Plate Tectonics

Plate Tectonic processes

Since the 1950s, several discoveries have led to a new understanding of how the Earth works. This includes Plate Tectonics, which explains the structure of the Earth's lithosphere (outer shell) and the forces that drive changes in its structure.

Tectonics comes from the Greek word tekton, meaning builder. The first scientist to propose that continents drift (a key to later Plate Tectonics) was the German meteorologist, astronomer and geophysicist, Alfred Wegener in 1912. Other important geologists who helped develop present theories were the two South Africans Alex du Toit and Lester King, and the Australian Samuel Carey. The theory of sea-floor spreading was outlined by Harry Hess of Princeton University, USA, and confirmed by F.J. Vine and D.H. Mathews from the UK.

Evidence for Plate Tectonics

When geologists in the late 1800s began to explore different areas of the Earth, they learnt that many stratigraphic successions had similar ock types, ages, fossils and depositional settings. After the 1950s, geologists realised that this also applied to the oldest parts on Earth that formed the stable cratons and metamorphic rock belts. Evidence of rocks forming in the same place and time and then being widely split apart, led to the ideas of cycles of super continents and new oceans. Evidence for ocean-spreading episodes includes symmetrical magnetic anomalies parallel to mid-ocean ridges, zones of large earthquakes, active volcanoes at some ocean margins, and the distribution of animals in the world (e.g. the restriction of certain mammal groups to Australia).

Based on mechanical properties, especially resistance to shearing force, the Earth can be subdivided into three main outer layers:

• Lithosphere: is the colder rigid, more resistant outer shell of the Earth. If it wasn't rigid, then mountains would simply level themselves out. It is made of both the crust and the uppermost part of the upper mantle. Under oceanic crust it extends down to 70 km, while under continental crust it extends down to 150 km. The word lithos is Greek for rock.
• Asthenosphere: this starts with a sharp decrease in shear strength known as the Low Velocity Zone (LVZ). The lower strength in Earth's mechanical properties means that this layer can flow more under stress. Within the asthenosphere, the Low Velocity Zone extends down to 250 km, below which shear strength increases progressively as a result of compression, until at 400 km depth there is a sudden rapid increase. The base of the asthenosphere is defined by the deepest known earthquakes (approximately 700 km) and is where descending lithospheric plates most likely bottom. The word astheno comes from the Greek combination of a - (means without) and stheno (which means strength).
• Mesosphere: Below the asthenosphere is the mesosphere which extends down to the outer core (at a depth of 2900 km). The mesosphere is composed of strong dense material and shear wave velocities increase rapidly with depth.

What are plates?

The lithosphere consists of rigid plates on a non-rigid subsurface (from approximately 70 km - 700 km in depth). Tectonic processes mostly take place at the plate edges. A plate moves as a single entity along the surface of the Earth over a plastic mantle. There are two types of plate:

• Oceanic plates form at the mid-ocean ridges. They thicken as they move away (at about 1 km for every million years) and have a basaltic composition.
• Continental plates are much more complex as their rocks vary in composition and thickness.

The differences between oceanic and continental plates go down to a few hundred kilometres.

The plates move under horizontal forces that cause them to collide, combine, break up, or, in the case of oceanic plates, to be drawn down (subducted). The forces that act on or near mid-oceanic ridges are called ridge-push force. It is driven by upwelling magmas from mantle regions that drive the plates away on either side of the ridge. Another driving force (particularly where oceanic plates are being subducted beneath continental plates) is slab-pull. This is produced by oceanic plates being significantly cooler and hence denser than the underlying mantle. They get pulled below the adjoining continental plate, dragging the remaining ocean plate after it. At the base of the plate, drag occurs as it moves over the underlying mantle.

Continental land masses and major ocean floors contrast strongly. For example, Mount Everest, the highest bit of continent, is 8 848 m, yet the Marianas Trench with a depth of 10 912 m, is vertically greater.

Oceanic crust

The oceans cover over 70% of the surface of the Earth and occupy large flat-bottomed basins traversed by 2 km high oceanic ridge systems. On their active margins, they have trenches adjacent to destructive plate margins (e.g along the Pacific Ocean) while on passive margins they have broad continental slopes up to 200 km wide (e.g along the Atlantic Ocean).

