Monday, August 23, 2010

Earth Quakes & Earthquake Zones

The continents slowly slip past one another, rising, falling, straining and distorting as they go.  Their passage is seldom smooth.  With little lubrication, strain builds up until the rock can withstand it no longer and the cracks. The resulting earth quake can release tremendous amounts of energy in the form of seismic waves that run through the ground, shaking buildings with destructive force. Movement along the fracture or fault line can extend for many meters, and can occur horizontally and vertically. Seismic seabed movement can generate seemingly harmless waves in deep water, but the waves travel at u to 800 Km/h. they are slowed down in shallow water but their amplitude increases so that they reach height of 50 meters. These tidal waves, or tsunamis, wreak great havoc on coastal areas.

Most earthquakes, like volcanoes, take place in specific zones that correspond to the boundaries of tectonic plates. Shallow earthquakes happen where one plate moves against another on the surface, as in California, and where displacement is horizontal. Deeper earth quakes occur where one plate is sliding beneath another, described as subduction zone. This is happening along the west coast of South America, where the ocean crust dives beneath the continent. Volcanoes occur in the same regions, as magma is produced at both constructive (where crust is being generated) and destructive (where it is disappearing) margins. The most violent eruptions occur at destructive margins, and  are known as Andesite volcanoes (after the Andes mountains).

Tuesday, August 17, 2010

Continetal waltz and Plate boundaries

The continental waltz
     Using the residual magnetism of particles locked in various rocks, geologists can work out their orientation when the rocks were formed. By comparing their magnetic alignment, scientists can trace how the continents have moved. Two hundred million years ago almost all the continents were joined together in the super continent of Pangaea. By 100 million years ago, Pangaea had split into Laurasia and Gondwanaland which themselves began to split. 50 million years ago the Atlantic was just opening in the 50 million years time the face of the earth will have changes one more.
Plate boundaries
      Tectonic plates are ringed with destructive boundaries marked by earthquakes and volcanoes, Most new plate forms under the sea at mid ocean ridges. The ridges system is, in effect, the longest mountain chain on earth at 70,000 Km long. Crust produced along a line will not fit over the spherical surface of the Earth so the ridges buckle and are punctuated by 90 degrees transform faults.
      When an ocean plate meets another plate, it takes a dive at what is termed a subduction zone. This can result in an ocean trench fringed with volcanic atoll above the descending slab. Ocean crust sub ducting beneath a continent scrapes off a accumulated sediment and carries sea water as it goes. This results in volcanic activity and mountain building. When continent runs into continent, massive buckling occurs. Neither continental plate can sink so the rocks are uplifted into high mountain ranges.
     As larva from mid ocean ridges cools, it freezes the prevailing magnetism. Over time the Earth’s reversing magnetic field results in a striped pattern of magnetism either side of a ridge, a record of a sea floor spreading.

Saturday, August 14, 2010

Earth Crust

The Earth’s crust only accounts for 0.6 percent of the planet’s volume, yet we have not penetrated right through even that. Oceanic crust is 5 to 10 Km thick and made of less dense rocks, including granite and thick accumulations of sediment. The continental crust resembles the scum on the surface of a big cauldron.
The oldest regions of continents, made of material that has been piling up for billions of years, have been “cooked” by heat and pressure and are made of crystalline metamorphic rocks. At the base of the crust is a boundary that reflects seismic waves, called the Mohorovicic discontinuity, or Moho. Beneath it are the rocky slabs of lithospheres mantle, composed mostly of iron and magnesium rich peridotite, on which the cruse floats.  The more the weight laden on the crust, the lower it sinks. Mountainous areas have “roots” within the mantle which are significantly greater than the height of the mountains above. The balance maintained is called Isostasy.
The crust is divided into relatively rigid plates, some made of ocean crust, some of continents. Over geological time the plates jostled around as the convection mantle moves beneath them triggering earthquakes. Ocean crust is created where molten rock wells up along mid ocean ridges, and is destroyed where it is subsumed under continental plates. Continental crust can be roasted, stretched and split by mantle plumes, or uplifted, warped and eroded at the surface, but ancient continental cores remain intact.
It is possible to trace the history of the movements of continents’ waltzes around the globe over hundreds of millions of years. Their edges fit together like the pieces of a jigsaw; the similarities between their fossils and rock strata, the past climates they experienced and the orientation of magnetic minerals, frozen in volcanic rocks like tiny compass needles, all record their travels. Precise laser measurements taken from satellites reveal the present rate of continental drift. It is roughly comparable to the rate at which finger nails grow- a mere 4 to 5 Cm in a year across the Atlantic, 12 to 14 Cm in a year across parts of the Pacific

