Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can be generated by bomb blasts, volcanic eruptions, sudden volume changes in minerals, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake prone areas, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth.

The point within the earth where the fault rupture starts is called the focus or hypocenter. This is the exact location within the earth were seismic waves are generated by sudden release of stored elastic energy.

The epicenter is the point on the surface of the earth directly above the focus. Sometimes the media get these two terms confused.

Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zig-zag trace that shows the varying amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, locations, and magnitude of an earthquake can be determined from the data recorded by seismograph stations.

 

Two of the most common methods used to measure earthquakes are the Richter scale and the moment magnitude scale.

The Richter scale is used to rate the magnitude of an earthquake, that is the amount of energy released during an earthquake.
The Richter scale doesn’t measure quake damage (which is done by Mercalli Scale) which is dependent on a variety of factors including population at the epicentre, terrain, depth, etc. An earthquake in a densely populated area which results in many deaths and considerable damage may have the same magnitude as a shock in a remote area that does nothing more than frightening the wildlife. Large-magnitude earthquakes that occur beneath the oceans may not even be felt by humans. Richter Scale of Earthquake Energy
The magnitude of an earthquake is determined using information gathered by a seismograph.
The Richter magnitude involves measuring the amplitude (height) of the largest recorded wave at a specific distance from the seismic source. Adjustments are included for the variation in the distance between the various seismographs and the epicentre of the earthquakes.
The Richter scale is a base-10 logarithmic scale, meaning that each order of magnitude is 10 times more intensive than the last one.

 

 

The uppermost outer solid and rigid layer of the earth is called crust. Its thickness varies considerably. It is as little as 5 km thick beneath the oceans at some places but under some mountain ranges it extends upto a depth of 700km. Below the crust denser rocks are found, known as mantle crust. This upper part of mantle upto an average depth of 100 km from the surface is solid. This solid mantle plus upper crust form a comparatively rigid block termed as lithosphere. Mantle is partially molten between 100 to 250 km depth. This zone is said to be asthenosphere, also known as Mohr discontinuity, a simplification of Mohorovicic, the name of the seismologist who discovered it.
The lithosphere is broken into several blocks. These blocks are known as plates, which are moving over asthenosphere. There are seven major plates.

 

While the continents do indeed appear to drift, they do so only because they are part of larger plates that float and move horizontally on the upper mantle asthenosphere. The plates behave as rigid bodies with some ability to flex, but deformation occurs mainly along the boundaries between plates.

 

 

 

The plate boundaries can be identified because they are zones along which earthquakes occur.Plate interiors have much fewer earthquakes.

There are three types of plate boundaries:

1. Divergent Plate boundaries, where plates move away from each other.
2. Convergent Plate Boundaries, where plates move toward each other.
3. Transform Plate Boundaries, where plates slide past one another.

Divergent Plate Boundaries

These are oceanic ridges where new oceanic lithosphere is created by upwelling mantle that melts, resulting in basaltic magmas which intrude and erupt at the oceanic ridge to create new oceanic lithosphere and crust. As new oceanic lithosphere is created, it is pushed aside in opposite directions. Thus, the age of the oceanic crust becomes progressively older in both directions away from the ridge.

Because oceanic lithosphere may get subducted, the age of the ocean basins is relatively young. The oldest oceanic crust occurs farthest away from a ridge. In the Atlantic Ocean, the oldest oceanic crust occurs next to the North American and African continents and is about 160 million years old (Jurassic)

. In the Pacific Ocean, the oldest crust is also Jurassic in age, and occurs off the coast of Japan.

Because the oceanic ridges are areas of young crust, there is very little sediment accumulation on the ridges. Sediment thickness increases in both directions away of the ridge, and is thickest where the oceanic crust is the oldest. Knowing the age of the crust and the distance from the ridge, the relative velocity of the plates can be determined.

Relative plate velocities vary both for individual plates and for different plates.

Sea floor topography is controlled by the age of the oceanic lithosphere and the rate of spreading.

If the spreading rate (relative velocity) is high, magma must be rising rapidly and the lithosphere is relatively hot beneath the ridge. Thus for fast spreading centers the ridge stands at higher elevations than for slow spreading centers. The rift valley at fast spreading centers is narrower than at slow spreading centers. As oceanic lithosphere moves away from the ridge, it cools and sinks deeper into the asthenosphere. Thus, the depth to the sea floor increases with increasing age away from the ridge.

