Theme D: The Restless Earth Cheat Sheet (DRAFT) by etait4502

The mantle of the earth is the thickest layer of the earth, made up of silicate rocks which due to high temper­atures (3500 degrees) remain molten and float about.

The theory of plate tectonics is credited to Alfred Wegener.

One constr­uctive margin is found in the middle of the Atlantic ocean. Here the Eurasian and North American plates are being pulled apart, moving away from one another. This means the Atlantic ocean is getting wider apart by approx. 3cm per year. The movement causes regular but weak earthquake activity. Magma wells up from mantle to plug the gap so there is often frequent gentle volcanic activity. This rising of material pushes up crust at either side slightly, thus creating a mid-oc­eanic ridge.

An example would be found in South America. Here the Nazca plate, made of Oceanic crust, is disapp­earing below the American plate. At plate margin, dense oceanic crust is pushed downwards. As it is dense it falls below its normal level, creating a deep ocean trench (Peru-­Chile). Movement is not smooth due to rough surface friction. Plates may become stuck for years until pressure is greater than friction. Will cause plates to jolt and move suddenly. Felt on Earth's surface as an earthq­uake.

Violent activity. An oceanic crust margin where oceanic crust and oceanic crust meet. Has many similar features to first type of destru­ctive margin. As the oceanic sinks into mantle, it melts and creates a less dense material than surrou­nding rock. Deep ocean trench forms where dense material pushed down into mantle. Can be very deep. Magma then rises upward and may erupt through crust to create volcanic island.

Where two contin­ental plates meet is a collision zone. Crusts of both plates buckle and fold upwards. Two sets of fold mountains overthrust one another, creating large range of fold mountains. Little material melting and that which does not melt cannot make it through high mountains to make volcano. Instead, magma forms large intrusions into mountain range. Magma cools slowly to form granite cores to mountains.

Violent activity. At conser­vative margins, such as San Andreas fault line (Calif­ornia) two plates try to slide past one another. Friction causes plates to stick, pressure is built up and is eventually released as an earthquake when plates jolt suddenly. Crust is neither created or destroyed so therefore no volcanic eruptions.

These include basalt and granite and they have been formed by cooling and solidi­fying of molten rock (magma) from underneath the earth's crust. This molten rock is called lava on the surface of the earth. Crystals are usually evident in the rock. However, if the rock cools quickly as the lava hardens on the surface, there will be little evidence of the crystals (basalt). If the magma is cooled slowly underg­round, then a cystalline structure will be more evident (granite).

Limestone and sandstone are formed by sediments being built up in layers, usually under water over a long period of time. As more and more eroded material is added, pressure pushes air and water out and the sediment gets cemented into rock.

Slate and marble have been changed through the addition of pressure or heat. The rocks would originally have been igneous or sedime­ntary - marble once was limestone. Metamo­rphic rocks are hard and resistant to erosion.

26th December 2004. Sumatra, Indonesia (West Coast). Magnitude of 9.2, duration of 10 minutes. Australian and Sunda plates respon­sible.

- Major fault line where Australian plate meets Sunda plate. Part of a subduction zone, 15m slippage along fault line in two stages causing prolonged earthq­uake.

- 125,000+ injured

- In Maldives, 17 coral attol islands had freshwater supply contam­inated. Inhabi­table for decades.

- Rebel group ceased fire against govern­ment.

- 30m high tsunami; countries affected on all sides of Indian ocean

- Afters­hocks continued for 3 to 4 months afterwards

- Raising of seabed reduced Indian ocean capacity and raised global sea level by 0.1mm

- Massive release of energy expected to shorten length of day by 2.68 micro seconds; change to earth's shape.

- Area affected used to minor quakes and volcanic eruptions but made up of poor LEDCs and lack resources to have scale and quality of response that MEDCs have.

- No early warning system to record underg­round quakes. Simeulue island evacuated coastal areas as tremors were felt; fled to inland hills.

- Sri Lanka, Indonesia and Maldives declared state of emergency. Strict laws implem­ented to keep order.

- Park and memorial site built around 2,600 ton ship that washed up on shore.

- New highways and vibrant night life.

- Country's location on border of a number of dangerous fault lines between tectonic plates means another large earthquake is inevit­able. One of the most notable is Sunda, megathrust fault line, parallel to Sumarta and Java Islands. Even an instan­taneous alert would not give residents enough time to reach high ground (30 minutes)

This occurs when rocks on either side of a weakness in the earth's crust (fault) causes the ground to vibrate and shake.

Seismic waves are recorded by a seismo­graph. During an earthquake the base of the seismo­meter will move horizo­ntally and the motion is converted into electrical voltage and recorded on paper.

Liquef­action occurs when an earthquake hits an area and shakes the wet soil. The shaking causes the water within the soil to start and rise up to the surface, and this process turns solid soil and rock into a liquid mud. Buildings will start to sink and tip over as the support for the founda­tions is waterl­ogged and cannot maintain the weight of the buildings.

A tsunami is a large wave which is created when an underwater earthquake sends shockwaves through the water, causing a surge of water to move towards the coastline. The energy can travel for thousands of miles across the ocean.

During an eruption a volcano may eject ash, hard bits of rock (volcanic bombs), lava or gasses.

(Forma­tion) Found at destru­ctive plate margins.

Lava builds up in a magma chamber underneath the volcano. This can be added to as more oceanic crust melts at the plate margin.

The pressure build-up causes an explosion which blows out ash, gas and lava.

The layers of lava become the sides.

Very violent eruptions (volcanic bombs)

Narrow base due to slow moving lava.

These are found on constr­uctive plate margins.

Shield volcanoes have gently sloping sides and are much wider than composite volcanoes.

The lava is very fast and runny - basic lava (basalt), with little ash. This spreads easily and cools to form the gentle sides.

- 90,000 may die from inhaling ash (cement like in human lungs). Even east coast immobi­lised by just 1cm ash.

- Water supplies undrin­kable.

- Air travel disrup­tions. Badly affected causing major disruption in other countries and to business.

- Could inject 2000mil tonnes of sulphur into earth's atmosp­here. Would cloak globe in 2-3 weeks. Aerosols reflect sunlight, reducing amount of energy reaching lower atmosphere and earth's surface, cooling them.

- Crop failures and 'little' ice age.

- It would take 10 years before any vegetation becomes re-est­abl­ished.