PLATE TECTONICS

3/11/2006

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PLATE TECTONICS

Decompression melting - if materials are heated to a sufficiently high temperature, they begin to melt. However the melting temperature is also a function of the confining pressure acting on the material. Consequently, it is feasible to melt material by lowering the pressure rather than raising the temperature. This is called decompression melting.

How do these three concepts help us explain the operation of the Earth? Well, the temperature of the asthenospheric mantle reservoir is increased by heat transferred from the molten core and by increments of heat generated by the decay of the radioactive elements U, Th, K, Rb, Sm, etc. At some critical temperature, the mantle will start to flow buoyantly towards the surface by the mechanism of deformation creep. Heat is therefore transferred by the process of convection, and because the confining pressure acting on the mantle decreases as it rises, at some critical depth the mantle will start to melt. Once a sufficient degree of melting has been achieved, the melt will separate and rise to form a body of magma (magma chamber) at the base of the lithospheric shell, from which it will find its way to the Earth surface via passageways created during the process known as sea-floor spreading. This is the principal way in which the Earth rids itself of its internal heat.

In contrast to the asthenosphere, heat transfer though the lithosphere is effected by conduction because the temperature of the lithosphere is too low to permit convection. As the average temperature of the Earth decreases, the lithosphere grows downwards and it becomes more effective as a thermal insulator. For this reason the rate at which heat is lost from the Earth decreases to a self-regulated minimum value. It is currently estimated that although the rate of radioactive heat production 3 billion years ago was twice the rate it is today, the mean temperature of the mantle at that time was only 150 degree K higher than its present value.

The uprising thermal currents must eventually turn over and descend back into the asthenosphere, and the zone of magma formation is therefore also coincidentally a zone of tensile stress allowing the magma easy egress to the surface via fractures created in the lithosphere by the laterally flowing asthenosphere. These fractures appear on the surface of the Earth as topographically elevated linear zones within ocean basins, and we know them as mid-ocean ridges. Where the rock magma comes into contact with sea-water it cools to form distinctively shaped and aptly named 'pillow lava' units, whereas within the fractures it cools as tabular bodies commonly referred to as 'sheeted diabase'. As the magma within the underlying magma reservoir cools, minerals crystallizing out of the melt either sink to the floor of the chamber to form layers of mineral 'sediments', or are added to a downward growing roof unit. As the mass of magma solidifies from top to bottom and bottom to top, the floor and roof of the chamber eventually meet and the wholly solid crust is carried away piggy-back by the laterally flowing asthenospheric mantle conveyor belt (i.e. sea-floor spreading). As long as the magma chamber is continuously fed with new batches of magma, oceanic crust is thus generated in a quasi-steady state manner.

The formation of an oceanic crustal reservoir at a Mid-ocean ridge

It is observable that the ridges are divided into a large number of segments separated from one another by fractures which geologists refer to as transform faults. The presence of `transforms' reflects the fact that the location of the magma chamber beneath the ridges tends to jump backwards and forwards along the ridge, and that the rate of spreading along the length of the ridge is not uniform. The variation in seismic activity along the transform provides remarkable confirmation of the process of sea floor spreading.

If some oceans are getting larger as a result of sea-floor spreading, then some must be getting smaller, otherwise the total volume of the earth would also have to increase commensurate with the increase in size of the surface area of the Earth. Since the Atlantic ocean is increasing in size whereas the Pacific is decreasing in size, the inference is that Pacific ocean crust is being consumed back into the asthenosphere at the margins of the Pacific. This process is called subduction, and it is intimately linked to the formation of volcanic island arcs, and eventually to the construction of continental crust.

Plate Tectonic Processes

Based on the distribution of mid-ocean ridges, subduction zones, and transform faults, the surface of the Earth can be represented as a set of moving plates, the relative movement along whose mutual boundaries may be extensional (mid-ocean ridges), compressional (subduction zones), or horizontal (transform faults).

Complete consumption of oceanic crust may lead to the collision of continental masses and the formation of collisional mountain chains such as the Himalayas, the Alps, or, closer to home, the Appalachians. In this way continents amalgamate to form ‘supercontinents’. Where arc systems participate in continental collision, they are also amalgamated to the continents and there is a consequent transfer of new arc material to the buoyant continental plate. The rate at which this process has varied over geological time is a matter of dispute, but it is this process that is thought to have led to formation of continents.

Continents are also destroyed by erosion and weathering brought about by the reaction of silicate minerals with bicarbonate-bearing rain water (the hydrologic cycle). As a consequence the oceans become the receptacle of weathered rock material and an intermediary in the subsequent transfer of material back to the continents or to the mantle, thus completing the material transfer cycle.

The conversion of thermal energy to chemical energy by the formation of hydrous minerals.

As the mid-ocean ridge basaltic material derived from the mantle cools, part of its heat energy is lost by conduction to the overlying sea water and part is converted to chemical energy by endothermic reaction of the basalt minerals (Cpx, Opx, Plagioclase) with sea water to form a new set of hydrous minerals (amphibole epidote and haematite (Fe2O3)). These minerals are then physically transported by the process of `sea-floor spreading' to zones of subduction where they pass back into the mantle. The water is supplied via hydrous convection cells which circulate within the cooling upper part of the ocean crust. The exit zones of the hydrous fluids are marked by the oft-publicised `black' and `white' smokers located on mid-ocean ridges.

The release of chemical energy and water and the formation of island arcs.

At depths of the order of a 100 km, the hydrous minerals produced by reaction of basaltic material with sea water at the ocean ridges undergo an exothermic dehydration reaction to form a high pressure anhydrous mineral (eclogite) assemblage (pyroxene (jadeite) + garnet) and a hydrous phase highly charged with metal ions. The hydrous fluid passes upwards into the overlying mantle, which as a consequence melts to produce an oxidized magma which rises to the surface to form the island arcs found adjacent to subduction zones.

Author: W.R. Church

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