How the theory of plate tectonics explains volcanism
The theory of plate tectonics provides the information that there are several huge slabs of tectonic plates floating on the earth’s surface, with the reason that it’s convection currents driving the plate tectonics’ movement through a combination of pushing and spreading apart at mid-ocean ridges, and pulling and sinking downwards at subduction zones. It also explains that there are 3 main types of plate boundaries : Divergent, convergent, and transform.
Most volcanoes form near the edges of the tectonic plates, mainly associated with two plate boundary types : Divergent and convergent margins.
Most volcanoes form near the edges of the tectonic plates, mainly associated with two plate boundary types : Divergent and convergent margins.
Diverging boundaries ( also known as constructive boundaries due to new crust being formed ) are where two plates are moving apart, creating mountain range such as the Mid Atlantic Ridge, fissure eruptions, earthquake activity, new seafloor, or widening ocean basin. Volcanism in this margin happens when the separating process things the crust, hence creating weaknesses in the crust. It typically breaks along parallel faults. It lowers the pressure on the underlying hot rocks of the asthenosphere, causing it to melt. Due to the gap between the two plates, magma from below finds its way up through the weaknesses in the crust to fill the gap. When the lava cools, it creates new crust and forms a mid-ocean ridge if it happens on an oceanic plate. Volcanoes form when the lava builds up, layer after layer to create mountains or ridges of mountains. ( Mapping Earthforms : Volcanoes, 2007 )
These boundaries causes the plates to grow wider as time passes. For example, the Pacific Ocean is growing wider by about 18cm per year as the plates diverge and the mid-oceanic ridge is built up. ( sciencelearn.org.nz, 2010 ) A well-known divergent boundary would be the Mid-Atlantic Ridge, which extends from the Arctic Ocean to beyond the southern tip of Africa. It’s growing wider about 2.5 cm per year. Although this ridge is mostly an underwater features, fragments of it had elevated enough to extend above sea level. And this particular elevated ridge is known as Reykjanes Ridge, running across Iceland. ( iat-sia.org, 2010 ) Iceland is an unusual country, crossing on both the Eurasian and North American plate boundary, as one of the few places where divergent plate volcanism occurs on land. The divergent causes the eastern and western half to move apart, and volcanism takes place in the narrow zone between. ( decodedscience.org, 2011 ) Notable volcanoes are Krafla and Eyjafjallajokull.
Converging boundaries ( Also known as destructive boundaries due to crust being destroyed ) are where two plates are colliding head on, creating an assortment of land features such as mountains, chains of islands, and underwater trenches. Volcanism in these margins happens when one of two plates subducts. Where subduction occurs, it creates weakness in the crust and generates a lot of heat.
As one of the plates subducts, the slab melts from the sufficiently hot depth. The magma chambers produced by this process are less dense than the mantle materials, hence it rises back upwards. This creates the volcanic activity.
When continental and ocean plates collide, the oceanic plate would be forced down the mantle due to it being thinner and more dense compared to the continental plate. Example : Cascade mountain volcanoes. On the case when two oceanic plate converges, the older plate would usually subduct due to its higher density. ( Geology.com, ) In the earlier stage, these volcanoes cones might still be beneath the ocean surface, but will later grow to be higher than sea level. Through continued development, the islands grows larger, creating a larger landmass. Example : Aleutian islands
Converging boundaries that occur in the ocean causes island arcs ( chains of volcanic island ) to form at the edges of tectonic plates . ( NSW Pearson Science 9, 2014 ) They are formed on the opposing edge of a subducted slab. Such examples would be the Lesser Sunda Island, the Kuril Island, or the Ryukyu Islands.
There is a region of continuous series of oceanic trenches, volcanic arcs, volcanic belts, and plate movements known as the Ring of Fire. This region is a 40000 km horseshoe-shaped basin around the edge of the Pacific ocean, with a chain of earthquake / volcanic activity. 75% of the world’s volcanoes and 80% of the world’s earthquakes are present in this region. ( universetoday.com, 2016 )
There’s an account of 452 volcanoes in this region. Examples of the volcanoes at this region are : Mount Ruapehu ( New Zealand, subduction of Pacific plate beneath the Australian plate ) , Krakatoa ( Indonesian, subduction of Australian plate beneath the Eurasian plate ), and Mount Fuji ( Japan, sitting on a triple junction interaction of the Amur Plate, Okhotsk Plate, and Philippine Plate ) |
Volcanic activity are typically not present on transform boundaries due to the reason that there’s no upwelling convection current or melting subducting plate creating magma sources. ( Geology.com, )
While majority of volcanism occurs at plate boundaries, intraplate volcanoes forms on top of hot spots. These hot spots are isolated weak spots in the crust where a lot of hot magma is created. Even though it is still undergoing research on why these hot spots exists, it is believed to have sources deeper down the Earth, described by scientists as hot mantle plumes. As the plate moves across these stationary hotspots, it will generate volcanoes which are only active as long as they are above the mantle plume. This is because when the plate moves away from the hot spot, the magma supply is cut off, hence becoming extinct. New volcanoes will still form on top of those hot spots on another area of the moving plate. ( volcanodiscovery.com, ) If these volcanic structures are built up quickly, they might be seismically active, due to the crust’s need to respond to that extra load by relieves the stress through tectonic activity.
A linear chain of island volcanoes such as the Hawaiian seamount chains and Mauna Loa are formed through intraplate volcanism.
A case study of the record of the world’s longest continental volcanoes chains, Cosgrove, belongs to a country that doesn’t sit at any boundaries. Australia has a 2000 km long chain of volcanoes that started forming 33 million years ago as the tectonic plate moved north ( at the rate of about 7 cm per year ). The mantle plume responsible for this is currently under Bass Strait. ( theguardian, 2015 )
These kind of volcanoes that are formed by stationary hot spots proves that the tectonic plates are slowly moving around.
Divergent boundaries shows that the movement of tectonic plates spreading apart at mid ocean-ridges causes volcanism to occur. Converging boundaries shows that when the movement of two plate collides, one will subduct and melt from the high temperature deep beneath, which the materials will then erupt back up to form a volcano. The theory helps reveals that volcanoes are formed due to the movement of tectonic plates. The formation of island volcanoes chains and intraplate volcanoes are also expounded through the movement of plate tectonics over the motionless hot spots. Consequently, we could also understand that most volcanoes form near the edges of tectonic plates. This observation provides further support for the theory of plate tectonics relating to volcanism.