By Dr. S.M. ALAM NIA, Tando Jam.
Aug 19 - 25, 2002


Earth is the third planet from the sun and the fifth largest: orbit 149,600,000 km from sun; diameter 12,756.3 km; mass - 5,97224 kg. Earth is the only planet whose English name does not derive from Greek/Roman mythology. The name derives from Old English and Germanic. There are, of course, hundreds of other names for the planet in other languages. In Roman Mythology, the goddess of the Earth was Tellus the fertile soil (Greek: Gaia, terra mater - Mother Earth). It was not until the time of Copernicus (the sixteenth century) that it was understood that the Earth is just another planet. Earth, of course, can be studied without the aid of spacecraft. Nevertheless, it was not until the twentieth century that we had maps of the entire planet. Pictures of the planet taken from space are of considerable importance; for example, they are an enormous help in weather prediction and especially in tracking and predicting hurricanes. And they are extraordinarily beautiful. Th~e Earth is divided into several layers which have distinct chemical and seismic properties (depths in km): 0 - 40 crust; 40 - 400 upper mantle; 400 - 650 Transition region; 650-2700 Lower mantle; 2700-2890 D" layer; 2890 - 5150 Outer core; 5150 - 6378 Inner core.

The crust varies considerably in thickness, it is thinner under the oceans, thicker under the continents. The inner core and crust are solid; the outer core and mantle layers are plastic or semi-fluid. The various layers are separated by discontinuities which are evident in seismic data; the best known of these is the Mohorovicic discontinuity between the crust and upper mantle. Most of the mass of the Earth is in the mantle, most of the rest in the core; the part we inhabit is a tiny fraction of the whole (values below x10^24 kilograms): atmosphere = 0.0000051, oceans = 0.0014, crust = 0.026, mantle = 4.043, outer core = 1.835, inner core = 0.09675.

The core is probably composed mostly of iron (or nickel/iron), though it is possible that some lighter elements may be present, too. Temperatures at the center of the core may be as high as 7500 K, hotter than the surface of the Sun. The lower mantle is probably mostly silicon, magnesium and oxygen with some iron, calcium and aluminum. The upper mantle is mostly olivine and pyroxene (iron/magnesium silicates), calcium and aluminum: We know most of this only from seismic techniques; samples from the upper mantle arrive at the surface as lava from volcanoes but the majority of the Earth is inaccessible. The crust is primarily quartz (silicon dioxide) and other silicates like feldspar. The Earth is the densest major body in the solar system.

The other terrestrial planets probably have similar structures and compositions with some differences: the Moon has at most a small core; Mercury has an extra large core (relative to its diameter); the mantles of Mars and the Moon are much thicker; the Moon and Mercury may not have chemically distinct crusts; Earth may be the only one with distinct inner and outer cores. Note, however, that our knowledge of planetary interiors is mostly theoretical even for the Earth.

Unlike the other terrestrial planets, Earth's crust is divided into several separate solid plates which float around independently on top of the hot mantle below. The theory that describes this is known as plate tectonics. It is characterized by two major processes: spreading and subduction. Spreading occurs when two plates move away from each other and new crust is created by upwelling magma from below. Subduction occurs when two plates collide and the edge of one dives beneath the other and ends up being destroyed in the mantle. There is also transverse motion at some plate boundaries (i.e. the San Andreas Fault in California) and collisions between continental pla~tes (i.e. India/Eurasia). There are (at present) eight major plates: North American Plate North America, western North Atlantic and Greenland; South American Plate South America and western South Atlantic; Antarctic Plate Antarctica and the "Southern Ocean"; Eurasian Plate eastern North Atlantic, Europe and Asia except for India; African Plate Africa, eastern South Atlantic and western Indian Ocean; Indian-Australian Plate India, Australia, New Zealand and most of Indian Ocean; Nazca Plate eastern Pacific Ocean adjacent to South America, Pacific Plate most of the Pacific Ocean (and the southern coast of California!).

There are also twenty or more small plates such as the Arabian, Cocos, and Philippine Plates. Earthquakes are much more common at the plate boundaries. Plotting their locations makes it easy to see the plate boundaries (right). The Earth's surface is very young. In the relatively short (by astronomical standards) period of 500,000,000 years or so erosion and tectonic processes destroy and recreate most of the Earth's surface and thereby eliminate almost all traces of earlier geologic surface history (such as impact craters). Thus the very early history of the Earth has mostly been erased. The Earth is 4.5 to 4.6 billion years old, but the oldest known rocks are about 4 billion years old and rocks older than 3 billion years arerare. The oldest fossils of living organisms are less than 3.9 billion years old. There is no record of the critical period when life was first getting started. 71 per cent of the Earth's surface is covered with water. Earth is the only planet on which water can exist in liquid form on the surface (though there may be liquid ethane or methane on Titan's surface and liquid water beneath the surface of Europe). Liquid water is, of course, essential for life as we know it. The heat capacity of the oceans is also very important in keeping the Earth's temperature relatively stable. Liquid water is also responsible for most of the erosion and weathering of the Earth's continents, a process unique in the solar system today (though it may have occurred on Mars in the past).

The Earth's atmosphere is 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. There was probably a very much larger amount of carbon dioxide in the Earth's atmosphere, when the Earth was first formed, but it has since been almost all incorporated into carbonate rocks and to a lesser extent dissolved into the oceans and consumed by living plants. Plate tectonics and biological processes now maintain a continual flow of carbon dioxide from the atmosphere to these various "sinks" and back again. The tiny amount of carbon dioxide resident in the atmosphere at any time is extremely important to the maintenance of the Earth's surface temperature via the greenhouse effect. The greenhouse effect raises the average surface temperature about 35 degrees C above what it would otherwise be (from a frigid -21 C to a comfortable +14 C); without it the oceans would freeze and life as we know it would be impossible.

