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The atmosphere,
hydrosphere, lithosphere, mantle and the
core are the five parts that make up the earth.
The
atmosphere, the first part, is a gaseous envelope that surrounds the
solid body of the planet. It has a thickness of more than 1100
kilometers; however, about half its mass is concentrated in the
lower 5.6 kilometers.
The
hydrosphere, the second part, is the layer of water that, in the
form of the oceans, covers approximately 70.8 per cent of the
surface of the earth. Although it chiefly consists of the oceans,
technically it includes all water surfaces in the world, including
inland seas, lakes, rivers, and underground waters. The average
depth of the oceans is 12,447 feet, more than five times the average
height of the continents. The mass of the oceans is approximately
1.35 quintillion metric tons, which is the equivalent of 1/4400 of
the total mass of the earth.
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The
third, fourth and the fifth layers, that is the lithosphere, mantle
and the core, are largely solid. The lithosphere, consisting mainly
of the cold, rigid, rocky crust of the earth, extends to depths of
100 kilometers. The rocks of the lithosphere have an average density
of 2.7 and are almost entirely made up of 11 elements, which
together account for about 99.5 per cent of its mass. The most
abundant is oxygen, which accounts for about 46.60 percent of the
total. Other elements include silicon, aluminium, iron, calcium,
sodium, potassium, magnesium, titanium, hydrogen and phosphorus. In
addition, eleven other elements are present in trace amounts of 0.1
to 0.02 per cent.
The
lithosphere comprises of two shells, the crust and the upper mantle.
These are divided into a dozen or so rigid tectonic plates. The
crust itself is divided into two parts. The
upper crust is referred to as Sial and the lower crust is
known as Sima.
The upper crust, which comprises of the continents, is made up of
igneous and sedimentary rocks whose average chemical composition is
similar to that of granite and whose density is about 2.7. The
simatic or lower crust, which forms the floors of the ocean basins,
is made of darker, heavier igneous rocks such as gabbro and basalt.
The
mantle and core are the heavy interior of the earth, making up most
of the earth’s mass. The mantle extends from the base of the crust
to a depth of about 2900 kilometers.
The
upper mantle is separated from the crust above by a seismic
discontinuity, called the Mohorovicic discontinuity, or
simply, Moho.
A weak zone, known as the asthenosphere, separates it from the lower
mantle below. The upper mantle is composed of iron and magnesium
silicates, as typified by the mineral olivine. The lower part may
consist of a mixture of oxides of magnesium, silicon, and iron.
Researchers say that the
core has an outer shell which is about 2225 kilometers thick.
Studies have shown that its outer surface has depressions and peaks,
the latter forming where warm material rises. In contrast, the inner
core, which has a radius of about 1275 kilometers, is solid. Both
core layers are said to consist largely of iron, with a small
percentage of nickel and other elements. The temperatures in the
inner core may be as high as 6650°C or 12,000°F. |
