Why are trenches bordering chains of volcanoes and island arcs? What happens at trenches? How do oceanic ridges form?

This article discusses the formation of trenches, volcanoes, island arcs, and oceanic ridges in geography. It explains why trenches border chains of volcanoes and island arcs, what happens at trenches, and how oceanic ridges form. The article also explores the origin of the magnetization pattern of the ocean floor and why earthquakes occur only in narrow zones such as mid-oceanic ridges and trenches.

Running Head: GEOGRAPHY
Geography
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Author Note

1GEOGRAPHY
Question 2- Why are trenches bordering chains of volcanoes and island arcs? What
happens at trenches? How do oceanic ridges form?
The Volcanoes can be considered as the vibrant manifestation of the plate tectonics
processes. These are extremely common along with the divergent and convergent plate
boundaries. These are also found in the lithospheric plates which are away from the plate
boundaries. The ocean trenches are the long and narrow depressions on the seafloor. These
are the chasms which are at the deepest part of the ocean and the natural sports too. These
trenches are found in also each ocean basin. The deepest trenches ring the Pacific ocean that
is called the “Ring of Fire” consisting of the active volcanoes. The island arcs on the other
hand form on the opposite edge of a subducted slab. Taken for example, there is a
connecting subducting slab along with a trench. One example of the island arc is the Kurli
Island Aleutian Isands of Alaska.
There are many happenings in the deep-ocean trenches. The oceanic crust bends downward
in order to form a deep ocean trench. Afterwards the oceanic crust starts sinking back to the
mantle through a special process named as subduction. The meeting point of continental crust
Deep ocean trench Island arc

2GEOGRAPHY
and oceanic crust develop the trench. At the same time, the oceanic crust sinks in the
continent and gets into the mantle having bent towards it. The oceanic ridges are the long
submarine mountain chains which extend approximately 80,000 kilometres. These are the
most significant parts of the ocean basins. As a whole, the oceanic ridge system can be
considered as the most prominent characteristic on the surface of the earth. The oceanic
crusts are developed at the seafloor which spreads the centres at those oceanic ridge crests.
The earthquakes at the ridge crests signify that oceanic crust is in high tension at that point.
Question 4- Explain the origin of the magnetization pattern of the ocean floor.
There are various types of evidence which support the sea-floor spreading theory of
Hess. The evidences are the eruptions of the molten materials, the magnetic strips in the rock
of the ocean floor and the rocks themselves. As per the theory of Hess, the oceanic crusts
form along the submarine mountain zones which are also known as the mid-ocean ridge
system. It gradually spreads out laterally away from them. This played a pivotal role in the
growth of the plate tectonics that is a theory which revolutionized geologic thought in the 20th
century. At the time when the scientists became aware of the patterns in the rocks of the
ocean floor, more support was found for the sea-floor spreading. It is surprising to note that
the magnetic poles of the earth have reversed themselves several times in the history of the
earth. About 8,000,00 years ago the last reversal had happened. The scientists have
discovered that rocks which make up floor of the ocean lie in the patterns of magnetic strips.
These are the strips which hold a record of reversals in the magnetic field of the earth.

3GEOGRAPHY
Question 5- Why earthquakes occur only in narrow zones of the Earth (i.e. mid-oceanic
ridges and trenches)?
The earthquakes do not occur everywhere across the globe. The seismologists have
found that the earthquakes mostly occur in the narrow seismic belts better called the seismic
zones. It coincides with the tectonic plate boundaries. These are plates which make up the
rocky crust at the surface of the earth and underlie both the oceans and the continents. The
oceanic crusts can be considered as something like the conveyor belt. However, the new crust
is consistently created at the ridges at the mid-ocean and devastated at the place where it
becomes invisible into the edges and the trenches. It is the place where the ocean is collided
with the continent. The trenches and the oceanic ridges are the places of earthquake
activity .Most of the seismic zones correspond with the plate boundaries and these
earthquakes are called the plate-boundary earthquake. It occurs in the narrow zones where the
rock masses move related to each other. Earthquakes occur in almost all the plate boundaries
but it occur more in the collision zones which consist of the oceanic trench than it occur in
the mid-ocean ridges. The rocks at the oceanic ridges are softer because of being hot. At the
trenches the crust is cooler and thicker giving more strain and leading to frequent
earthquakes.
Magnetic Strips

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Global seismic centres

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Bibliography
Baes, M., Sobolev, S.V. and Quinteros, J., 2018. Subduction initiation in mid-ocean induced
by mantle suction flow. Geophysical Journal International, 215(3), pp.1515-1522.
Gamage, S.S.N., 2017. Seismic activity near the Sunda and Andaman Trenches in the
Sumatra subduction zone. International Journal of multidisciplinary Studies, 4(2).
Green, D.H. and Falloon, T.J., 2015. Mantle-derived magmas: intraplate, hot-spots and mid-
ocean ridges. Science Bulletin, 60(22), pp.1873-1900.
Scholz, C.H., 2019. The mechanics of earthquakes and faulting. Cambridge university press.
Usgs.gov (2019). This Dynamic Earth--Contents [USGS]. [online] Pubs.usgs.gov. Available
at: https://pubs.usgs.gov/gip/dynamic/dynamic.html [Accessed 9 Jul. 2019].