Geography: Tectonic Hazards

Plate tectonics theory

• Convection currents - hot rock lower in the mantle (asthenosphere) rise due to it being less dense and the denser rock higher up sinks due to gravity. Contineous circulation, which moves the crust.

• sea floor spreading - occurs at mid-oceanic ridges where new crust is being developed. This crust eventually moves away from the ridge.

Evidence

Palaeomagnetism in Mid-Atlantic ridge: alternating polarity of the rocks. Iron particles cool when erupted and allign with the earths magnetic field. Earth's Polarity changes roughly evry 40,000 years. The result is a series of magnetic 'stripes' with rocks aligned alternately towards the north and south pole.

Fossils (Mesosaurus): Reptile found both in south america and southern africa. unlikely the reptile could migrate or developed in both areas.

Jigsaw - shape of Americas & Europe/Africa

Geology: Rock sequences in NW highlands and newfoundland, Canada. indicateing they were laid down under the same conditions in one location.

Destructive margin - Dense oceanic crust is subducted beneath the less dense continental lithosphere. As the oceanic crust descends it is partially melted, destroyed. The magma then rises into chambers and may result in explosive volcanoes. ocean trenches and island arcs and fold mountains. volcanoes and earthquakes.

Subduction zone - Challenger Deep, Mariana Trench - 36,000 ft deep (pacific and eurasion plate)

If Continental/continental it creates fold mountains. lower density and therefore not much subduction. no volcanoes.

Young fold mountain - Annapurna Range, Himalayas - central nepal, south asia. (Indo-Australian Plate and the Eurasian Plate)

If oceanic/oceanic it is the same process as first stated.ocean trenches and island arcs volcanoes and earthquakes.

Island arc - Mariana islands, NW Pacific (pacific and eurasion plate)

Constructive - where plates move apart in oceanic areas they produce mid-oceanic ridges. Where they move apart in continental crust they from rift valleys. space between the diverging plates is filled with basaltic lava (low in silcia) coming from below.

Oceanic ridges

Mid-ocean ridge that runs along the centre of the Atlantic Ocean, parallel to its edges, for some 14,000 km – almost from the Arctic to the Antarctic. Like other ocean ridges, the Mid-Atlantic Ridge is essentially a linear, segmented volcano.

Rift valleys

The brittle crust fractures at sections as it moves apart. crust drops down between parrallel faults and forms rift valleys.

African rift valley is the largest rift valley extended for more than 4,000 km from Mozambique to the Red sea and from the Red sea to Jordan, which is a total of 5,500km

Conservative margin - two crustal plates slide past one another parrallel to the plate margin. No creation or destrucion. No subduction and therefore no volcanoes. Causes earhquakes as the plates can become locked and eventaully released causing shallow-focus earhquakes.

Example: Los Angeles, 1994. occure due to the san andreas fault, where the pacific and north american plate move parrallel to one another.

This is an area of localized swelling and cracking of the earth's crust due to an upward welling of magma. Volcanoes form above hot spots: the Hawaiian islands are cited as an example. One theory suggests that, as plates move across the hot spot, a line of volcanoes is formed.

The volcano above the Hot Spot is the only active volcano with the previous volcanoes becoming extinct. This shows that the Pacific plate is moving NW and therefore also being a means to shows that the earth's crust is moving.