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by S. Antony Norbert Senior Lecturer in Geography, University of Colombo

The science of Earth is an incredibly fascinating subject. It concerns the Earth, its origin, history, and the dynamics of how it changes.

Geologists, Geophysists and Physical Geographers study such diverse phenomena as continental drift, volcanoes, earthquakes, and even the history of life. People have, for centuries, tried to understand the earth, to reconcile its restless processes, such as seismicity and volcanism with its ageless geology.

The Earth is large, and throughout the history of mankind, little was known about how it functions as a dynamic planet. The forces that move continents and cause them to collide are powered by the energy sources deep within the earth.

These forces are not influenced by the surface patterns of temperature, winds, precipitation, vegetation, or soils. Thus, an understanding of structure of the earth and plate tectonics is important to our understanding of the global patterns of natural hazards.

The structure of the Earth

The earth as a whole is an almost spherical body approximately 6400 km in radius. The centre is occupied by the core, which is about 3500 km in radius.

The layers of the earth's interior include the crust, mantle, liquid core, and solid core. The density of these layers is controlled by temperature and pressure. Temperature melts or softens rocks.

What lies deep within the earth? The outermost and thinnest of the earth shell is the crust, a layer normally about 8-14 km thick.

This surface layer is known as the lithosphere and includes the crust and the upper mantle. It is formed largely of igneous rock, but it also contains substantial proportions of both sedimentary and metamorphic rock.

The earth's crust is commonly divided up into two main types - continental crust and oceanic crust. In continental areas silica and aluminum are very common.

When combined with oxygen they make up the most common rock and granite. By contrast, below the oceans the crust consists mainly of basaltic rock in which silica, iron and magnesium are most common.

Mantle is composed of mafic mineral similar to olivine (a silicate of iron and magnesium). Temperature in the mantle range from about 2800 degrees C near the core to about 1800 degrees C near the crust.

Below the lithosphere is a layer, scientists know as the asthenosphere, which is a zone of relative weakness. the asthenosphere extends for about 200 km and is a zone of melted rocks capable of flowing.

Scientists have concluded that the core consists of two parts, a liquid outer core about 2200 km deep and a solid inner core extending for about 1300 km. Based on different kinds of data, it has long been inferred that the core consists mostly of iron, with some nickel.

The core is very hot; its temperature ranges from 2800 degrees C to 3100 degrees C. The study of the earth's deep interior is a fascinating one and one that is critical to comprehend the processes that affect the earth's surface. Many questions remain unanswered.

In 1912, a German meteorologist and geophysicist named Alfred Wagener presented the basic tenets of continental drift in two articles. His major work on the subject appeared in 1922.

Wagener used his observations to hypothesize that all of the present-day continents were once part of a single super continent called Pangaea. Wagener put forward the notion that the continents plow their way through the crust beneath the oceans.

The mechanism, however, was easily dismissed by many American geophysicists, who understood enough about the nature of the earth's crust to know that continents could not push their way through the floor of the oceans without breaking apart.

However, Wagener had loyal supporters in Europe, South Africa, and Australia. Several lines of hard geologic evidence presented by Wagener and his followers strongly favoured the existence of Pangaea.

These arguments remain strong today. But the actual physical process of separation of the continents was strongly criticized on valid physical grounds. Arthur Holmes, a Scottish geologist proposed a mechanism in 1928. He believed that heat trapped in the Earth caused convectional currents.

Tragically, Wagener died during a meteorological expedition to Greenland in 1930. Wagener died just as oceanic research vessels were beginning to acquire high-quality seafloor topography (bathymetric) data that would provide scientists a vastly better view of the character of the ocean floor.

In the late 1930s Harry Hammond Hess, a geologist at Princeton University started to develop Wagener's continental drift hypothesis, as well as a hypothesis proposed in 1928 by British geologist Arthur Holmes.

