The Spreading Oceans

Evidence for Sea Floor SpreadingAs more research has been done on the ocean floor, data are constantly improving. New technologies have allowed the deep sea floor to be filmed from submersibles capable of working in the high pressures at depth. These submersibles have also measured water and rock temperatures and collected samples. Drill ships have taken cores of sediments and underlying igneous rocks, and analysis of these has given the age of the ocean floor. Ships have measured the variations in gravity and magnetism that occur in the ocean basins. Satellite measurements and visual images can help still further, giving us a detailed information about the oceans. Satellite altimetry is an essential ool for mapping the ocean floor and has added detail to the original maps, which filled in the areas between echo sounding lines by exploration.Mid-Ocean RidgesIn all the oceans' basins are large mountain chains running down the centre, called mid-oceanic ridges. They are clearly seen on the oceans map. In the middle of the MOR is an axial rift, a deep valley with steep-sided mountains on either side. Occasionally the MOR rises above sea level, forming islands such as Iceland on the Mid-Atlantic Ridge. Closest to the ridge the crust is hot and expanded, so forming the ridge itself. Away from the ridge, the crust cools and contracts, creating the flatter areas of the ocean basins.High Heat Flow and Volcanic Activity with Gravity AnomalyMORs show the highest heat flow directly above the axial rift. This suggests convection currents in the mantle are rising under ocean ridges, bringing magma close to the surface. This forms fissure volcanoes in and near the axial rift. The eruptions are of non-explosive, low viscosity basaltic lava. If the basalt cools quickly under water, the outside rapidly solidifies while the inside remains liquid for longer, producing pillow lavas. Gravity anomalies show a similar pattern to the heat flow, with a positive anomaly peaking in line with the axial rift. The broad positive anomaly at the MOR means there is an excess of mass due to the large volcanoes and rising magma. Transform Faults and EarthquakesShallow-focus earthquakes occur along the length of the MOR, caused by the rising magma. They also occur along linear transform faults, which have large horizontal displacement, are perpendicular to the ridge and can be thousands of kilometers long. The transform faults divide the MOR into sections, with movement taking place in opposite directions, causing many small, shallow earthquakes. Sea floor spreading proceeds section by section with periods of activity alternating with periods with no eruption or movement. Measuring the Width of the OceansSatellites can be used to measure the exact location of a series of fixed receivers on the surface of the Earth. The positions of these relative to each other is recorded and their relative movement can be calculated. When this is used in Iceland it proves that the two halves of the island are moving away from each other at a rate of abour 2.5cm a year.

Magnetic AnomaliesThe Earth's magnetic field undergoes complete reversal, so that the north becomes south.and vice versa. This may happen up to four times in a million years. The sea floor acts as a tape recorder dor these changes in the Earth's magnetic field. As new magma is erupted at a MOR the iron particles line up parallel to the Earth's existing magnetic field. As the rocks cool down the remain permanently magnetised by the field. The result is a striped magnetic anomaly pattern on the sea floor, showing normal and reversed polarity of the Earth. They form a symmetrical pattern parallel to the MOR. These zones are not all the same width, varying with the length of time during which the magnetic field stayed in one direction, and with the amount of lava erupted during any particular period.The important observation is that the pattern of reversals is symmetrical on either side of the MOR. The only possible interpretation for this symmetry is that the crust on the two sides of the ridge is moving apart. Basalt erupts at the ridge to form new oceanic crust, which then spreads away from the ridge equally on both sides. Ocean basalts can be radiometrically dated to give a series of absolute dates from which the rate of spreading can be calculated. This is conclusive proof for sea floor spreading.Dating SedimentsThin oozes and other fine-grained sediments on the ocean floor can be dated from core samples by pelagic microfossils, which sank to the sea floor after . At the base of the core of the sediment which was deposited and therefore shows the date when the sedimentation started on that part of the newly formed basalt sea floor. Measuring the distance from the MOR to the sediments of known maximum ages is another method for finding the rate of sea floor spreading.The thickness of the sediments increases away from the ridge. This is because the ocean floor's age is greater away from the ridge, so the sediments have more time to be deposited away from the ridge and are therefore thicker. Sediment forms very slowly (1mm per 1000 years) from the remains of microscopic plankton such as radiolaria or foraminifera. The age pattern is symmetrical, ranging from zero at the MOR, where there is no sediment to 200 Ma furthest away from the ridge, where it can be 2000m thick.The Structure of the Ocean CrustWhenever you drill into the oceanic crust, the overall structure is always the same. This is because all the igneous rocks form as a result of the magma rising up at the MOR, cooling and the rocks then moving apart symmetrically. basaltic pillow lava is the material from volcanoes that cooled rapidly under water, forming fine crystals dolerite dykes are the vertical feeder pipes from the magma chamber supplying the volcanoes. The dolerite cools fairly slowly, so the crystals are medium sized. gabbro cools slowly, forming coarse crystals in the magma chamber peridotite is part of the upper mantle within the lithosphere, where the magma originates by partial melting To work out the rate of spreading: (distance moved in cm)/(number of years)

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Spreading Oceans 2