Why Tectonic Processes Produce a Variety of Contrasting Landscapes
To answer this question, I will analyse the formation of various intrusive (plutonic), extrusive (volcanic), and seismic landforms, using the theory of plate tectonics. My justification for the use of this currently ubiquitously accepted theory is the abundance of evidence, including the discovery of identical rock types across oceans, e.g. in Wales and the northeast coast of the USA, the existence of coal in Antarctica, which is unlikely to have formed at its current latitude, as it requires tropical conditions and extensive vegetation, and the discovery of identical fossils in various continents, such as Glossopteris across the southern hemisphere.
Alfred Wegener presented his idea of continental drift in 1912, but was met with criticism, due to his lack of a mechanism for the movement. In the 1960s, linear magnetic anomaly patterns were found in the Atlantic Ocean; this record of the Earth's periodically changing magnetic field in magnetite could only be explained by sea-floor spreading – a product of continental drift. The mechanism, convection currents of magma within the Earth's mantle, was proposed after the discovery of a low velocity layer, deep within the Earth; seismic waves change velocity at the Mohorovičić discontinuity, inferring a layer of different density.
Intrusive landforms are formed by magma rising toward the surface, but cooling and forming solid igneous landforms, before reaching the surface. The overlying country rock is often less resistant to erosion, and can lead to the landforms becoming exposed.
Extrusive landforms are similar, with the distinction of the magma solidifying after reaching the surface.
Seismic landforms are the result of plate movement, and vary depending on the type of boundary.
- The Isle of Arran, Scotland (intrusive landforms, e.g. batholith, sills, and dykes).
- The Mid-Atlantic Ridge (oceanic constructive plate boundary).
- The East African Rift Valley (continental constructive plate boundary).
- Deccan Plateau, Northern India (flood basalt plateau).
- Edexcel A2 Geography (Dunn et al, 2009).
- Advanced Geography (Nagle, 2000).
- Global Geomorphology (Summerfield, 1991).
- United States Geological Survey.
- 10 Things You Didn't Know About Volcanoes (BBC documentary, 2006).
- Geography Factsheet 164, Simon Ross, Curriculum Press.
Source 1 is extremely useful, as it is aimed at people with my level of knowledge, and all content is likely to be highly relevant to this topic. However, it may fail to go into much background detail. Conversely, source 3 has an abundance of detail, but the concepts described are aimed at university level students, and so could be misinterpreted, thus reducing the validity of my conclusions. Source 4 is updated regularly, whereas source 3, having been produced around 25 years ago, could now contain erroneous information.
Intrusive landforms result from the solidification of magma underground. The Isle of Arran in Scotland displays many examples; dykes in the north radiate from the underlying granitic batholith, and appear much higher than the softer sedimentary rock. Sills in the south have also become exposed, via weathering of the aureole, and have formed beaches, where sediment has become trapped at their base, pushed by the sea.
Dykes and batholiths are discordant, whilst sills and laccoliths (dome-shaped landforms, similar to sills, which can push the overlying ground up, in a similar way to the batholith) are concordant, exploiting lines of weakness between layers of country rock, i.e. discordant landforms cut through multiple layers.
Plutons include batholiths (over 100 square kilometres of surface exposure) and stocks (less than 100 square kilometres of surface exposure). Batholiths are fundamental to orogenesis, and can support fold mountains, such as the Himalayas.
On the Isle of Arran, the sills are more resistant to erosion than the surrounding rock, and have formed cliffs, where the overlying country rock has been worn away; Drumadoon Point has a 50 metre sill cliff.
Extrusive landforms result from magma solidifying on the surface of Earth's crust. The type of magma is determined by the plate boundary; destructive boundaries tend to form andesitic lava, as magma is forced to rise through continental crust, which is high in silica; the added silica makes the magma more viscous, and so it flows a shorter distance before cooling and solidifying, forming steep domes.
As shown above, a variety of volcano types exist at plate boundaries with disparate magmas. Magma ranges from basic to acidic, depending largely on the silica content, as shown below.