During the 1940s and 1950s, great advances were made in our knowledge of the seafloor and in the magnetic properties of rocks. Both of these fields of study provided new evidence to support continental drift.

In the years following World War II, new methods of mapping the ocean floor were developed and ushered in a mass of new data and the stage was set for a new paradigm.

In 1950s, a seismologist, a scientist who specializes in the study of earthquakes, showed that the global system of mid-ocean ridges was also an active seismic belt, or zone of earthquakes. The seismic corresponded to a trough, or rift, system similar to the trough known at the crest of the Mid-Atlantic Ridge.

The Mid-Atlantic Ridge is 2000 meter above the sea floor, which is at a depth of about 6000 meter below sea level. In all, the oceanic ridges and their rifts extend for more than 60,000 km in all the world's oceans.

Sea-floor spreading

Hess, a member of the U.S. Naval Reserves, was commissioned as captain of an assault transport ship during World War II. With the cooperation of his crew, he conducted echo-sounding surveys to map out seafloor depth as his warship cruised the Pacific Ocean.

Back at Princeton after the war, Hess presented a hypothesis known as Sea-floor Spreading. In 1959, he presented his first draft and circulated widely among the geophysical community, but it was not published.

Hess proposed that new ocean floor is formed at the rift of mid-ocean ridges. The ocean floor and the rock beneath it, is produced by Magma that rises from deeper levels. The ocean floor moved laterally away from the ridge and plunged into an oceanic trench along the continental margin.

In his study, he suggested that convectional currents would force molten rock (magma) to well up in the interior and to crack the crust above and force it apart.

In the late 1950s, scientists mapped the present-day magnetic field generated by rocks on the floor of the Pacific Ocean.

The volcanic rocks which make up the sea floor have magnetization because, as they cool, magnetic minerals within the rock align to the Earth's magnetic field. Scientists detected magnetic anomalies or differences in these magnetic field from place to place. They found positive and negative magnetic anomalies.

For the purpose of measuring these magnetic anomalies, they used magnetic instruments (magnetometers) which were adapted from airborne devices developed during World War II to detect submarines and they began to recognize odd magnetic variations across the ocean floor.

In 1963, Vine and Mathews presented their hypothesis to explain this patten and further, they proposed that lava erupted at different times along the rift at the crest of the mid-ocean ridges preserved different magnetic anomalies.

They published the results of a magnetic survey over part of the Carlsberg Ridge in the Indian Ocean. Thus, this observed pattern of ocean floor magnetic anomalies provided strong support for the concept of sea floor spreading.

In 1965, a Canadian geologist Tuzo Wilson linked together the ideas of continental drift and sea floor spreading into a concept of mobile belts and rigid plates, which formed the basis of plate tectonics.

In 1968, Morgan proposed that the earth's surface consists of twenty rigid plates that move relative to each other.

Pichon simplified the concept of plate tectonics by dividing the Earth's surface into six major plates, and a few small ones. He also published a synthesis showing the location and type of plate boundaries and their direction of movement.

Since mid 1960s, the plate tectonic model has been rigorously tested and it is now called the plate tectonic theory that is accepted by almost all geologists.

Plate tectonics

Plate tectonics is a relatively new theory that has revolutionized the way geologists think about the Earth. Tectonics is a noun meaning "the study of tectonic activity". Tectonic activity refers to all forms of breaking and bending of the entire lithosphere, including the crust.

According to the theory, the surface of the Earth is broken into large plates. The size and position of these plates change over time. The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes and mountain building.

Plate tectonics is a combination of two earlier ideas, continental drift and a sea-floor spreading. Continental drift is the movement of continents over the Earth's surface and their change in position relative to each other.

Sea-floor spreading is the creation of new oceanic crust at mid-ocean ridges and movement of the crust away from the mid-ocean ridges.

Geologists measure the plate velocity with the use of satellite laser ranging techniques and rock magnetism, echo-sounders and small explosions to detect shock waves in the earth's interior.

