What is Geophysics?

Geophysics is a branch of science that applies physical principles to the study of the earth. Study of Earthquake, or Seismology, is a sub-branch of Geophysics.

Earthquakes represent an enormous threat to the Nation. Although damaging earthquakes occur infrequently, their consequences can be staggering. As recent earthquakes around the world have demonstrated, high population densities and development pressures, particularly in urban areas, are increasingly vulnerable. Unacceptably high loss of life and enormous economic consequences are associated with recent global earthquakes, like the one in Kashmir in Jan 2006, when 80,000 people lost their lives, or the submarine earthquake of Banda Aceh of the 26th of December, 2004, in which the Tsunami killed more than 300,000 people in Asia and Africa.

It is very important to improve the understanding of earthquakes and their effects. A large number of earthquakes are felt all over the globe every year. The small ones are unnoticed while the large ones are felt over thousands of kilometers. The big quakes are not necessarily the most destructive. Earthquakes have damaged and destroyed man’s work since time immemorial.

Here, I have tried to explain the nature, distribution, causes and effects of this terrifying natural event.

Natural Calamities and Disasters

Natural calamities are the phenomenon which can’t be prevented, but we can take precautions. Natural Calamities such as Floods, Tornadoes, Volcanic Eruptions, Tsunamis, Earthquakes, causes disturbance to our day-to-day life. Today, with the help of Science and Technology, we may be able to counteract these natural events and avoid disasters, or even reduce its impact on people.

Tornadoes & Hurricanes

Tornado
Tornado

A Tornado is a violently rotating column of air extending from within a thundercloud down to ground level. The strongest tornadoes may sweep houses from their foundations, destroy brick buildings, toss cars and school buses through the air, and even lift railways from their tracks.

Floods

Floods
Floods

Floods mean an increase in water level in an area due to heavy rainfall or melting snow.

Floods are the natural disaster caused due to overflow of rivers due to heavy rainfall or melting of snow which causes a large destruction in cities and villages.

Earthquakes

Earthquake
Earthquake

Shaking of the Earth’s surface caused by rapid movement of the Earth’s rocky outer layer is known as an Earthquake. Earthquakes occur when energy stored within the Earth, usually in the form of strain in rocks, is suddenly released.

What do you mean by Geophysics?

Geophysics is a branch of science that applies physical principles to the study of the earth. Geophysicists examine physical phenomena and their relationships within the earth; such phenomena include the earth’s magnetic field, heat flow, the propagation of seismic (earthquake) waves, and the force of gravity

Subdivision of the wide-ranging subject matter of geophysics into various branches involves categorizing specific endeavors. Strictly speaking, however, the discipline embraces all fields devoted to researching the earth’s interior, atmosphere, hydrosphere, and ionosphere.

Birth of the Universe, Solar System and Earth

Our Universe

All existing matter, space & other phenomena constitute the Universe. In other words, everything that exists anywhere is included in the universe.

The Solar System includes dozens of moons and countless pieces of rocky & icy debris along with planets. The Sun is one of the hundreds of billions of stars that form our galaxy, the Milky Way. There are several hundreds of billions of galaxies in the Universe, and scientists see them in every direction of the sky.

Andromeda Galaxy, another spiral galaxy just like our Milky Way Galaxy
Andromeda Galaxy

How was Universe created?

Big Bang Theory

The big bang theory proposes that the universe was once extremely compact, dense, and hot. Some original event, a cosmic explosion called the big bang, occurred about 13.7 billion (1.37 x 1010) years ago, and the universe has since been expanding and cooling.

According to the big bang theory, the universe expanded rapidly in its first microseconds. A single force existed at the beginning of the universe, and as the universe expanded and cooled, this force separated into those we know today: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.

Intelligent Design Theory

Some scientists say that the Big Bang theory cannot explain everything. The Universe is too perfect to have come into existence by chance. So, they propose that there has to be an Infinite Intelligence who has designed the Universe. God has created this Universe. This theory is known as Intelligent Design Theory.

Other Theories

Several scientists, including the Georges Lemaitre in Belgium, Willem de Sitter in Holland, and Alexander Friedmann in Russia, gave different theories, and the universes described by the different solutions varied. De Sitter’s model had no matter in it. This model is actually not a bad approximation since the average density of the universe is extremely low. Lemaitre’s universe expanded from a “primeval atom.” Friedmann’s universe also expanded from a very dense clump of matter, but did not involve the cosmological constant. These models explained how the universe behaved shortly after its creation, but there was still no satisfactory explanation for the beginning of the universe.

Formation of Solar System

Many scientists favor the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except for Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.

Within the planetesimal Earth, heavier matter sank to the center and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the center contained radioactive material, an important source of Earth’s internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed.

Our solar system began forming about 4.567 billion (4.567 x 109) years ago, when a cloud of gas and dust between the stars in our Milky Way Galaxy began contracting. The gravitational energy caused by this contraction heated the solar nebula. As the cloud became smaller, it began to spin faster, much as a spinning skater will spin faster by pulling in his or her arms. This spin kept the nebula from forming a sphere; instead, it settled into a disk of gas and dust.

In this disk, small regions of gas and dust began to draw closer and stick together. The objects that resulted, which were usually less than 500 km across, are the planetesimals. Eventually, some planetesimals stuck together and grew to form the planets. Today, there are thousands of Asteroids, innumerable Comets, hundreds of moons and about 12 Planets in the Solar System.

Formation of Earth

During Earth’s early formation, some 4.567 billion years ago, it formed and was enlarged by collision with many smaller bodies, including asteroids, meteorites, and comets that crashed into the early planet at high velocity. When a large asteroid comes to a halt in collision with Earth, the asteroid’s energy is released as heat and is stored within the planet. One such giant impact occurred early in Earth’s history, when a Mars-sized object struck Earth and tore off a piece that is now the Moon.

