Practice report for question two, Unit 4 Tectonics

Explain why Tectonic processes produce a variety of contrasting landscapes. 70mrks

Introduction

Tectonic processes are the varying types of events that are caused by movement of magma in the mantle and outer core. As a result of heat from the inner core, many eras ago the Earth’s crust split into plates driven in different directions by the resulting convection currents.  All landscapes and landforms are affected by past or present tectonic processes somehow. Many of these examples began on plate boundaries, in magma chambers within the ground or are rare results of tectonic hotspots. The size, shape and material of the resulting landscapes vary greatly depending on the type of tectonic process and the scale of the event. In this essay I will be examining the differences between these processes and their resulting landscapes, comparing the above details of each.

Seismic Activity

Earthquakes commonly occur on conservative plate boundaries but epicentres can also be found in areas of great pressure such as subduction zones on destructive boundaries or in the central vent of an explosive volcano. Seismic activity rarely has a very high impact on the landscape though as not being igneous activity it does not create dramatic new landforms. However in extreme cases, they do have the potential to change landscapes entirely. In Sichuan, China, May 2008 an Earthquake measured 7.8 on the Richter scale, so powerful that it was felt in Bangkok around 2000km from the epicentre. Immediately the landscape was only changed by collapsed buildings, however when the earthquake hit Beichuan city the secondary impacts were landslides that dammed river valleys in the basin causing extensive flooding and fast rising lakes in place of the river. Man-made dams and reservoirs were also affected, 30 dams were damaged and several reservoirs burst altogether causing low lands to flood. As a result of the earthquake new lakes have been formed, valleys filled and mountainsides destroyed, hence changing the shape of the landscape.

This earthquake was caused by the friction between the Indian plate and the Eurasian plate on a destructive boundary. The epicentre was caused by a slip in the mid margin, known as the Yingxiu-Beichuan fracture. As a result of the pressure Fold Mountains have been formed and are vulnerable to subsequent landslides. This information has been adapted from the Times 13^th May, 2008 and geology.com.

Intrusive Landforms

As shown in Sichuan, tectonic processes don’t necessarily have to involve lava to create new landscapes. Intrusive landforms can also change the shape of the landscape to varying levels of extent.  Dartmoor is the largest outcrop of granite in Britain with and area of 275mi and is proposed to be part of the exposed Cornubian batholith which is also cause to many other outcrops, some of which are found in Bodmin nearby or the isles of Scilly. Dartmoor is 17km thick in places but only a few hundred metres are exposed above ground to make wide flat moor lands and dense, impermeable granite tors.

The batholith was formed in the early Permian period by the pressure of a magma plume beneath the crust. As the pressure was so powerful the magma pushed into gaps in the crust to create a magma chamber. From here the magma could not find a way to the surface and eventually after the crust had moved away from the plume the magma began to cool. As the magma cooled underground the process was slow so granite rocks with large crystals were able to form, making a batholith. As the country rock above the batholith had changed over time water seeped through to the batholith and weathered the granite causing it to crack, eventually the country rock eroded away enough to expose the tors and make a new landscapes, some areas of the batholith were either eroded or are still covered meaning several different landforms appear separate but are actually part of the same batholith.

However not all intrusive landforms have any effect on the landscape at all. From recordings of underground granite, less than 20% of the Cornubian batholith’s surface is estimated to be exposed. In order to prevent a magma chamber from becoming a volcano the above country rock must be strong enough to withstand the pressure of the magma over 1000c in temperature. Batholiths can only ever create new landscapes with severe erosion over millions of years. Many batholiths remain underground without exposure. This information was found on Wikipedia and Geocases2.co.uk.

There are other types of intrusive landforms many of which don’t have the potential to create whole landscapes. An example includes the volcanic plug on which sits Edinburgh Castle. This is only around 50m higher than the land now and only large enough to accommodate the castle, creating a round hard granite mound that sits near the city centre. The plug used to be the central vent of a volcano that when made extinct cooled inside the volcano. As time passed the outer volcano eroded and sediment was deposited evenly to leave a thin column of granite. This has a rather different effect from the hundreds of square miles of granite in Dartmoor but shows the potential for volcanoes to create new landforms even when they are extinct. Information found from Geocases2.co.uk.

There are also sills and dykes which act as extensions of batholiths. When magma chambers are formed the magma pushes through the country rock in an attempt to reach the surface. Dykes push vertically through faults and folds and sills push through bedding planes horizontally. These cool in a similar way to the batholith to form rocks with large crystals that mix with the surrounding country rocks to sometimes form different rock types. Also like the batholiths these rocks are exposed when the above country rocks are eroded. An example of this is the Whin Sill in northern England on which Hadrian’s Wall was built. This sill appears on the landscape like a thin long series of hills covering the thin band of Dolerite in a wall structure beneath. The hills are significantly high but no wider than 60ms and stretch over 20km. As the sill is the result of more than one batholith the Whin Sill creates multiple paths and features including one of Britain’s most famous waterfalls, the High Force.  Though smaller than batholiths sills and dykes have the potential to create varied landforms within the landscape.