The presence of free oxygen is quite remarkable from a chemical point of view. Oxygen is a very reactive gas and under "normal" circumstances would quickly combine with other elements. The oxygen in Earth's atmosphere is produced and maintained by biological processes. Without life there would be no free oxygen. The interaction of the Earth and the Moon slows the Earth's rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 48118-hour days in a year. Earth has a modest magnetic field produced by electric currents in the outer core. The interaction of the solar wind, the Earth's magnetic field and the Earth's upper atmosphere causes the auroras (see the Interplanetary medium). Irregularities in these factors cause the magnetic poles to move and even reverse relative to the surface; the geomagnetic north pole is currently located in northern Canada. (The "geomagnetic north pole" is the position on the Earth's surface directly above the south pole of the Earth's field).

The Earth's magnetic field and its interaction with the solar wind also produce the Van Allen radiation belts, a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around the Earth. The outer belt stretches from 19,000 km in altitude to 41,000 km; the inner belt lies between 13,000 km and 7,600 km in altitude. Earth, fifth largest planet of the solar system and the only planet definitely known to support life. It is the third in order from the sun. This mean distance from the earth to sun is 93,000,000 miles. The earth rotates from west to east about a line (its axis) that is perpendicular to the plane of the equator and passes through the centre of the earth, terminating at the north and south geographical poles. The earth revolves about the sun once in a period of a little more than 3651/4 days (a year). The surface of the earth is divided into dry land and oceans, the dry land occupying 57,469,928 sq. miles and the oceans 139,480,841 sq. miles. The interior of the comprises solid rock to a depth has been estimated at from 2,000,000 to over 4,000,000 years.

The chemical composition of the earth's crust (% by weight) is: Oxygen (46.71); Silicon (27.69); Aluminium (8.07); Iron (5.05); Calcium (3.65); Sodium (2.75); Potassium (2.58); Magnesium (2.08); Titanium (0.62); Hydrogen (0.14) and total composition is (99.34). The remaining 80 -odd elements comprise of the order of 0.66% and include important plant nutrients such as N, B, Co, Mn, Cu, Zn, S, P. What there is of these elements tends to be concentrated in isolated deposits. These elements exist in combination as minerals, and minerals exist in combination as rocks. But, because of their varying modes of origin rocks may differ other than in their mineral composition.

As a result of the forces at work controlling this cycle rocks differing in form and composition arise. Two processes are involved: i) Denudation and deposition under the control of external forces such as climate; ii) Earth movements due to forces of internal origin - pressure and temperature. The earth has passed through phases of flat monotony when i) dominant or of bold relief when ii) dominant. That is geological ages are characterized by either sedimentary of igneous activity.

For a while the rock, as it were, is loaned to the soil and whilst in the soil may be further resorted as a result of pedological weathering. This phase in the massive geochemical cycle is that upon which Man largely depends for his food. The cycle as a whole is a continuous weathering, sifting, sorting, and reconsolidation cycle the chemistry of which is the interest of the geo-chemist and the dynamics that of the geophysicist. For few soils in Britain are formed the weathered residue of the underlying rock. Most are formed within a mineral skeleton which is an assortment of residues assembled of resorted by the geomorphological processes of gravity, water, wind and ice.

Most minerals are crystalline in form and may be identified by their optical properties. Their chemical composition may vary within limits due to the phenomenon of isomosphous replacement. Thus, during crystal formation one eIement in abundant supply may substitute for another within the crystal lattice providing its ionic diameter is within 10-15% of that required. It is seldom possible, therefore, to give precise formulae for particular minerals.

MINERALS OF IGNEOUS ROCKS: Quartz: SiO2; Felspars: a) Microcline and Orthoclase Group - KalSi3O8 - resistant to weathering, b) Plagioclase Group e.g. Albite - NaAI2Si3O8 - resistant, Anorthite - CaAl2si3O8 - less resistant; Felspathoids: e.g. Nepheline NaAISiO4 Leucite KalSi2O6; Micas: a) Muscovite characterized by lack of divalent cations - Kal2(AlSi3)O10 (OH)2 - resistant, b) Biotite - K(Mg, Fe)3 (AlSi3)O10 (OH)2 - less resistant, Ferro-magnesium Minerals: generally less resistant; Olivine: (Mg, Fe)2SiO4; Amphilboles: complex silicate of Ca, Mg, Fe, Na, A1.; Pyroxenes: a) Enstatite - Hypersthene Series - (Mg, Fe) SiO3, b) Augite - complex silicate of Mg, Fe, Ca, A1; Iron Oxides: Magnetite Fe304, Ilmenite FeOTiO2.

The nature of the minerals present in an igneous rocks depends on the bulk composition of the part of the magma crystallizing. The texture of an igneous rock depends on the manner of cooling: Slow large crystals; very rapid glassy if cooling time insuff'cient for crystal formation; variable porphyritic a few large crystals in a fine - textured matrix.


Igneous Rocks: Acid > 66%; Intermediate 66-52%; Basic 52-45% and Ultra - Basic <45%; Sedimentary rocks: Conglomerate, sandstone, mudstone, shale, limestone; Metamorphic rocks: Conglomerate schist, quartzite, mica schist, marble, serpentine.

All these rocks body break into small pieces through weathering (physical, mechanical, chemical and biological) processes resulting in the formations of different types of soil all over the world.