Structure of Earth

Earth’s interior consists mainly of the mantle and the core. The mantle and core make up by far the largest part of Earth’s mass. The distance from the base of the crust to the center of the core is about 6,400 km.

Scientists have learned about Earth’s interior by studying rocks that formed in the interior and rose to the surface. The study of meteorites, which are believed to be made of the same material that formed the Earth and its interior, has also offered clues about Earth’s interior. Finally, seismic waves generated by earthquakes provide geophysicists with information about the composition of the interior. The sudden movement of rocks during an earthquake causes vibrations that transmit energy through the Earth in the form of waves. The way these waves travel through the interior of Earth reveals the nature of materials inside the planet.

The mantle consists of three parts: the lower part of the lithosphere, the region below it known as the asthenosphere, and the region below the asthenosphere called the lower mantle. The entire mantle extends from the base of the crust to a depth of about 2,900 km. Scientists believe the asthenosphere is made up of mushy plastic-like rock with pockets of molten rock.

The term asthenosphere is derived from Greek and means “weak layer.” The asthenosphere’s soft, plastic quality allows plates in the lithosphere above it to shift and slide on top of the asthenosphere. This shifting of the lithosphere’s plates is the source of most tectonic activity. The asthenosphere is also the source of the basaltic magma that makes up much of the oceanic crust and rises through volcanic vents on the ocean floor.

The mantle consists of mostly solid iron-magnesium silicate rock mixed with many other minor components including radioactive elements. However, even this solid rock can flow like a “sticky” liquid when it is subjected to enough heat and pressure. The core is divided into two parts, the outer core and the inner core. The outer core is about 2,260 km thick. The outer core is a liquid region composed mostly of iron, with smaller amounts of nickel and sulfur in liquid form. The inner core is about 1,220 km thick. The inner core is solid and is composed of iron, nickel, and sulfur in solid form. The inner core and the outer core also contain a small percentage of radioactive material. The existence of radioactive material is one of the sources of heat in Earth’s interior because as radioactive material decays, it gives off heat. Temperatures in the inner core may be as high as 6650°C.

Interior of Earth
Interior of Earth (c) Encarta

Earth’s Crust

Crust is the outermost layer of Earth. The crust is solid and relatively thin, and it lies below both landmasses and oceans. The dry land of Earth’s surface is called the continental crust. It is about 15 to 75 km thick. The oceanic crust is thinner than the continental crust. Its average thickness is 5 to 10 km. The crust is very thin in relation to the rest of Earth.
If a trip to the center of Earth at 100 km/h were possible, it would take 64 hours, of which only the first 15 to 45 minutes would be in the crust.

Upper Mantle

Earth’s Upper mantle is about 650 km thick and features two distinct layers. Directly beneath the crust is a solid layer that, combined with the crust, forms the lithosphere, which makes up the earth’s plates. Beneath this layer is the asthenosphere, where semi-molten rock flows slowly like hot tar. It is believed that convection currents, which move within this area like boiling water, drive the overlying plates.

Lower Mantle

Earth’s lower mantle is about 2300 km thick. Even though temperatures are higher here, this part of mantle is solid. Tremendous pressures keep the rock material from melting.

Outer Core

Earth’s liquid outer core is about 2300 km thick. As a result of extremely high temperatures, this region is made up of molten iron and nickel. The liquid material helps produce Earth’s magnetic field.

Some scientists theorize that the flow of liquid iron in the outer core sets up electrical currents that produce Earth’s magnetic field. Known as the dynamo theory, this theory appears to be the best explanation yet for the origin of the magnetic field. Earth’s magnetic field operates in a region above Earth’s surface known as the magnetosphere. The magnetosphere is shaped somewhat like a teardrop with a long tail that trails away from the Earth due to the force of the solar wind.

Inner Core

Earth’s inner core, is about 1200 km thick and is made up of solid iron and nickel. Temperatures of the inner core may reach 6650°C. It is under very high pressure and has very high density.

Lithosphere and Asthenosphere

Earth's Crust and Asthenosphere

The solid, outermost section of the mantle and the solid crust together form the lithosphere. The lithosphere is approximately 65 to 100 km thick and covers the asthenosphere

Interior of Earth

Earth's Layers

The asthenosphere is approximately 100 to 350 km thick. It consists of rocky material that is softer and less rigid than that in the lithosphere. This softer, less rigid state results from higher pressures and temperatures, which cause the rocks partially to melt and become soft.

Continental Drift Theory and Formation of Continents

In the 1960s geologists began to uncover proof that the continents not only float—that is, move up and down within the crust—but that they also travel, or drift, laterally. If you take a good look at a World map, you can see that the continents “fit into” one another. Today, scientists say that continents are moving a few centimeters every year.

Tectonic Plates
Tectonic Plates

The theory was first advanced by Alfred Wegener. It said that Earth’s continents were originally one land mass. Pieces of the land mass split off and migrated to form the continents

Tectonic Plates Movement – Chronology

Pangaea
Pangaea

About 240 million years ago, there was only a single Mega-Continent, called Pangaea.

Pangaea
Laurasia and Gondwanaland

About 160 million years ago, Pangaea split into two parts, viz, Laurasia and Gondwanaland.

India separated from Gondwanaland
India separated from Gondwanaland

About 140 million years ago, India separated from Gondwanaland and stated moving northwards.

Continents Today
Continents Today

As we see today, all the continents fit into each other. They have achieved this state after millions of years of movement.

This theory is known as the Continental Drift Theory.