 

Convergent Plate Boundaries

When a plate of dense oceanic lithosphere moving in one direction collides with a plate moving in the opposite direction, one of the plates subducts beneath the other. Where this occurs an oceanic trench forms on the sea floor and the sinking plate becomes a subduction zone. The Wadati-Benioff Zone, a zone of earthquakes located along the subduction zone, identifies a subduction zone. The earthquakes may extend down to depths of 700 km before the subducting plate heats up and loses its ability to deform in a brittle fashion.

As the oceanic plate subducts, it begins to heat up causing the release water of water into the overlying mantle asthenosphere. The water reduces the melting temperature and results in the production of magmas. These magmas rise to the surface and create a volcanic arc parallel to the trench. If the subduction occurs beneath oceanic lithosphere, an island arc is produced at the surface (such as the Japanese islands, the Aleutian Islands, the Philippine islands, or the Caribbean islands

Transform Plate Boundaries

Where lithospheric plates slide past one another in a horizontal manner, a transform fault is created. Earthquakes along such transform faults are shallow focus earthquakes.

Most transform faults occur where oceanic ridges are offset on the sea floor. Such offset occurs because spreading takes place on the spherical surface of the Earth, and some parts of a plate must be moving at a higher relative velocity than other parts One of the largest such transform boundaries occurs along the boundary of the North American and Pacific plates and is known as the San Andreas Fault. Here the transform fault cuts through continental lithosphere

Triple Junctions occur at points where thee plates meet.

Hot Spots

Areas where rising plumes of hot mantle reach the surface, usually at locations far removed from plate boundaries are called hot spots. Because plates move relative to the underlying mantle, hot spots beneath oceanic lithosphere produce a chain of volcanoes. A volcano is active while it is over the vicinity of the hot spot, but eventually plate motion results in the volcano moving away from the plume and the volcano becomes extinct and begins to erode.

Because the Pacific Plate is one of the faster moving plates, this type of volcanism produces linear chains of islands and seamounts, such as the

• Hawaiian – Emperor chain, the Line
• Islands, the Marshall-Ellice Islands,
• and the Austral seamount chain

 

 

 

Alfred Wegner was a German Meteorologist in the early 1900s who studied ancient climates. Like most people, the jigsaw puzzle appearance of the Atlantic continental margins caught his attention. He put together the evidence of ancient glaciations and the distribution of fossil to formulate a theory that the continents have moved over the surface of the Earth, sometimes forming large supercontinents and other times forming separate continental masses. He proposed that prior to about 200 million years ago all of the continents formed one large land mass that he called Pangea .

According to Alfred Wegener, the entire landmass of the globe was together about 280 million years ago. It was termed as Pangea, a super continent. The huge water body surrounding the Pangea was known as Panthalasa. From 80 to 150 million years ago, Pangea was broken latitudinally into northern and southern parts known as Laurasia (Angaraland) and Gondwanaland, respectively. Both of them drifted away and in between a shallow sea emerged by filling up the water from Panthalasa. It was known as Tethys sea. Later on Laurasia and Gondwanaland rifted and finally drifted to form the present day distribution of land and water on the earth .

 

Wegener’s explanation of continental drift in 1912 was that drifting occurred because of the earth’s rotation. Fossil records from separate continents, particularly on the outskirts of continents show the same species.

The evidence which gave rise to the theory of continental drift includes the following:

• The coasts of the continents surrounding the Atlantic ocean could, if the continents were moved closer, fit together like a jigsaw puzzle.
• Living animals in widely separated lands are similar. For example India and Madagascar have similar mammals, which are quite different from those in Africa, even though it is now near to Madagascar.
• Fossil plants in India, South Africa, Australia, Antarctica and South America are similar to each other. This so-called Glossopteris flora is quite different from plants found in other parts of the world at the same time.
• There are numerous geological similarities between eastern South America and western Africa.
• Apparent Polar Wandering: Paleomagnetism tells us how far from the poles rocks were when they formed, by looking at the angle of their magnetic field. The story told by different continents is contradictory, and can only be explained if we assume the continents have moved over time.There are ridges in the floors of the main oceans.Paleomagnetism shows that the sea floor has spread away from these ridges. Distinct patterns of stripes can be seen in the magnetism of rocks on either side of the ridges.

