Evidence for Sea Floor Spreading
As 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 Ridges
In 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 Anomaly
MORs 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 Earthquakes
Shallow-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 Oceans
Satellites 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.

	Created by siobhan.quirk almost 13 years ago

Undergraduate Geology - Part 1 (Continental Drift) Note on The Spreading Oceans, created by siobhan.quirk on 14/05/2013.

The Spreading Oceans