Plate Tectonics- Plate Movement

Plate Margins

• Towards each other: Convergent (destructive or collision)
• Away from each other: Divergent
• Alongside each other: Transform

Constructive Margins

Plates move away from each other, for example, N. American and Eurasian plates, creating mid-ocean ridges such as the Mid Atlantic 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 from the rest of the continent. 

Destructive Margins

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.

Earthquakes are associated with all these types. Shallow, intermediate and deep earthquakes are associated with oceanic/continental and oceanic/oceanic convergences, but only shallow earthquakes are found in continental/continental collisions.

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.

Conservative margins

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. 

Hot Spots

These are examples where volcanic activity is not linked to plate margins as in the case of 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 then find their way to the surface, similar to solar flares on the sun, volcanoes are formed and as the plate moves over the hot spot it creates a chain of islands and over time they are eroded by the sea and weathering.

Plate Tectonics- Plate Movement

Plate Tectonic Theory

It is believed that millions of years ago (200 million), all continents were joined to one land mass, Pangaea, which later split into Laurasia in the north and Gondwanaland to the south.

Alfred Wegener put forward a theory regards their movement in 1912 with his theory of Continental Drift. His theory was based on observations such as:

Biological:

• Some fossils (e.g. Mesosaurus) are found in both Africa and South America
• Fossils of a fern (Glossopteris) are found widely across the southern continents.

Geological: 

• Rock types and geological structures are similar on both sides of the Atlantic. The Appalachian Mountains (N.America) and the Caledonian Mountains (Scotland) both have the same sequence of igneous and sedimentary rocks. 
• Coal is found in UK but needs warm, wet, humid conditions to form.
• Late-Carboniferous glaciations evidence exists in India, South America and Antarctica

Observational:

• The shapes of countries appear to 'fit' one another, for example, S. America and Africa. Especially when you look at the continental shelf rather than the present coastline.

In the 1940's-1960's the theory was revised and Plate Tectonic theory emerged, based on the premise that the lithosphere is divided into plates, which are moved by convection currents coming from the earth's core.

Further Evidence

Mid-Atlantic Ridge & Sea Floor Spreading

Sea floor spreading; where new ocean crust is being continually created in zones in the middle of oceans, such as the Mid-Atlantic ridge which is moving the Eurasian plate and North American plate apart by 2.5cm/year or 25 km in a million years.

Mid-Atlantic Ridge - A submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip of Africa. Seafloor spreading over the past 100 to 200 million years has caused the Atlantic Ocean to grow from a tiny inlet of water between the continents of Europe, Africa, and the Americas into the vast ocean that exists today. 

In 1947, a team of scientists led by Maurice Ewing confirmed the existence of a rise in the central Atlantic Ocean, and found that the floor of the seabed beneath the layer of sediments consisted of basalt, not the granite which is the main constituent of continents. They also found that the oceanic crust was much thinner than continental crust. The new data that had been collected on the ocean basins also showed particular characteristics regarding the bathymetry. One of the major outcomes of these data sets was that all along the globe, a system of mid-oceanic ridges was detected. An important conclusion was that along this system, new ocean floor was being created.

Palaeomagnetism

Palaeomagnetism is the examination of the polarity of the rocks that make up the ocean floor. Grains of magnetite -- behaving like little magnets -- can align themselves with the orientation of the Earth's magnetic field. This means each time the polarity switches it results in magnetic stripes in the sea floor, which is exactly mirrored either side of the mid Atlantic ridge.

Ocean Trenches

Ocean trenches are where large areas of ocean floor are pulled downwards and destroyed, the opposite to sea floor spreading.

Plate Tectonics- Plate Movement

Earth Structure

The Earth is comprised is of three layers:

The Core

The core comprises of dense rocks, especially of iron and nickel alloys. The inner core is solid and the outer core is liquid. The outer core is where the Earth’s magnetic fields originate, due to electrical currents flowing in the metallic core. Fluid moves due to convection currents. It is thought there is a radioactive decay happening in the core which powers the convection currents.

The inner core is 1100km deep. The outer core is 2400km deep

The Mantle

The mantle is less dense than the core as the rocks are comprised of lighter elements such as silicon and oxygen with varying densities depending on the proximity to the core.

The junction between Core and Mantel is known as the Gutenberg Discontinuity.

The Crust

The junction between the Mantle and Crust is called the Mohorovičić or Moho discontinuity.

The density of the crustal rocks is lower than the core and mantle because of lighter elements such as oxygen, silicon, aluminium, potassium and sodium. There is a large difference in the thickness of the crust depending on what is on the surface. It can range from only 5km thick below the oceans to up to 65km think below mountain ranges.

Lithosphere

The Lithosphere is the crust and the rigid upper part of the mantle forming a layer which is 80km thick; this is divided in to 12 larger plates and numerous smaller ones.

Asthenosphere

The asthenosphere is semi-molten material below the lithosphere on which the plates of the lithosphere ‘float’. The asthenosphere is several hundred kilometres thick.