 

Most of the knowledge we have about Earth’s deep interior comes from the fact that seismic waves penetrate the Earth and are recorded on the other side.  Earthquake ray paths and arrival times are more complex than illustrated in the animations, because velocity in the Earth does not simply increase with depth. Velocities generally increase downward, according to Snell’s Law, bending rays away from the vertical between layers on their downward journey; velocity generally decreases upward in layers, so that rays bend toward the vertical as they travel out of the Earth . Snell’s Law also dictates that rays bend abruptly inward at the mantle/outercore boundary (sharp velocity decrease in the liquid) and outward at the outer core/inner core boundary (sharp velocity increase).

Major Points to remember about P S and Love waves

• P wave or primary wave. This is the fastest kind of seismic wave, and, consequently, the first to ‘arrive’ at a seismic station.
• The P wave can move through solid rock and fluids, like water or the liquid layers of the earth.
• P waves are also known as compressional waves.
• S waveor secondary wave, which is the second wave you feel in an earthquake. An S wave is slower than a P wave and can only move through solid rock, not through any liquid medium.
• Travelling only through the crust, surface wavesare of a lower frequency than body waves, and are easily distinguished on a seismogram as a result.

 

Earth’s Layers – Earth’s Composition

The Crust of Earth

It is the outermost and the thinnest layer of the earth’s surface, about 8 to 40 km thick. The crust varies greatly in thickness and composition – as small as 5 km thick in some places beneath the oceans, while under some mountain ranges it extends up to 70 km in depth.

The crust is made up of two layers­ an upper lighter layer called the Sial (Silicate + Aluminium) and a lower density layer called Sima (Silicate + Magnesium).The average density of this layer is 3 gm/cc.

The Mantle of Earth

This layer extends up to a depth of 2900 km.

Mantle is made up of 2 parts: Upper Mantle or Asthenosphere (up to about 500 km) and Lower Mantle. Asthenosphere is in a semi­molten plastic state, and it is thought that this enables the lithosphere to move about it. Within the asthenosphere, the velocity of seismic waves is considerably reduced (Called ‘Low Velocity

The line of separation between the mantle and the crust is known as Mohoviricic Discontinuity.

 

The Core of Earth

Beyond a depth of 2900 km lies the core of the earth.The outer core is 2100 km thick and is in molten form due to excessive heat out there. Inner core is 1370 km thick and is in plasticform due to the combined factors of excessive heat and pressure. It is made up of iron and nickel (Nife) and is responsible for earth’s magnetism. This layer has the maximum specific gravity.The temperatures in the earth’s core lie between 2200°c and 2750°c. The line of separation between the mantle and the core is called Gutenberg­Wiechert Discontinuity.

 

 

 

 

 

 

Beginning of the Universe started about 13.6 billion years ago,when the Big Bang created the universe from a point source.
During this process, light elements, like H, He, Li, B, and Be formed. From this point in time, the universe began to expand and has been expanding ever since.
Concentrations of gas and dust within the universe eventually became galaxies consisting of millions of stars.
Within the larger stars, nuclear fusion processes eventually created heavier elements, like C, Si, Ca, Mg, K, and Fe.
Stars eventually collapse and explode during an event called a supernova. During a supernova, heavier elements, from Fe to U, are formed. (See figure 1.9 in your text).
Throughout galaxies clusters of gas attracted by gravity start to rotate and accrete to form stars and solar systems. For our solar system this occurred about 4.6 billion years ago.
The ball at the center grows dense and hot, eventually nuclear fusion reactions start and a star is born (in our case, the sun).
Rings of gas and dust orbiting around the sun eventually condenses into small particles. These particles are attracted to one another and larger bodies called planetismals begin to form.
Planetesimals accumulate into a larger mass. An irregularly-shaped proto-Earth develops.
The interior heats and becomes soft. Gravity shapes the Earth into a sphere. The interior differentiates into a nickel-iron core, and a stony (silicate) mantle.
Soon, a small planetoid collides with Earth. Debris forms a ring around the Earth.The debris coalesces and forms the Moon.
The atmosphere develops from volcanic gases. When the Earth becomes cool enough, moisture condenses and accumulates, and the oceans are born.