Plate Tectonics - Formidable Forces
Plate tectonics is the fundamental mechanism that drives geological processes in the geosphere. Plate tectonic theory is based on an understanding of the Earth's internal structure, the different types of tectonic plates and plate boundaries, and the driving forces of plate movements. The occurrence of earthquakes and volcanoes, the distribution of different rock types, and the Rock Cycle, as well as the processes of mountain building, continental rifting and seafloor spreading, can be concisely explained by plate tectonic processes.
- Structure of the Earth
- Plate Tectonics and Earth Surface Processes
- Volcanoes in different Plate Tectonics Settings
Structure of the Earth
The Earth is an irregular sphere, with a radius that varies between 6,356 and 6,378 kilometres. This solid sphere is chemically divided into layers that become less dense from the centre towards the surface. Three main layers are recognised: the core (which comprises an Inner Core and an Outer Core), the mantle, and the crust. Each layer has a distinctive chemical composition, and a different density (Figure 1).
Figure 1: Generalised structure of the Earth.
Scientists infer that the dense core is primarily composed of the heavy elements iron and nickel. The outer core is made of molten iron, which produces the Earth's magnetic field.
Dynamic Structure of the Earth
The crust and upper part of the mantle of the Earth is further subdivided into the lithosphere and the asthenosphere (Figure 2). The slow movement of the lithospheric plates over the mobile asthenosphere is known as plate tectonics, a process that maintains the surface of the Earth in a dynamic and active state.
Figure 2: Generalised structure of the crust and upper mantle.
What drives Plate Tectonics?
Plate tectonics is driven by the internal heat energy of the Earth. This comprises the heat left over from the initial formation of the Earth, combined with heat from the decay of radioactive minerals contained in the rocks.
Figure 3: Mantle convection as the driving force for plate tectonics.
Types of Plate Boundary
There are three types of plate boundary: convergent, divergent, and transform plate boundaries.
Plate Tectonics and Earth Surface Processes
Plate tectonics is a process that maintains the surface of the Earth in a dynamic and active state. Consequently, it drives such processes as:
Within regions of continuous continental crust, the process of mantle upwelling leads to thinning of the crust and to the development of rift valleys. The Great African Rift Valley in Kenya is an example of this type of continent-continent breakup.
Figure 7: Development of a back-arc rift zone behind
Magma is derived from deep within the Earth by partial melting of the upper mantle or lower crust. It is composed of a high temperature mixture of silicate material (magmatic liquid), water, and dissolved gases. Magma may also contain crystals that formed during cooling of the magma, and rock fragments incorporated into the magma during its ascent towards the Earth's surface.
Solid mantle material rises close to the Earth's surface at divergent plate boundaries (Figure 8). This process is accompanied by a decrease of pressure, which lowers the melting temperature of the mantle material. Thus, when the solid mantle reaches a certain level, it begins to partially melt to form a magmatic liquid. Buoyancy causes the magmatic liquid to rise towards the Earth's surface, away from the zone of melting, leaving behind unmelted solid material. Partial melting, caused by the reduction in pressure associated with the upward movement of mantle, is termed decompression melting.
Figure 8: Plate tectonic settings.
Decompression melting is also responsible for the production of magma at hotspots (Figure 8). A hotspot is a location where an active convection system occurs within the mantle. At these locations, the solid mantle materials are hotter and less dense, and are moving upwards towards the crust in plumes.
Figure 9: Active volcanoes, plate tectonics and the“Ring of Fire”.
Volcanoes in different Plate Tectonics Settings
Magma erupted from a volcano is generally a mixture of three main components, crystals, magmatic liquid, and dissolved gases (mostly water vapour). Different types of volcanoes occurring in contrasting plate tectonic settings will exhibit varying degrees of explosivity depending upon the composition of the erupting magma, in particular the dissolved gas content. When a magma is erupted at the surface, any dissolved gas comes out of solution and is released in the form of gas bubbles.
