Friday, September 24, 2010

Rock Records

Rock Records

The rocks beneath our feet are mostly built up from layers. Each one resembles a page in a history book, recording conditions on Earth during its formation.  In a sedimentary rock the grains size and shape can record the energy of the environment in which it was deposited. Coarse pebbles or rock fragments indicate rapid erosion of a nearby land mass by waves and weather. Fine clays and silts indicate slower accumulation in still waters. Angular grains may come from a desert, rounded ones from a river, chemical deposits, such as lime stones, imply accumulation away from a source of eroded sand and mud. Many rocks contain fossils, visible by eye or by microscope and these can suggest the conditions in which the creatures lived. There are chemical clues too, to the salinity of seas, and the temperature and even composition of the atmosphere at the time of rock formation.
Molecular Paleontology

If the theory of evolution is right, every living thing contains molecular clues to its past. Genes are living fossils. If there is a constant rate of random mutation, DNA is like a molecular clock; the genetic differences between species record the time since they shared an ancestor. The differences also reveal which species are related; humans are genetically only one percent different from chimpanzees.
Genes and proteins have been recovered from extinct species from museum specimens of the marsupial wolf, and from a woolly mammoth in Siberian ice. DNA has even been found in a 100 million year old weevil preserved in amber. Dinosaur DNA may survive in bone but not in sufficient quantities to create a Jurassic Park.
Atomic messengers

                Fossils and most sediments contains from the atmosphere at the time they were laid down. These atoms carry massages. For example, oxygen comes in different form or isotopes- sea water contains O16 and O18. O16 is lighter and evaporates more easily to fall as snow; it thus becomes locked in polar ice caps, and proportion of O18 or O16 remaining in the shells of tiny fossils indicates the amount of polar ice. Carbon too, comes in different forms. Plankton take up C12 more readily than C13 , If that carbon continues down the chain to bottom dwelling species, the atmosphere is depleted in C12, as are subsequently plankton. Thus a comparison of C13, in surface and bottom dwelling microfossils reveals biological activity at the time.

Monday, September 20, 2010

Rock Forming Minerals

The bulk of the rocks of the Earth’s crust consist of silicates. They have a plethora of names, compositions and complex structures, but the most important ones can be grouped into families.

                These are the most abundant of all minerals at the Earth’s surface: spiral chains of silicate tetrahedral linked at the corners often transparent and crystalline. Acid rocks are at least ten percent quartz.

                These make up to 50-60 percent of the mass of all igneous rocks: 3D frame work of silicate tetrahedral with aluminum and varying amount of potassium, sodium, calcium and aluminium. There are two families of feldspars: orthoclase feldspars have varying amounts of aluminum and potassium; plagioclase feldspars have varying amounts of sodium and calcium.

This dense, ultra basic family of minerals is common in the Earth’s mantle and igneous rocks originating from a deep source: dense, close packed silicate tetrahedra containing magnesium and iron; grassy in appearance.

                 This is a family of chain silicates in igneous rocks containing magnesium (in the case of enstatite), calcium and magnesium (in diopside) or iron and aluminium (in augite).

                These have double chains of silicates containing iron and magnesium. Many amphiboles are described as fellomagnesium and one of the best known examples ins hornblende.

                Mica consists of layered silicates that cleave easily into flakes or sheets, and include muscovite (white mica) and biotite (dark mica); a common constituents of gneiss and granite.

Tuesday, September 14, 2010

Rock Cycle

The moon is dead. With no atmosphere and a cold, solidified interior, most features on the surface are several billion years old. The earth is very different and little survives the aeons intact. Even great mountain ranges arise and are eroded over a few hundred million years. The rocks on the Earth’s surface are perpetually being recycled. Magma rises from deep inside the Earth. Some is trapped underground and hardens into intrusive igneous rock. Some erupts onto the surface as extrusive igneous rock. Some erupts onto the surface as extrusive igneous rock. Pressure and heat from below cook or metamorphose the rocks and colliding continents push them up to the surface. Wind rain and ice erode the rock and with the help of gravity carry away the sediment. Rivers deposit it along their flood plains or at the bottom of lakes and seas, where it builds up in layers and hardens under pressure into sedimentary rocks. These sink into the ground and are metamorphosed by head and pressure, or folded and uplifted again by more tectonic activity, continuing the cycle.

The rock cycle is powered from above and below. Heat from within the earth ultimately derived from radioactive decay and the slow solidification of the inner core causes the upwelling of intrusive and extrusive rocks and produces the process of metamorphism of rocks. It also drives the drifting continents, uplifting mountain ranges. The energy of the Sun heats and expands rocks at the surface and ultimately drives the wind, waves and precipitation that cause erosion. Gravity causes landslides which contribute to the circulations of rocks.

