PLATE TECTONICS

1. PLATE TECTONICS

2. The measurement of seismic waves passing through the Earth, which has a radius of about 6500 km, indicates that the Earth is made up of : 1) a partly molten core composed largely of iron; 2) a mantle, largely composed of oxygen, magnesium and silicon in the ratio of 4:2:1, divided into two shells, an inner shell called the asthenosphere, and an outer shell called the lithosphere; 3) an outer crust composed of two components, one represented by the sea floors and the other by the continents; 4) a discontinuous hydrosphere and polar ice cap; and 5) a continuous atmosphere. These constitute the main material or chemical RESERVOIRS of the Earth. The boundaries between the reservoirs are relatively sharp but the reservoirs themselves may be heterogeneous in composition. To understand how material and energy are transferred between these reservoirs it is necessary to first grasp the concepts of material creep, thermal convection, and pressure-release melting.

4. Material Creep - the earth seems to be a very solid and elastic body when subjected to short term stresses, but when the stresses are imposed over long periods of time it behaves more like a plastic or viscous material capable of flowing like a thick liquid - and the higher the temperature, the greater the propensity of the material to behave as a plastic material. This kind of time-dependent deformation is known as creep.

6. Thermal convection - in solid material heat is transferred from regions of high temperature to regions of low temperature by the process of thermal conduction. Heat can also be transferred by the process of radiation, as in the case of the heat we receive from the Sun, or by convection, as in the case of rising hot air. All three processes are involved in the transfer of heat from the interior parts of the Earth towards the surface but, surprisingly, the process of thermal convection is the most important.

7. Three ways by which heat is transferred

8. 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.

9. The contradictory effect of temperature and pressure on the melting of rock material

10. How do these three conceptshelp us explain the operation of the Earth? Well, the temperature of the asthenospheric mantle reservoir is increased by heat transferred from the molten coreand 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, andbecause 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.

11. 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.

12.     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.

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

14. Nature of the Atlantic oceanic crust based on seismic experiments

15. Drilling of the Pacific sea floor demonstrated that the age of the oceanic volcanic rocks increased westwards from the East Pacific Rise towards the western reaches of the Pacific.

16.     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.

17. Earthquakes are only observed in the red segment of the transform fault, where the crustal sections on either side of the fault are moving in opposite directions.

18.     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.

21. Distribution of island arcs around the Pacific ocean; note the lack of arcs in the Atlantic ocean other than in the Pacific extrusion zones represented by the Antilles and Scotia arcs.

22. Islands arcs may form on oceanic crust (Marianas) or on pre-existing continental crust (Japan, Andes). As you will note on the following two slides, continental crust becomes progressively thickened above subduction zones. Some arcs such as Japan and the Marianas separate from the continents on which they were initiated, and ‘float’ eastwards into the Pacific. Where the arcs become totally separated, new oceanic crust forms between the arc and its source area.

26. Plate Tectonic Processes • The formation of ocean crust at mid-ocean ridges is balanced by the destruction of oceanic crust at subduction zones

27. 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).

28. The mid-ocean ridges are displaced by -east-west trending transform faults

29. Continents may aggregate to form ‘Supercontinents’ which may in time break up to form a dispersed set of smaller continental fragments. The latter may then reassemble to form a new supercontinent but with the fragments arranged in a new pattern.

32.     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.

34. 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.

35. The following two figures illustrate the vastly different stories represented by the physiographic (surface) and geologic maps of North America.

38. The complex nature of the geology of Australia is easily discernable in terms of the variable magnetic nature of the rocks.

40. The geologic history of North America dates back to almost 4 billion years ago and records a complicated history of oceanic consumption, island arc development, and continental collision.

42. The line marked Iapetus Suture on the following geological map of Newfoundland represents the collisional boundary between North America and Europe about 400-300 million years ago. Geologically speaking south-east Newfoundland is more akin to Europe than it is to North America.

44. The yellow area marks the newly developing Atlantic ocean during the Jurassic; the green line is the line of closure of the earlier Iapetus ocean separating North America and Europe.

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

46.     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.

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

48.     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.