Oceanic landforms
The earth’s landscape varies extensively around the planet. Abyssal plains cover the majority of the ocean floor at depths of 6,500 feet below sea level and are the most featureless landscape on the planet. They are formed when new ocean crust is produced by sea floor spreading; the new crust gradually gets overlain with sediment which eventually produces vast flat areas of sea floor, they can extend for thousands of kilometres and 1000km2 only tend to vary in slope gradient by less than one foot(The Blue Planet BBC, 2005). The largest example of an abyssal plane is the ocean floor of the Atlantic (The Geological Society; Lyell collection) which extends for 900,000km2.
At the centre of these abyssal planes are mid-ocean ridges. Mid ocean ridges are formed when the convection current in the mantle push up the asthenosphere and cause crustal thinning to occur within a plate which then splits the plate in half forcing the two sides of the ridge to diverge. This process is known as the Wilson cycle and can be seen at different stages around the planet: The East-African Rift Valley is currently in the embryonic stages of the cycle; the Red Sea is in the juvenile stage; the mid-Atlantic ridge is in the mature stage and the Pacific ring of fire is the declining stage. This sea floor spreading is part of the plate tectonics theory discussed previously and is the reason for the supercontinent cycle. This process has resulted in the largest underwater mountain chain on the planet; the mid-Atlantic ridge. The mid-Atlantic ridge stretches across 16,000km separating the North American plate and the Eurasian plate and spanning the entire length of the Atlantic Ocean. Iceland is one of the few places where the mountain range reaches above sea level. The island (and the whole of the mountain range) was formed by the active volcanism along the ridge where the magma rising to the spreading centre caused basaltic lava to erupt regularly giving birth to active volcanos and consequently, Iceland.
Seafloor spreading however isn’t the only way mountain ranges can be formed on the sea floor. A mantle plume in the centre of the pacific plate combined with the movement of the Pacific plate over the hot spot has given rise to a chain of extinct volcanos called the Hawaiian-Emperor Seamount Chain. The chain was formed over millions of years as the pacific plate has been driven by plate tectonics at the East Pacific Rise diverges, forcing the pacific plate to travel over the mantle plume which remains stationary in the mantle as a constant stream of superheated magma. As the pacific plate has moved over the hotspot, gradually new islands and volcanos have been formed directly above the mantle plume as the older volcanos have drifted too far from the source of magma and become extinct.
Continental landforms
Where the convection currents drag the earth’s tectonic plates down into the mantle, the older and therefore colder and denser oceanic crust begins to subduct beneath either the more buoyant continental crust or the less dense oceanic crust. At the point where the two plates make contact the downward flexure forms a trough, known as an ocean trench (geology.com). The Mariana Trench is the deepest ocean trench on planet earth stretching 2,550 km and is only 69km wide. It marks the convergent boundary where the Pacific plate subducts beneath the Mariana plate. The trench reaches depths of down to 35,840 feet below sea level at Challenger Deep, which is the deepest part of the ocean. This subduction not only causes huge crevasses in the ocean floor but also shapes the land above sea level. Where the denser plate subducts it undergoes partial melting, where some of the plate is heated and rises to the surface directly above the site of melting. This hot magma rises to the surface of the sea floor and pushes its way through the crust to form a volcano. These underwater volcanos become larger over time and form a volcanic island arc. Examples of island arcs include Japan and the Caribbean islands.
Where two continental plates converge fold-mountains are produced. Continental crust contains more silica rich minerals such as quartz and feldspars than oceanic crust and therefore neither subduct. As the two plates collide, the rocks get forced upwards causing them to undergo compressional stresses and fold/crumple under the immense pressure; as the pressure is applied over millions of years the folding and crumpling builds up to form mountain ranges such as the Himalayas. The Himalayas began to be formed when the Indian plate collided with the Eurasian plate 45 million years ago. 225 million years ago the India what a solitary island situated off the coast of Australia and the Tethys Ocean separated it from the Asian continent. India began to travel northwards due to mantle convection after the split of Pangea. About 80 million years ago India was located ~6,400km south of Asia, moving northward at a rate of about 9 meters per century (Two Continents Collide USGS). When India collided with Asia it’s northward progress slowed by about half; the associated decrease in plate movement is thought to mark the beginning of the rapid uplift of the Himalayas.
Conservative boundaries are where two tectonic plates move past each other and neither plates are destroyed. Where these boundaries are visible through continents they produce a large visible tectonic fault line, which characterised the landscape by causing the boundary where the plates meet to be uplifted and a trench to form through the middle of the uplift. The most famous place to see this is along the San Andreas Fault in California. The San Andrea's fault extends for roughly 1,300km and is caused where the North American plate is moving northwards at a relatively faster rate than the Juan de Fuca plate.
Plate tectonics can be more complex than where just two plates meet; 90 million years ago the African plate began to subduct beneath the Eurasian plate at the same time as the Arabian plate was moving past them this contrast in relative movement of the three plates formed the island Cyprus to be formed from a mixture of compressional and shear stresses forcing part of the sea floor above sea level. The different stresses that forged the island can be seen throughout the Troodos mountain range where the ophiolite suite exhibits the structure of the sea floor, including dolerite dykes and basalt pillow lavas. The compressional stress which acts upon the earth has also allowed folding to be exposed throughout the island, where the rock strata has been deformed to produce a series of anticlines and synclines which characterise the landscape.
External igneous land formations
Landscapes can be influenced by other igneous featured which have not been formed due to plate tectonics. The Dartmoor granite Tors formed 280 million years ago, when the granite which forms the tors cooled very slowly in a batholith producing coarse crystals. Over time the overlying sediment got eroded away exposing the granite batholith, as the granite cooled it contracted producing cooling joints which when exposed to weathering act as planes of weakness within the structure allowing for more rapid weathering known as hydrolysis (OCR AS &A2 Geology Textbook 2008). Granite is particularly susceptible to kaolinisation, which is the rapid break down of feldspar minerals (feldspars make up 30-40% of granite) into kaolinite (china clay). Over time this weathering lead to the formation of the Dartmoor tors (Dartmoor National Park Tor Formation Factsheet).
Other tectonically unrelated formations include The Great Win Sill in the North Pennines, which was formed when molten magma intruded into the crust and cooled concordant to the rock strata 295 million years ago. The magma was injected in-between layers of limestone, shale and sandstone and covers most of North East England. It’s over 80km thick and over millions of years of erosion and weathering has become exposed at the surface.
There are many igneous intrusions which characterise landscapes across the earth, Whin sill and the Dartmoor tors are two examples; however extrusive igneous features also characterise vast areas. Roughly 250 million years ago the earth experienced the largest volcanic eruption in the history of the planet. At the end of the Permian, the Siberian traps fissure volcano thought to be fuelled by a mantle plume erupted 1 – 4 million km3 of basaltic lava, enough to cover the whole of Europe in only 200,000 years (University of Rochester 1991).
Conclusion
To conclude, tectonic processes produce a variety of landscapes around the planet, which can be on a global (Pangea); national (the collision of India with Asia); or a local scale (the formation of folding in the Troodos Mountains, Cyprus). They result from the stresses and pressures the earths ridged lithospheric plates are put under from their movement on the asthenosphere, by convection in the mantle. These stresses and strains cause the tectonic plates to react in different ways depending on the rock’s properties (whether a plate is dense and subducts or buoyant and does not). Although tectonic processes have been important in shaping the earths varying landscapes over millions of years they are not the only influence acting upon the surface structures of the planet, as explained through the affects that igneous intrusions and extrusions have on the landscape formations such as through the erosion of batholiths and sills.