Plate Movement • Gravity: Oceanic lithosphere thickens as it moves away from a spreading centre and cools, a configuration which might tend to induce plates to slide under the force of gravity, from a divergent margin towards a convergent margin. • Since the plates are slightly denser than the underlying asthenosphere, they tend to sink. This sinking action is known as slab-pull because the sinking plate edge pulls the remainder of the plate behind it. • Descending plates: This hypothesis suggests that a cold dense plate descending into the mantle at a subduction zone may pull the rest of the plate with it and thus cause plate motion. • Convection:the movement of molten material circulating deep within the earth. • A convection current occurs when hot, molten, rocky material floats up within the asthenosphere, then cools as it approaches the surface. As it cools, the material becomes denser and begins to sink again, moving in a circular pattern. • Thermal Plumes: vertical columns of molten material that rise up through the asthenosphere and appear on the surface of the earth as hot spots. (more associated with Island formation than plate movement) • Magma Injection This hypothesis invokes the injection of magma at a spreading centre pushing plates apart and thereby causing plate movement.
Plate Movement (older outdated) • Centrifugal Force: The spinning of the earth is the force which moves the plates. Alfred Wegener proposed this “engine theory” • Lunar Pull:This hypothesis suggests that the moons influence on the oceans also influences the movement of the plates. The movement of water is a huge force, and therefore could cause plates to move as well • Myths - … • Catfish, Elephants Turtle, Boar etc • Great Internal Winds • Others?
Magma • How does the magma form?Partial melting of the asthenosphere to form large amounts of magma is largely due to the reduction of confining pressure on the upper mantle (known as mantle decompression) - this is rather like releasing the pressure in a bottle of fizzy drink by unscrewing the cap. Mantle decompression is caused by the doming and thinning of the overlying lithosphere as it is stretched. Rising convection currents of heated mantle material raises the temperature of this region, and also brings up volatiles such as carbon dioxide and sulphur dioxide into the mantle rocks below the developing rift system, which helps lower the melting point of the mantle rocks. The combination of pressure reduction, increased heat flow and the introduction of volatiles result in the partial melting of the asthenosphere to produce massive quantities of basaltic magma which segregate and collect in magma chambers below the developing spreading line.
The birth of a new ocean Continental rifting eventually leads to formation of new ocean crust in the split between the two halves of continental plate. This process doesn't happen overnight - it takes millions of years! The simplest way to understand it is to break the process down into a series of stages... Stage 1 Continental rifting all starts with hot, rising convection currents in the mantle. As the currents reach the upper part of the mantle, they spread outwards and away from each other (diverge). The rising mantle material is hot and buoyant, and it causes the overlying plate to bulge upwards and stretch. At the same time, the diverging currents drag on the bottom of the plate, also causing the plate to stretch. Eventually, cracks start to appear in the plate.
Stage 2 The diverging mantle convection currents continue to diverge (move apart), causing the plate to stretch until it splits. Along the line of the split, huge blocks of crust start to sink downwards (subside) due to gravity. Think about what happens when you pull a blob of warm blue tak or bubble gum apart! The release of pressure on the mantle directly below the broken plate enables magma to form and well up into the cracks (faults) between the blocks. Huge amounts of basalt (a type of volcanic rock) can pour out onto the floor of the valley during this stage.
Stage 3 The plates continue to be pulled apart, and a rift valley forms in the central part, which is still subsiding. Magma from the underlying mantle feeds numerous volcanoes, and shallow earthquakes are caused by the edges of the blocks rubbing together. Stage 4 At this stage, the plates are still being pulled apart by the mantle currents, but injection of magma along the central crack (rift) also pushes the plates apart. The central rift valley has now sunk down to below sea level and has been flooded with seawater to create a new ocean.
Stage 5 Magma injected along the central rift spills over and forms a chain of underwater mountains and volcanoes. This is now a fully functional mid-ocean spreading ridge, with new ocean crust being injected into the central rift. The ocean will continue to grow wider until the continental crust at the edges because consumed by subduction, or continental collision occurs.
Constructive Plate Boundaries Constructive plate margins are areas where new crust is formed. This type of margin develops in situations where rising and diverging mantle convection currents create forces which pull or push the plates apart. Magma is injected into the lithospheric plates along the spreading line to form new plate material. These types of plate margin are characterized by the birth and growth of ocean basins which are formed by the divergence of the plates. Constructive margins occur both on land and under the sea.
Continental constructive margins • This type of margin is developed where rising mantle convection currents occur below a continental plate. • The rising hot mantle material causes the overlying continental plate to dome up and stretch. Stretching of the relatively rigid plate causes brittle fractures to develop, and the plate eventually cracks. • As the mantle convection currents diverge, the plate is gradually pulled apart and the central blocks between the faults sink downwards, forming a rift valley. • In the centre of the rift valley, magma from the underlying asthenosphere wells up to fill in the crack between the two halves of the broken plate. Rift valleys usually produce large amounts of volcanic lava, which form extensive lava plateaus and volcanic cones.
Magma Injection Magma Injection (along divergent boundary) May be why plates pulling apart at different rates. Like a series of channels to the surface