Silica content (%)
Constructive boundaries and oceanic hotspots
Destructive boundaries and continental hotspots
This table shows that temperature and gas content are also important in determining viscosity, which in turn influences the landforms created.
Both a high silica content and a high temperature prevent gas from escaping; silica forms long molecular chains, which get tangled, increasing the viscosity. As the magma rises through the lithosphere, gas bubbles within the magma, consisting of mostly water vapour and carbon dioxide, cannot escape, unlike in basaltic magma, where they can easily rise and create fire plumes at the crater. In rhyolitic magma, as the pressure drops to 101 kPa (atmospheric pressure), the bubbles of gas expand rapidly, creating very explosive eruptions.
At constructive boundaries, the low silica content is due to magma not being forced through continental plates, but rather between them. The high temperature means that atoms possess a high amount of kinetic energy, so they spread out, and make the magma less dense, thereby reducing its viscosity, and leading to effusive eruptions, which allow wide and low shield volcanoes to form.
However, the presence of water can cause explosive eruptions at both constructive and destructive plate boundaries. These phreatic eruptions can blow part of a cone from a volcano, such as Mount Pinatubo in the Philippines; this forms a caldera.
Lava dome volcanoes can form from viscous lava; they are occasionally found within, or on, stratovolcanoes, e.g. in Mount St. Helens' cone, USA.
Mount St. Helens
Cinder cones, along with lava domes, can form parasitic (secondary) cones on larger volcanoes. Cinder cones, however, are formed by tephra, most commonly lapilli. The types of pyroclast (solid material ejected from an eruption) are shown below.
Ash ----------------------- < 2mm
Lapilli --------------------- 2-64mm
Blocks and bombs ---- > 64mm
Cinder cones therefore contain small, unconsolidated particles, making them unstable, but steep; as they are unstable, they are rarely over 700 metres tall.
Basaltic magma can form vast flood basalt plateaus, such as the Deccan Plateau in India, as the lava is not very viscous. Rivers can cut into the relatively soft basalt to form gorges; the Manjira River has cut into the Deccan Plateau.
Seismic landforms include horst and graben; these are primarily formed by faulting at constructive and collisional boundaries. The type of fault determines the presence of certain landforms; conservative boundaries do not produce horst and graben, as there is no tensional or compressional force. The three types of fault are normal, reverse, and strike-slip, as shown below. A thrust fault is similar to a reverse fault, but with the dip of the fault plane at less than 45°.
The two blocks are called the hanging and foot walls; at constructive boundaries, the hanging wall slumps, and is generally the graben, with the foot wall, rising relative to the graben, becoming the horst. A cliff can be formed at both normal and reverse faults, called a fault scarp; these do not form at strike-slip boundaries, giving an explicit example of a tectonic process influencing the landforms created. However, faults are rarely of a single type, e.g. the San Andreas strike-slip fault in northwest USA is in fact 5% reverse faulting, and so some fault scarping can occur.
At destructive boundaries, the subducted plate is the one with the higher density. Therefore, oceanic plates almost always subduct beneath continental plates, since they have a density of around 3,000 kg/m3, compared to around 2,700 kg/m3 for continental crust. The top of the oceanic crust may be scraped off, forming an accretionary prism, also known as an accretionary wedge, such as the Nankai Trough, in Japan.
The trench formed can be vast, such as the Mariana trench, which was formed by the Pacific Plate subducting beneath the Philippine Sea Plate.
My analysis is based on the assumption that the theory of plate tectonics is correct; whilst there is an abundance of evidence supporting the theory, new technology in the future could disprove it, rendering my report redundant.
The analysis section of this report found that the type of extrusive landforms created, i.e. volcanoes, is largely determined by the viscosity of the magma. This, in turn, is influenced by the silica content, and therefore the plate boundary setting. However, gas content and temperature also play an important role in determining viscosity. Beyond all of these tectonic factors, the most significant influence may be from the existing landforms themselves, as shown by the side-by-side existence of pahoehoe and a'a lava in Hawaii, which have no difference in chemical composition.
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