) Over time, the new crust is dragged apart and even more new crust forms between it.

5) When this happens at a plate margin under the sea, the sea floor gets wider.

6) This process is called sea floor spreading.

7) It creates structures called mid-ocean ridges- ridges of higher terrain on either side of the margin.

8) A similar process of spreading occurs at land margins where the plates are moving apart.

9) When plates move together, sometimes the crust is destroyed and sometimes it is pushed up into mountains.

New crust formation at mid-ocean ridges doesn't happen at the same speed everywhere- at some, it is less than 10mm per year and at others is more than 100mm per year.

17^th Century

• People first noticed that South America and Africa looked like they could fit together like a jigsaw.
• there were suggestions that the continents might once have been joined together before moving apart, but most people believed the continents were fixed in place.

1912

• Alfred Wegener proposed the theory of continental drift.
• he suggested that all of the continents were once joined as a super-continent (called Pangaea) which drifted apart (250 million years ago).
• Wegener based his theory on geological evidence and fossil records but he couldn't back it up with a mechanism that explained how the continents moved.

1950’s

• Arthur Holmes provided evidence that supported continental drift, through paleomagnetism.

1960’s

• Harry Hess discovered the process of sea floor spreading- providing a mechanism for continental drift.

1960’s- Present Day

• Continental drift theory was developed further by scientists after these findings. it grew into the theory of plate tectonics.

Geology

1) Areas of South America and Africa have rocks of the same age and composition - if you fit these continents together, the distribution of the rocks matches up.

2) You can also match up the age, rock type and distribution of some mountain ranges. e.g. Mountains in Scotland, Norway Sweden, and Finland are similar to those found on the east coast of North America.

3) These rocks and mountains must have formed under the same conditions and in the same place in order to match so well - this would only be possible if the continents were once joined.

Fossil Records

1) By fitting land masses together you can match up the distribution of some fossils e.g. fossils of mesosaurus (a reptile) found in both South America and southern Africa; Glossopteris (a plant)- existed when coal was being formed and has only been located in India and Antartica and Fossil Brachiopods found in Indian limestones which can be compared to similar fossils in Australia.

2) It's very unlikely that these species migrated across thousands of miles of water, or that they evolved in different places.

3) So the fossil records suggest that these places were joined together when these organisms were alive- hundreds of millions of years ago.

Living Species

1) The same living organisms can also be found on different continents- like with fossilized organisms, it is unlikely that some of them migrated across the oceans or evolved in different locations.

2) For example, earthworms of the family Megascolecidae are found in New Zealand, parts of Asia and North America. This suggests that the continents were once joined, allowing the earthworms to travel between them.

Climatology

There is evidence that the past climates of some continents were similar, despite being thousands of miles apart now. This suggests that they were located together and in a different place on Earth where they are now. Here are some examples:

1) Similar glacial deposits are found in Antartica, Africa, South America, India, and Australia. By fitting these places together you can match up the distribution of the deposits, which suggests that they were joined together millions of years ago and located close to the South Pole.

2) Large coal deposits that were formed in tropical conditions have been found in North America and parts of Europe.This suggests these regions were once closer to the equator than they are now- they've drifted over time.

Palaeomagnetism

1) Palaeomagnetism is the study of the history of the Earth's magnetic field.

2) Once every 200 000 years or so, the Earth's magnetic field reverses polarity (the magnetic north and south poles switch).

3) Palaeomagnetism has provided evidence for the process of sea floor spreading.

4) As magma erupts from mid- ocean ridges, magnetic minerals in the molten rock align themselves with the direction of the Earth's magnetic field.

5) When the new crust has solidified, the alignment is fixed.

Palaeomagnetism (continued)

6) The magnetic minerals in the crust created in periods of normal polarity (magnetic north near the North Pole) are aligned in the opposite direction to those in the crust created in periods of reverse polarity (magnetic north near the South Pole).

7) This creates a series of alternating magnetic stripes along the sea floor. The stripes show that the crust is older the further away from a mid- ocean ridge you go - this means that the plates are moving apart.