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Plate Tectonics According to theory of plate tectonics, Earth is an active planet — its surface is composed of many individual plates that move and interact, constantly changing and reshaping Earth's outer layer. Volcanoes and earthquakes both result from the movement of tectonic plates. In this interactive activity produced for Teachers' Domain with images from NASA, see the relationship between earthquakes and volcanoes and the boundaries of tectonic plates.
According to theory of plate tectonics, Earth is an active planet — its surface is composed of many individual plates that move and interact, constantly changing and reshaping Earth's outer layer. Volcanoes and earthquakes both result from the movement of tectonic plates. • In this relationship between earthquakes and volcanoes and the boundaries of tectonic plates.
The main features of plate tectonics are: • The Earth's surface is covered by a series of crustal plates. • The ocean floors are continually, moving, spreading from the center, sinking at the edges, and being regenerated. • Convection currents beneath the plates move the crustal plates in different directions. • The source of heat driving the convection currents is radioactivity deep in the Earths mantle.
Convection Current • Convection current is very slow movement of molten rock within Earth’s mantle caused by heating and cooling.
Crustal plates • These are irregular-shaped slabs of material in the lower crust, of varied size • Continental plates have great thickness of old continental material. Continental crust is composed of lighter rock of granitic type. • Oceanic plates are thin layers of rocks under the ocean. Oceanic crust consists of much younger and denser rock of basaltic composition.
A feature present at plate margins is volcanic activity • Rocks below the crust have a very high temperature, but the great pressure upon these keeps them in a semi-solid state. If the pressure weakens then some of the rocks become liquid. This liquid is called magma. Magma forces its way into cracks of the crust and may either reach the surface where it forms volcanoes or lava flows, or it may collect in the crust where it forms batholiths, laccolith, sills and dykes. Magma reaching the surface may do so quietly or with great violence. Whichever happens it eventually cools and solidifies.
What is a Volcano? • A volcano is a geological landform (usually a mountain) where magma (rock of the earth's interior made molten or liquid by high pressure and temperature) erupts through the surface of the planet.
Extrusive Features (Features formed on the Surface) • Lava ConesThe slope of the cones depends upon weather the lava is fluid or viscous. Viscous lava cannot easily spread outwards. It may form a steep-sided dome. There are many viscous lava domes in St Lucia and Dominica, formed by extinct volcanoes. Fluid lava gives rise to gentle sloping cones.
Ash and Cinder ConesIn some violent volcanic eruptions, molten material is thrown up into the atmosphere to great heights. It breaks up, cools and solidifies before falling back to earth to build up a cone. This material - known as tephra - varies from fine 'ash' to large 'bombs' which may weigh as much as several tonnes. If there are several falls of tephra and no lava, the result is a symmetrical ash cone.
Composite ConesThese are made up of alternate layers of lava and ash. It is quite common for a volcano to produce both ash and lava eruptions - one after the other. One eruption produced a layer of ash. The next produces a layer of lava. The result is a composite volcano. One example' is St Vincent Soufriere; another is the famous Italian volcano, Vesuvius.
In an ash eruption, the finest particles are often carried a considerable distance by the wind. When the St Vincent Soufriere erupted in 1902, 3.6 million tonnes of volcanic ash fell on Barbados, 180 km away. After the Krakatoa eruption in Indonesia in 1883, the ash cloud rose to 80 km in height, and spread right round the world. There were spectacular red sunsets for several months, and particles of ash
Calderas A caldera is a large volcanic crater created by explosion or internal collapse of a volcanic cone. During the most destructive volcanic events, a huge mass of incandescent material called ignimbrite bubbles out of a fissure or vent in the ground. When the ignimbrite cools, it has an ashy or sandy consistency. When a volume of ignimbrite is ejected by a volcano, a large cavity is opened up beneath the cone. The upper part of the mountain cannot be supported any more - and collapses inwards to form a big circular depression called a caldera.
Two ignimbrite eruptions are known to have occurred in historic times, both in uninhabited regions - in eastern Siberia in 1956, and near Mount Katmai in Alaska in 1912. In this eruption, 10 km3 of material flooded out of the volcano in only 20 hours, completely filling a river valley, and turning it into a flat plain 4 km across. This was known as the Valley of Ten Thousand Smokes, because for several years after the eruption it was dotted with jets of steam which spurted out into the cold air. Explorers who tried to cook their meals over the hot steam found that it contained acid - which burned holesin their pots and pans.Crater Lake in the State of Oregon in the USA was created about 6,000 years ago after a major ignimbrite eruption. A volcano 3,600 m high subsided to form a depression 9 km wide and 1,200 m deep. Wizard Island, in the centre of the lake, is a more recent volcanic cone.