Because oceanic crust spreads away from the mid-oceanic ridges, the oldest oceanic floor is found at the ocean margins. The oldest oceanic rocks are only 200 million years old but most are less than 100 million years old. This comes from older oceanic crust being subducted at destructive plate margins. In rare cases, some older ocean floor was pushed up (obducted) onto the adjacent continental crust forming alpine-type serpentinite and ophiolite belts (e.g. in the Alps across Europe).

Continental crust

In contrast to oceanic crust, continental crust has constantly formed throughout much of the Earth's history, with the oldest rocks from Greenland dating back to 4.2 billion years, and the oldest known mineral, a zircon from the Pilbara region of Western Australia dating at 4.5 billion years. Three main types of continental crust are:

• Exposed continental shields (or cratons) of mainly Precambrian (older than 570 million years) crystalline igneous and high-grade (formed at high temperatures and pressures) metamorphic rocks.
• Continental platforms mainly of gently folded younger low-grade (formed at low temperatures) metamorphic rocks overlying Precambrian basement rocks.
• Young, mainly Cenozoic (less than 65 million years old) mountain belts that contain deformed metamorphic rocks and later igneous (both volcanic and plutonic) rocks.

Heat in the Earth

Heat flows outward from the interior of the Earth. Most of the heat generated at present results from decay of long-lived radioactive isotopes (especially of the elements potassium, uranium and thorium). However, early on (greater than four billion years ago), some of the heat may have come from decay of short-lived radioactive isotopes, meteorite impacts and gravitational compaction.

As potassium, uranium and thorium are concentrated in the upper continental crust, the heat produced by their decay accounts for some 40% of the average continental heat flow. In contrast, heat flow within oceanic regions largely comes from the mantle (i.e. through the magmatism along mid-oceanic ridges).

Heat flow measurements are given as milliwatts per square metre (mWm2) and at the surface of the Earth vary from 30 to over 200, with an average of 80 mWm2. Heat flow is highest in volcanic areas, such as mid-oceanic ridges (especially in areas of rapid sea-floor spreading) and young volcanic areas on continents (e.g. the East African Rift). The lowest heat flows occur on old ocean floor and trenches, and in rapidly subsiding basins on continental crust. Continents have an average heat flow of 55, whereas oceans have an average heat flow of 95. Over 60% of heat flow loss from the Earth escapes from new oceanic crust at mid-oceanic ridges.

Convection

Most heat in the Earth is transferred by convection. This comes about when heating from below decreases the density so that heated material rises. When it reaches the top of the heated column the heat spreads sideways and cooling occurs. On cooling, density increases once more and the material sinks. The materials that have the greatest influence on convection within the Earth are sea-water, groundwater in the crust, and magmas.

The Geothermal Gradient

As heat wells up from the interior of the Earth, the temperatures must increase with depth. This rate of heat flow with depth is the Geothermal Gradient. It is usually calculated from measurements in mines and drill holes and ranges from 8° C / km up to 65° C / km but average some 35° C / km in continental crust. These figures only apply to the top part of the Earth as lower down, calculations become complex because of changes in composition and physical properties. Geothermal gradients are lower under old cratonic regions and higher in fold belts (thicker crust) and volcanic regions.

Pressure within the Earth

Pressure within the Earth increases with depth. The standard unit for pressure is the Pascal (Pa) which is too small for geological purposes. Instead, bars are used, with one bar being almost equal to one atmosphere ( 1 atm = 101 325 Pa). A common measurement of pressure within the Earth is the kilobar (kb or 1000 bars).

Pressure comes from the weight of overlying rocks. As the density of crustal rocks averages 2.8 gm / cm3, so pressure within them increases downwards by 0.28 kb / km. Mantle rocks have higher density, which further increases pressure downwards.

Isostasy

Although relatively rigid and strong, the outer shell of the Earth (lithosphere) cannot support the stresses imposed upon it by either the positive weight of a mountain belt or the more negative weight of an ocean basin. For these features to exist, a compensating mechanism is required. The principle of isostasy is that below the depth of compensation, the pressures from overlying materials are equal everywhere.

Isostasy was recognised by French scientists working in the Peruvian Andes during the eighteenth century while trying to determine the shape of the Earth. They had to make a correction to their vertical measurements to take into account the horizontal deflection caused by the gravitational attraction of the Andes. However, they also found this correction was less than the calculated deflection. This discrepancy (called Bouguer anomalies) arises from a negative mass anomaly below the Andes which compensates the positive mass of the mountains.