Volcanic Landscapes

However when considering Dartmoor and the Whin Sill in comparison to all tectonic made landscapes, they are very small. The Indian Deccan traps are also large land masses of igneous rock only instead of the 275 square miles of Dartmoor; the Deccan traps cover 200,000 square miles, engulfing western central India. The traps are also as much as 2kms thick, so large they are theorised to have changed the climate of the entire planet. The traps are made from flat lying flood basalts and have created giant soft sloped mountains with edges with a step like appearance as the lava cooled in layers.

 The Deccan traps were created by a constructive tectonic plate boundary between 60-65million years ago. As the plates tore apart the crust faulted leading to multiple fissure eruptions from the mantle. As the faults were large and created without intense pressure the lava erupted in the form of flood basalts. The lava was pure, hot and non-viscous, allowing it to disperse over the large spaces of North West India until the fissures were blocked by the volcanic rocks.

As the lava cooled quickly outside of the mantle it formed small grained basalts that have been eroded easier than coarse grained granites formed intrusively. The Deccan traps were predicted to cover an area almost four times as large before erosion which suggests that even though they may create a larger landform than Dartmoor now, the impact on the landscape may be less over time as the resistant granites of the Cornubian batholith are likely to last over a longer time than the flood basalts of the Deccan Traps. This information was found on the website for the Oregon State of Education: volcanoes.

However there are varying types of volcanic activity to be considered. Hawaii is a series of volcanic islands in the Northern Pacific ocean stretching in an arc shape over 5,800kms long of at least 129 volcanoes. Mauna Loa is an example of these volcanic islands and covers around 90km2. This makes Mauna Loa the largest current shield volcano in the world and it rises as far as 17kms above sea level. This information was found on volcanolive.com.

The landscapes of Hawaii were created by the pressure of a magma plume and weaker oceanic crust. The magma plume was proposed by Wilson to be a fixed point of intense heat energy in the mantle. As heat rises extreme pressure is pushing up through the crust until it breaks through to erupt in pure basaltic lava floods, similar to those that formed the Deccan traps. However these eruptions form a volcano as they occur through a central vent. As the magma is non-viscous and pure it builds on the sea floor to form a wide smooth sloped shield shaped volcano with wide flat landscapes. As volcanoes move away from the hotspot they also decline into new landscapes as they erode. These are atolls, submerged islands that often host coral reefs. Eventually each island declines into a sea mount (some in Hawaii as old as 47million years) meaning that like the Deccan traps, each landform has no lasting impact on the landscape. However the hotspot is still active meaning it continues to create new landforms unlike the declining Deccan traps. The information on the formation of hotspots was found on a Geo Fact sheet and Wikipedia.

Continental Rifting

Tectonics can further create new landscapes through the effects of continental rifting. A famous example of this is the African Great Rift Valley which stretches over 5,000kms long. The rift valley is a dramatic stretch in which steep sided hills drop into deep basins that are either filled by lakes, the Red Sea or remain as land. Mountains also line the basin with both shield and composite volcano types, examples include Mt Kilimanjaro and Mt Kenya (steep sided slopes).  

Eventually all basins will fill with the Red Sea as the Danakil depression is 100ms below sea level. The total area of the basin is over 92,000km2 and annual surface run off is less than 1km3. Combining the hot, dry African climate with an enclosed basin, with high levels of volcanic activity, the Danakil desert is the hottest place on Earth and irrigation potential is zero.

The Great Rift Valley was created when the African and Arabian plates parted on a divergent boundary 35million years ago. As the plates were torn apart a section of land in between the plates sank downwards to create a new basin, lava then joined the plates back together forming the new valley sides. As the basins are left on weak crust shield volcanoes are common here. In addition to this there is also a magma plume causing the hills to bulge and create mountains. These processes formed the contrasting hills and basins of the Great Rift Valley. This information was found in the article East Africa’s Great Rift Valley: A Complex Rift System and a blog on scientificamerican.com.

Conclusion

In conclusion there are many varying landscapes produced by different tectonic processes. As much as the landscapes created will be different between the tectonic influences of a batholith and a volcano different volcanoes in different locations will also produce contrasting landscapes. In the Great Rift Valley there are a variety of contrasting landscapes created by multiple tectonic processes; this is also possible as one area isn’t necessarily limited to one example of tectonic activity.