Mid-ocean ridges

Standing on the seashore and looking out to sea, we are met with a continuous surface of moving water that covers 75% of the Earth’s surface. This is the ocean and what lies beneath it determines the history of our planet. While the continents form the dry land that we humans consider our home, the ocean floor lies at an average depth of 3790m below the sea surface. This is well beyond the reach of sunlight or our ability to see it. But why is there the distinction between land and ocean? The answer lies at the heart of geology!

A 21st Century view of the Earth – the hidden ocean floor revealed by gravity. Unlike all previous images of the Earth, this one shows the shape of the ocean floor: criss-crossed by chains of volcanoes and mid-ocean ridges and bound by deep-ocean trenches,. (After Sandwell and Smith, 1992).












Ultimately, the shape of the surface of the Earth is a product of energy dissipation. The heat from the interior of the Earth is constantly escaping to space. Deep inside the Earth, hot material convects slowly. This material - the Earth’s mantle - extends downwards for 3000km. It is the most abundant rock type on Earth. At the immense pressure and temperature within the Earth, this mantle rock behaves plastically and is in constant slow motion.

Above the mantle is the Earth’s crust. This is the brittle lid that forms over the mantle. On the continents, the crust is made of low-density siliceous materials that ride high on the denser mantle. Like a raft on water, the continental crust is carried by the churning currents beneath it, colliding at its edges and sometimes splitting in two (rifting). This continental crust is very old, up to thousands of millions of years old. It contains the history of movement, collision and rifting. The results are high mountain belts, deep basins, volcanoes and faults.

Simplified diagram showing a cross-section through the Earth. The thick brown lines are the continental crust, the thinner grey lines are oceanic crust and the thin white lines are flow lines indicating convective movement in the mantle.



















The oceanic crust is quite different. Being made of denser volcanic rock, it rides much lower on the underlying mantle. So low, in fact, that it is covered by the sea. As the mantle churns, so the crust above moves. Giant plates containing continental and oceanic crust together move at the rate your fingernails grow. Where the plates diverge, the resulting crack fills with molten rock forming new oceanic crust. These cracks are known as the mid-ocean ridges and are where new ocean crust is born. Where the giant crustal plates collide, ocean crust is consumed back into the mantle forming the deep-ocean trenches. This continuous process of recycling regenerates the ocean crust approximately every 160 million years. That is, ¾ of the Earth’s surface is renewed every 160 million years.

Age map of the ocean floor based on magnetic data: red is youngest and dark blue is oldest. After: Müller, R.D., Roest, W.R., Royer, J.-Y., Gahagan, L.M., and Sclater, J.G., A digital age map of the ocean floor. SIO Reference Series 93-30, Scripps Institution of Oceanography.
















Mid-ocean ridges are where new oceanic crust is generated. They form a volcanic ridge 50,000km long that stretches twice around the world. Here, 90% of the Earth’s active volcanoes are found. Mid-ocean ridges are the most dynamic geological environments on the planet. Vast quantities of heat are dissipated along the ridges, carrying huge quantities of chemicals and forming mineral deposits and hydrothermal vents that host unique chemosynthetic ecosystems supporting life without the need for sunlight.

Research into mid-ocean ridges at the NOC forms part of an international scientific effort coordinated under the InterRidge programme. Recent studies by the NOC group have focused on the way volcanic systems work at slow spreading ridges: those ridges that spread at less than 3cm per year. These represent the majority of ridges and are found in the Atlantic, Indian and Arctic oceans.

Bathymetry (left) and sonar imagery (right) of the Mid-Atlantic Ridge showing a deep axial valley (blue) with shallower ridge flanks (yellow and red) with an axial volcanic ridge in the centre (green). The light grey ‘lumpy’ texture in the centre of the image (right) are recent volcanic eruptions. (scale: left image is 60km wide; right image 25km wide).


















3D image of magnetic intensity anomalies draped over bathymetry for the 45°N segment of the Mid-Atlantic Ridge showing the highest magnetic intensities (yellow and red) located along the axial volcanic ridge indicating the most recent volcanic activity (scale: image is 60km wide).


















A collapsed lava tube on the sea floor of the axial valley of the Mid-Atlantic Ridge at a depth of 3200m.

















The Marine Geosciences Group is currently researching a number of localities on the global mid-ocean ridge system. These are oceanic core complexes, where mantle is exposed on the seafloor at 13°N on the Mid-Atlantic Ridge; axial volcanic ridges and hydrothermal activity at 45°N on the Mid-Atlantic Ridge, and the deepest known hydrothermal vents at 5000m on the Mid Cayman Rise in the Caribbean Sea. In addition, we are actively researching the interaction of the Iceland plume on the Reykjanes Ridge, looking at variations in plume activity as recorded by southward closing V-shaped ridges that dominate the bathymetry and gravity field of the North Atlantic. These features have had a fundamental effect  on the ocean circulation in he North Atlantic for at least the past 12 million years.

Pulses of hot mantle rise beneath Iceland in the form of a plume and spread laterally outwards like ripples in a pond. Where the hot ripples meet the spreading ridge, thicker crust is produced generating a trail of V-shaped ridges on the floor of the North Atlantic. The hot pulses also cause the surrounding region to rise and fall, resulting in changes to the Greenland-Iceland-Scotland ridge depth. This ridge acts as a gateway to North Atlantic Deep Water  flowing from the Arctic Ocean southwards. Such deep water is a major contributor to driving the ocean circulation in the Atlantic and has fundamental consequences for climate variability.

The ocean crustal research undertaken by the Marine Geosciences Group at the NOC is funded through ‘blue-skies’ responsive mode grants awarded by the Natural Environment Research Council (NERC) via a competitive bidding process. The success of the Marine Geosciences Group in securing these grants is a reflection of the outstanding quality of this research. The work fits with NERC’s Delivery Plan for supporting high quality research into fundamental Earth processes. It also benefits from continuing technological innovations in robotic underwater vehicles and sensors and national and international collaboration.

Figures showing the gravity field for the North Atlantic (above) in which the v-shaped ridges close southwards from Iceland indicating flow of mantle at a rate of 10-20cm per year beneath the Reykjanes Ridge. The cartoon (below) is a graphical representation of the flowing ‘ripples’ of hot and cold mantle and their affects on North Atlantic Deep Water flow over the Greenland-Iceland-Scotland ridge.
















A pulsating plume model for Iceland and its effect on ocean circulation


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