Around the world, a complete spectrum of magma compositions can be found. These range from magmas rich in magnesium and iron aluminium silicate minerals (basalt), to those rich in sodium, potassium, and calcium aluminium silicate minerals (rhyolite), with a range of intermediate magmas occurring in between (e.g. andesite).
The magma in basaltic volcanoes is relatively hot, and flows easily. This allows any gas bubbles to escape, resulting in largely non-explosive eruptions. In contrast, the magma in rhyolitic volcanoes is relatively cooler and more viscous. Any gas bubbles in the rhyolitic magma have great difficulty escaping, and are explosively released, shattering the magma. Thus, rhyolitic volcanoes are more explosive than basaltic volcanoes, producing greater quantities of ash. Volcanoes that generate roughly equal proportions of lava and ash produce the characteristic stratovolcanoes that occur around the Pacific “Ring of Fire” (Figure 10).
Three common types of volcanoes that occur in different tectonic settings are shield volcanoes, stratovolcanoes, and caldera volcanoes.
Most basaltic eruptions in intraplate or mid-ocean spreading centre settings form low surface slope shield volcanoes. Typically, lava fountains from the vent in these eruptions (Figure 11). Large volumes of low viscosity basaltic magma pour out as rivers of lava. This style of eruption is characteristic of Hawaiian-type volcanoes.
Volcanoes associated with continental margin arc or island arc tectonic settings are dominantly andesitic or rhyolitic in composition. These volcanoes typically form stratovolcanoes (composite volcanoes), and the eruption style is generally more explosive than Hawaiian-type volcanoes. A typical stratovolcano has a steep cone shape, made up of alternating layers of ash and lava (Figure 12).
Depending upon the amount of dissolved gas contained in the erupting magma, the eruptions at stratovolcanoes may vary from minimally explosive (Strombolian-type) to highly explosive (Plinian-type). The greater the dissolved gas content, the more explosive an eruption will be, and the larger the volume of erupted volcanic ash.
Figure 10: Mount St. Helens volcano, USA (Photo courtesy of USGS/Cascades Volcano Observatory).
Figure 11: Kilauea volcano, Hawaii (Photo countesy of the USGS/Cascades Volcano observatory).
Rhyolitic volcanoes in convergent boundary tectonic settings may form caldera volcanoes. Caldera volcanoes are ones in which the top part of the volcano has been explosively destroyed following collapse into an underlying drained magma chamber (Figure 13). Caldera volcanoes are the result of very violent eruptions.
The volcanic rocks that occur in Hong Kong are predominantly of rhyolitic composition, consisting mainly of ash. These rocks were formed by violent eruptions of ash from caldera-type volcanoes. Ash clouds from caldera-type volcanoes are commonly so large, that they collapse under gravity and flow over the landscape like a fiery fluid.
(A) Explosive eruption partially empties the underlying magma chamber;
(B) The summit of the volcano collapses into the empty space left in the magma chamber, forming a caldera.
Hazards created by volcanoes can be divided into those that are produced directly from volcanic activity (primary hazards), and those that are produced indirectly from volcanic activity (secondary hazards) (Figure 12).
Primary hazards include: lava flows, pyroclastic flows, ash-falls, and gas emissions.
Secondary hazards include: lahars (mud flows), floods, fires, and tsunami (large sea waves).
Earthquakes are associated with all types of plate boundary. They result from the sudden release of energy that occurs when the stress that builds up between two adjacent, moving tectonic plates finally overcomes the friction that holds the two plates together (Figure 14).
At transform plate boundaries, such as the San Andreas Fault in California, lateral displacement between two adjacent plates may occur in sudden catastrophic movements that leads to splitting of the Earth's crust at the surface. These ruptures in rock are known as faults.
At divergent or convergent plate boundaries, the movement between the tectonic plates may occur at any depth below the Earth's surface. However, some earthquakes that occur in subduction zones may be associated with faults that reach the surface. In some cases, these can result in catastrophic vertical movements of the ground and the generation of tsunamis, such as the one that occurred following the December 2004 Indian Ocean earthquake.