Divergent plate margins: • Basalt PlateauxOn divergent plate margins, eruptions usually produce basalt lava. Basalt lava flows smoothly, and the gas it contains can escape easily. Volcanic eruptions are not nearly as violent as on convergent plate margins. When there is a large scale lava flow the hills and valleys of the pre-existing landscape gets smothered. A wide, flat plateau, is the result. The world's largest basalt plateaux in India, South America, and Africa, each cover an area larger than Guyana. They were formed by a series of different eruptions - there are layers of lava with gravel and fossil soil trapped between them.
Iceland is a large island which is located on the Mid- Atlantic Ridge, which is a divergent plate margin. In 1783, a 25-km crack, known as the Laki fissure, opened up in central Iceland. Over a period of five months, more than 12 km3 of lava flowed out to the surface. One tongue of molten rock, 56 km long and 100 m deep, filled up an entire river valley. There are many lava flows in the Caribbean, but almost all of them are formed by viscous lava. A viscous lava eruption produces a steep sided volcano; a basalt lava eruption produces a broad plateau.
Shield volcanoesThe Hawaiian islands are an isolated group of volcanoes in the central Pacific which produce basalt lava. Repeated eruptions of smoothly flowing lava from a series of vents have formed shield volcanoes which rise gradually with a slope of between 2° and 10° from the ocean floor to as much as 4,000 m above sea level.
Batholith • This is a large mass of magma extending to great depth with its upper portion dome-like in shape these usually form the root a mountains. It is made of granite and can be exposed on the surface by the removal of the overlying rocks by erosion
Laccolith • The laccolith: A body of igneous rocks with a flat bottom and domed top. It is parallel to the layers above and below it. The laccolith is smaller in size than the batholith and us formed closer to the surface.
Sill • A sill is a sheet of magma that lies along the bedding plane. Some sills form ridge like escarpments when they are exposed by erosion. Others may give rise to waterfalls and rapids where they are crossed by rivers
Dykes • A dyke is a sheet of magma that cuts across the bedding plans and forms a wall like feature. Dykes may be vertical or inclined. Some dykes when exposed on the surface resist erosion and stand up as ridges or escarpments. Others are easily eroded and form shallow depressions.
Hot Springs, Geysers, and Fumeroles • A geyser is a jet of hot liquid or gas (steam) that shoots from a crack in the crust of the earth • A hot spring is a place where warm or hot groundwater issues from the ground on a regular basis • A fumerole is a hole from which superheated gas and steam discharges under pressure.
Lava Flows • Lava flows move down slope away from a vent and bury or burn everything in their paths.
Pyroclastic Flows • Pyraclastic flows are high speed avalanches of hot rock, ash and gas that are formed by the collapse of lava domes. They can move up to 160 kmh-1 and have temperatures to 815°C
Volcanic Activity • ACTIVE - A volcano that has erupted in the recent past and is expected to erupt again. • DORMANT - A “sleeping” volcano. A volcano that has not erupted in the recent past, but could erupt again. • EXTINCT - A volcano that is not expected to erupt again.
Earthquakes • Earthquakes • A fault is a crack or fracture in the Earth's crust along which movement has occurred. Movement along a fault produces earthquakes and seismic waves. • An earthquake is the vibration of the Earth, produced by the rapid release of energy. Energy radiates out from the focus. • The focus is the place within the Earth where the rock breaks, producing an earthquake. • Energy moving outward from the focus of an earthquake travels in the form of seismic waves. • The epicenter of an earthquake is the point on the ground's surface directly above the focus.
What causes the rock to break? • Stress - a force or directed pressure acting on a rock • Types of stress: • Compressional stress - pushing together • Tensional stress - pulling apart • Shear stress - sliding • When stresses are applied to a rock, the rock will be deformed. Once the elastic limit of the rock is exceeded by the stress, the rock will: • Deform plastically, resulting in permanent changes in size and shape (such as folding), or • Fracture (i.e., break).
Brittle materials break during elastic deformation. • Rocks will deform plastically and will fold under conditions of high temperatures and pressures, like those deep within the crust of the Earth. • Rocks nearer the surface of the Earth, where temperatures and pressures are lower, rocks will behave like a brittle solid and will fracture once their elastic limit is exceeded. This produces a fracture or fault. • Compressional stress leads to the folding of rocks, reverse faults, and thrust faults (see types of faults, below). • Tensional stress leads to the formation of normal faults . Shear stress leads to the formation of transform faults .