The seismic waves are also used to examine the theory of plate tectonics. By determining the speed and the path of these shock waves through the earth, geologists are able to identify the density and thickness of rocks that lay thousands of kilometres within the earth's interior.

In addition with increasing distance from the ridge the rocks were older. This supported the idea that new rocks were being created at the centre of the ridges and the older rocks were being pushed apart.

Lithospheric plates

There are two very different kinds of lithospheric plates. Plates that lie beneath the ocean basins consist of Oceanic lithosphere (See plate A and B in the Figure 1), which is comparatively thin and about 50 km thick.

Plates that lie beneath continental crust are made up of Continental lithosphere (Plate C in the figure), which is much thicker (about 150 km). The lithosphere can be thought of as "floating" on the soft asthenosphere.

The distribution of earthquakes, volcanoes, and mountain ranges define six large plates and twenty smaller plates.

The Nazca and Juan de Fuca plates consist of only oceanic lithosphere. The pacific plate is mostly oceanic lithosphere only a small slice of continental lithosphere in Southern California and Baja Mexico. Most of the other plates consist of both oceanic and continental lithosphere.

The speed at which new ocean floor is created varies from one location on the ocean ridge to another. Between North America and Europe, the rate is about 3.6 cm per year.

At the East Pacific rise, which is pushing a plate into the west coast of South America, the rate is 32.2 cm per year. The Arctic Ridge has the slowest rate that is less than 2.5 cm per year.

The India plate is moving in a northeastward direction at about 6 cm per year relative to the Burma plate. Plate motions can be determined by magnetic measurements and satellite laser ranging techniques.

The measured velocities between stations are very close to the average velocities estimated from the magnetic measurements (See Figure 2). According to these measurements, the distance between a point on the Nazca Plate and point on the Pacific Plate increases on the average of 16.1 cm per year.

Seismic activity occurs primarily near lithospheric plate boundaries. The greatest intensity of seismic activity is found along converging plate boundaries where oceanic plates are undergoing subduction.

When this down slanting contact of these two plates take place, strong pressures are relieved by sudden fault slippages that generate earthquakes of large magnitude.

This mechanism explains the great earthquakes experienced in Japan, Alaska, Chile, and other narrow zones close to trenches and volcanic arcs of the Pacific Ocean basin.

Earthquakes also occur at scattered locations over the continental plates, far from active plate boundaries. In many cases, no active fault is visible, and the geologic cause of the earthquake is uncertain.

The Austral-Indian plate takes the form of a long rectangle. It is mostly oceanic lithosphere but contains two cores of continental lithosphere-Australia and peninsular India.

The December 26, 2004, 9.0 magnitude earthquake is a shallow and thrust-type one, occurred off west coast of northern Sumatra at the interface between the India and Burma Plates (See Figure 3).

The India plate begins its decent into the mantle at the Sunda Trench, which lies to the west of the earthquake's epicentre. The sea floor overlying the thrust fault is uplifted by several meters as a result of the earthquake.

Approximately, 1000 kms of the plate boundary slipped as a result of the earthquake. After this 9.0 earthquake, after shocks rattled along much of the shallow plate boundary between northern Sumatra to near Andaman Island.

Conclusion

The general idea of plate tectonics is now widely accepted, many aspects still continue to confound and challenge scientists. The earth-science revolution launched by the theory of plate tectonics is not finished.

But tsunami disasters which have been generated by earthquakes pose a major threat to the coastal populations of the world.

In the past four decades alone, tsunamis have been responsible for the loss of thousands of lives and of millions in property damage. Earthquakes are considered as geomorphologic hazards which cause severe damage to human life and physical property.

To mitigate these hazards effectively in future, countries like Sri Lanka should have established effective international cooperation in early warning systems. It is obvious that the best way of mitigating the tsunami hazards is with a programme of public education and awareness.

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