Monday, September 13, 2010

Types of Eruption

Apart from hotspots over mantle plumes, volcanoes frequent crustal plate boundaries, making a “ring of fire” around the Pacific. The ocean ridge system is a chain of submarine volcanoes.  Where it breaks the surface, in Iceland for example, it coincides with a mantle plume. The volcanoes of the Rift Valley of east Africa represent a new ocean trying to open. Where ocean crust dives beneath a continent, it takes with it water locked in minerals. As the rocks heat and melt, the wet magma rises like uncorked champagne to produce some violent eruptions.
Mt. St. Helens in Washington State is one of many volcanoes above the sub ducting Pacific plate. The wet magma ascends periodically like a pressure cooker letting off steam. Up to May 1980 geologists had monitored 10,000 small earthquakes in the region and had used lasers to measure the growing bulge on the mountains north flank. By May 12 parts of the bulge were 138 m higher than before and very unstable suddenly on May 18, the entire north flank collapsed in three great landslides only seconds apart. The second exposed pressurized molten magma which erupted in a tremendous lateral blast, flattening trees up to 30 Km away, the third block to slide exposed the top of the magma column itself, which erupted upwards sending ash more than 19Km high and coating 50,00Km2 with 540 million tons of ash.

Saturday, September 11, 2010


Although the earth’s mantle is solid, it can still flow slowly in the same way as a glacier does. Plumes of hot mantle rock rise and as they do so the pressure drops and some of the minerals begin to melt. Not everything melts, so the composition of the melt or magma is different from the bulk composition of the mantle. If the angles between the remaining grains (the dihedral angles) are big enough, the magma can flow out and upwards, accumulating in large volumes called magma chambers. The nature of the subsequent volcanic eruptions depends on the source and chemistry of the magma.
The ratio different helium isotopes in bubbles of gas contained in the mantle plume suggests that some of them come from great depth, possibly the base if the lower mantle. These produce the vast basalt flows on which the Hawaiian Islands are built. In the past they have produced even bigger eruptions. Sixty five million years ago millions of cubic kilometers of basalt erupted over what is now western India. The effects on climate of large amounts of volcanic gases are considerable, and may have caused the demise of the dinosaur. Such shield volcanoes produce copious quantities of runny, alkaline lava which spreads over a wide area. Acidic lavas are more viscous and produce more explosive eruptions and ash clouds, particularly if they contain a lot of water or dissolved gas.

Friday, September 10, 2010

Prediction and Prevention of Earth quakes

An earth quake can happen anywhere and anytime. Clearly they are most likely near faults, but prediction that a major quake is likely sometime in the next 50 years is not much use. A few faults seem to be regular; quakes had occurred in California every 22 years but that was expected in 1989 has still not come. Short term prediction is notoriously very difficult. A small isolated quake looks like the foreshock to a major one. In 1975, observations of natural phenomena in China led to a successful prediction and many lives were saved, but a year later another Chinese quake killed 240,000. Accurate predictions only give a warning of seconds; sensors can raise the alarm at the speed of light, while shock waves take longer to travel; that may be enough to stop trains and lifts, save computer data and stop pumping dangerous chemicals.

No one can prevent an earth quake from happening, but water pumped into boreholes can lubricate a fault, thus reducing friction and releasing stress. This could still result in damage, however, with expensive legal consequences, and is an option not without its own risk.
The way a building is constructed can reduce potential damage by earthquakes. Houses made of adobe and inadequately reinforced concrete can fall easily with great loss of life, especially if built on soft sediments. Modern earthquake resistant buildings are stiffened or built on rubber bearings to dampen out vibrations. Some Japanese buildings have active systems that move weights to cancel out the effects of seismic shock waves.

Saturday, September 4, 2010

Epicenter of Earthquake

Although displacement in an earthquake is usually along the plane of a fault, seismic waves appear to radiate out in all directions from a single point. The epicenter of a quake is the point on the ground directly above that focus, which is called the hypo center. The focus may be many kilometers deep within the Earth. Quakes on subduction crustal plates can occur at such a depth that the surrounding rock is more molten and therefore too soft to sustain a brittle fracture. These quakes happen when minerals suddenly change into a denser phase as a result of increases in temperature and pressure.
The San Andreas Fault in California is the most famous crack in the world. Since the great earthquake of 1906 in San Francisco, no one has doubted the fault’s destructive power, released as the pacific crustal plat e slides slowly north past the North American plate. In places the fault lines cross urban and industrial areas where earth quake activity could be potentially devastating.

Our knowledge of why earthquakes happen is a major step towards improved prediction. In 1992 a 7.4 magnitude earthquake in California was mapped by combining satellite radar images take before and after the shock. The closest contours around the fault show the zone of maximum ground displacement. Multiple faulting caused the confuse zone near the epicenter. Radar mapping is more accurate than field surveys, which require monitoring equipment to be set up before shock.