Reading 1 – The Ring of Fire

Did you know that 90% of the world’s earthquakes and 81% of the world’s largest earthquakes occur along the Ring of Fire? So what on earth is the Ring of Fire? It is a zone of frequent earthquakes and volcanic eruptions that encircle the basin of the Pacific Ocean.

The Ring of fire is 40,000 km long and is shaped like a horse shoe. It is associated with a nearly continuous series of oceanic trenches, island arcs, volcanic mountain ranges, and plate movements.

The Ring of Fire is a direct result of plate tectonics and the movement and collision of crustal plates. The plate tectonic theory explains the forces within the earth that cause earthquakes. Geologists use the term tectonics to describe the deformation of the earth’s crust, the forces producing such deformation, and structural features that result.

The outer layer of the earth, the lithosphere, is broken into about twenty pieces which are known as tectonic plates. These are fitted together like in a jigsaw puzzle. These plates slowly slide around, periodically colliding with each other. Oceanic plates are denser and heavier than continental plates so when the two collide, the oceanic plate slides under the continental plate and is driven into the earth’s interior where it melts to form mountain ranges and volcanoes at the edge of the continental plate; as it is driven upwards a trench forms at the point where the oceanic plate dives under the continental plates.

When two oceanic plates collide one of the two plates sub-ducts (i.e. slides under the other) creating an oceanic trench at the interface. There is tremendous energy created when these plates collide. The subducting plate easily melts as it penetrates the earth’s interior and some of the magma from this rises back to the surface forming volcanic island chains parallel to the oceanic trenches. Volcanoes are temporary features on the earth’s surface. (The world has some 1500 active volcanoes; almost 90% are in the ring of fire; 75% of the world’s dormant volcanoes are also in this region.) The Aleutian Islands of Alaska are an example of this.

When the two continental plates collide one is driven beneath the other. Since both plates are less dense than the oceanic plates they are not sufficiently heavy to sink and melt into the earth’s interior. The crust in the collision area thickens and mountains form. Himalayas in Asia is an example of such mountain.

These plates which are constantly in motion clash into or move away from each other creating stresses and pressure at their margins. These stresses are released through volcanic eruptions when the molten rock is ejected as magma, through fissures in the crust or via earthquakes, then the pressure causes the crust to buckle and move violently. Some scientists say such eruptions help to release pressure and perhaps prevent even bigger explosions happening.

Questions

a)      Why is the Ring of Fire called by that name?
b)      What is the Ring of Fire compared to?
c)      What would a tectonics expert be able to explain us?
d)      What are tectonic plates?
e)      Why does the oceanic plate slide under the continental plate?
f)        When they collide with each other?
g)      Where do most number of volcanoes take place?
h)      Mention a place where volcanoes found in past and not at present?
i)        According to the passage how was Himalayas formed?
j)        What is magma?
k)      Name the trench that lies just on the equator.

Based on the information you have decided to make a list of four points that you would present at a discussion ‘The Geographical changes that take place as a result of tectonic movement and collision of crustal plates’.