Friday, August 13, 2010

The earth Magnetism

The earth has a strong dipolar magnetic field, as if there was a large bar magnet in the core. In fact, electrical currents in the churning, liquid outer core produce a self sustaining dynamo. The influence of the magnetic field extends beyond the planet. The field meets charged particles streaming from the sun in a shock wave about 50,000 Km above the Earth and slows them from 400 to 250 Km/s. the solar wind, as these particles are known, sweeps the Earth’s magnetic field into a six million Km tail. Solar flares on the surface of the Sun result in magnetic storms on Earth two days later. There are belts of solar particles trapped in the magnetic field about 3000 and 25000 KM above equator, called the Van Allen belts.
Aurora occur where charged particles trapped by the Earth’s magnetic field stream towards the poles. The earth’s magnetic axis is 11degree west of the North Pole and is slowly wandering Eddy currents in the core and magnetism trapped in rocks produce local anomalies. In the geological past, the magnetic field has reversed many times. Between 118 and 83 million years it did not reverse at all. For the last few million years it has reversed about once in every 220000 years, revealed by the magnetic alignment in volcanic rocks.  We are overdue for another reversal.

Thursday, August 12, 2010

Earth's Mantle convection and seismic activity

Heat rises through the mantle in convection currents like those in a pan of water on a stove. But does semi molten rock circulate through the whole mantle or are there two separate circulations like a double boiler? Density estimates suggest the upper and lower mantles are chemically different and are therefore not mixed by whole mantle circulation. But seismic evidence shows slabs of cold materials descending through the lower mantle: some volcanic rocks carry chemical signatures of the lower mantle. There is a barrier to flow about 650 KM down. To descend further, rocks may denser.
As the seismic waves travel through the Earth’s crustal layer, the velocity of seismic waves is highest in hard, dense rocks. However, seismic waves travel faster through the semi molten mantle than the solid lithosphere above it. This apparent mystery is explained by the Earth’s gravity, which compresses the planet into a ball, both making the interior extremely hot and subjection it to great pressure. The immense pressure generated within the mantle means that the hot rock of which it is composed can remain molten but also be incredible dense at the same time. The density changes encountered with increasing depth can cause traveling seismic waves to be refracted.

Tuesday, August 10, 2010

Seismic clues and Earth quakes

Earthquake Zones INDIA
Earth quakes produce shock waves that reverberate outwards through rock from a central point of energy release. Different kinds of waves can be identified and their paths can be plotted. Rayleigh waves and love waves only travel around the surface of the Earth. Primary waves and Secondary waves travel through the Earth, although Secondary waves cannot cross the boundary of the molten outer core. They reflected off that boundary, thus revealing deep structure. Waves picked up hundreds of receivers are used to build up a structural cross section.
The continents slowly slip past one another, raising, falling, straining and distorting as they go. Their passage is seldom smooth. With little lubrication, stain builds up until the rock can withstand it no longer and the ground cracks. The resulting earthquake can release tremendous amounts of energy in the form of seismic waves that run through the ground, shaking buildings with a destructive force. Movements along the fracture or fault line can extend for many meters and can occur horizontally and vertically. Seismic seabed movement can generate seemingly harmless waves in deep waves in deep water, but the waves travel up to 800Km/ hour. They slowdown in shallow water but their amplitude increases so that they reach heights of up to 160fts.  These tidal waves , or tsunamis, wreak great havoc on coastal areas.

Monday, August 9, 2010

Origin of the Earth

Earth science examines our planet’s structure and the geological processes at work within it and on its surface. It considers rock formation and composition, and the geological evolution of the earth. Land sculpting agents, weather and mapping conclude the study.
Planet earth is a small, dense, rocky planet that is unusual in several aspects. It has free oxygen in its atmosphere; water is present in three phases- ice, liquid and vapor, and surface temperatures are maintained in a relatively narrow range. Life forms play a key role in maintain the balance of conditions on Earth. The interior is layered. Radioactive decay in the interior releases heat and which slowly rises to the surface. Convection currents within the semi solid mantle produce volcanic hot spots. The currents create new ocean crust and push the continents around like floating rafts. About 4700 million years the young sun began to shine, blowing the remaining material of the solar nebular into a ring of dust, gas and ice. Rocky silicates condensed out nearest the sun; gases and ices were driven further.
Gravity and impacts helped the rocky material clump together. Radio activity and the heat of impacts melted the larger proto planets so that iron could sink and form a core; So that the iron could sink and form a core. Depression in the young earth’s surface that became the ocean basins may have been created by impacting comets.  The first atmosphere was composed of gases expelled by volcanoes. A Mars sized object hitting the planet may have produced moon.