Similar observations were made in the Himalayas in the nineteenth century and this subsurface compensation has been confirmed by variations in the Earth's gravitational field over broad regions. Anomalies are generally negative over elevated continental areas, and positive over ocean basins.

Mid-oceanic ridges

These transect the major ocean basins of the world and are active sites for generating new crust from magmas rising from the mantle below these linear fractures. On a bathymetric map, they form topographic highs within the deep oceans. Most volcanism on Earth occurs here and they are also sites of copper-zinc-lead-gold mineralisation. Because of complex interactions of the different plates, spreading ridges have different spreading rates, as do the plates themselves. In general, mid-oceanic ridges divide into those with high spreading rates, and those with low spreading rates. These differences in the rates of magma rise cause unique patterns of rock types.

Subduction zones

As all the new crust created at the mid-oceanic spreading ridges does not seem to expand the Earth, the older crust must somehow be returned back into the mantle. This occurs at the plate edges along subduction zones. As oceanic crust cools down, it becomes denser so that on its boundary with adjacent continental crust, it sags below the continental crust under the push from the spreading ridges. Subduction zones also occur between oceanic plates, particularly oceanic plates of differing age. In some regions behind the oceanic plate, spreading starts when new magmas well up from the subduction of the down-going oceanic plate to give back-arc basins. The back-arc basins themselves influence the down-going plate and deflect it towards the back-arc spreading. Down-going oceanic crust contains hydrous minerals (and some trapped oceanic water), and these are released at depth. This released water initiates more melting so that new magmas rise up through the plate and produce large volcanoes along an arc, forming island arcs.

Earthquakes and large faults

Robert Hooke in 1705 recognised that earthquakes came from land movements and Mallet (1810-1881) realised earthquake damage resulted from waves (called seismic waves) generated by the movement.

A wave propagates strain through a material, which then tends to become restored to its original state (Newton's Law of Equilibrium in the Universe). Earthquakes simply release strain built-up within the Earth, from plate tectonic movement or movements of molten rock.

Although seismic waves spread out from disturbances of the ground, only earthquakes and nuclear explosions are large enough sources of waves to be detected around the world.

There are two types of waves that can be used to study the Earth's interior:

• P-waves are Primary or Compressional Waves. Faster waves that have speeds of 5 km/s at the top of the crust, 8 km/s at the top of the mantle, and 14 km/s at the bottom of the mantle. They can travel through liquids and so can pass through Earth's liquid outer core.
• S-waves are Shear or Transverse waves. Slower than P-waves, they cannot travel through liquids and do not pass through the Earth's core.

Movement of Seismic Waves through the Earth

When seismic waves are generated, they spread out spherically but become distorted by regions of different density or elastic properties. The faster P-waves are the first to arrive at recording stations and the time delays between P- and S-wave arrivals help determine where the earthquake originated. It takes several such seismic stations to determine the location and size of an earthquake accurately.

When two tectonic plates converge, huge frictional forces build up with time until at a critical point rupturing occurs, producing a fault and an earthquake. Earthquakes within the ocean basins produce tsunamis but those on the continents also devastate by releasing energy in rebounding waves. The largest earthquakes are often along the collision zone of two continental plates. Most large earthquakes around the world occur along plate margins, particularly around the edges of the Pacific Plate. A well-known fault is the San Andreas Fault of the western USA.

Hotspots

Many of the world's volcanoes occur along the plate margins (e.g. Pacific Rim of Fire), but some occur wholly in isolated pockets within plates (e.g. Yellowstone National Park in the USA). The latter form from hotspots of upwelling magma that pierce the crust, forming voluminous volcanic outpourings. These stationary upwellings underlie the plate movements, so they form a line of volcanoes that migrate in the opposite direction to the plate movement. Perhaps the best known hotspot chain is the Hawaiian Islands, in which the older volcanoes finally disappear below sea level as subsided mounts. Some of the hotspots seem to come from the Earth's core, others from less deep layers in the mantle. At present about 50 hot spots are known on Earth. Hotspots have erupted along the margin of eastern Australian and in the Tasman Sea.

Tectonics and mineral deposits

Because plate tectonics is a large-scale process that transfers heat, water and magmas, it underpins the formation of many mineral deposits. Since these deposits form in special plate tectonic settings, we can use our knowledge of present plate tectonic processes to search for deposits formed in the past in similar tectonic settings.