Plate tectonics and associated hazards

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Plate tectonics and associated hazards

The structure of the earth

·         Crust:

o   2 types of crust:

§  Oceanic – an occasionally broken layer of basalt rocks known as sima (because they are made up of silica and magnesium)

§  Continental – bodies of mainly granite rocks known as sial (because they are made up of silica and aluminium)

·         Lithosphere:

o   Together the crust and the upper mantle are known as the lithosphere

·         Mantle:

o   Widest section of the earth 2900 km thick

o   Made up of liquid rock

o   The upper mantle is more solid

o   The lower mantle moves slowly

·         Core:

o   Hottest part of the earth – above 5000oc

o   Made of iron and nickel it is 4 time denser than the crust

o   The inner core is solid

o   It is thought that it creates the earth’s magnetic field

o   The core generates convection currents within the mantle

Plate tectonics

·         Theory made by Alfred Wegener

·         Wegener’s theory based on:

o   The jigsaw fit of South America and Africa

o   Matching rock sequences linking Scotland and eastern Canada

o   Coal found in Antarctic ice cap

o   Permian fossil brachiopods in India and Australia

o   A unique Permian fossil reptile called Mesosauras if found only in southwest Africa and Brazil

o   Glacial deposits and striations on rocks in Brazil match those in west Africa

·         Paeomagnetism:

o   Paeomagnetism confirms sea-floor spreading

o   Paeomagnetism is the study of the earth’s changing magnetic fields

o   Every 400000 years or so the earth’s magnetic field swaps polarity

o   The magnetic particles in the rock cool facing different ways so you can tell which way the magnetic north was at the time it cooled

o   This allowed you to get a rough age of the rocks

·         Tectonic plates:

o   Seven major and several minor tectonic plates

o   The plates are floating on the mantle

o   Continental plates have a low density so they are less likely to sink

o   Oceanic plates are more dense so sink more often

o   The plates are continuously moving each year at different rates

o   The plates are moved by convection currents within the mantle

Constructive plate margins:

·         When 2 plate diverge they from a constructive margin

·         There are 2 types of divergence:

o   In oceanic areas, sea-floor spreading occurs on either side of mid-ocean ridges

o   In continental areas stretching and collapsing of crust creates rift valleys

·         It is at constructive plate margins that some of the youngest rocks on the earth’s surface are found

Mid-ocean ridges:

·         Oceanic divergence forms chains of submarine mountain ridges which extend for thousands of km across the ocean floor

·         Regular breaks called…


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