20 / Basics of Environmental Science
rotated. The inner planets formed by accretion. Small particles moved close to one another, were drawn
together by their mutual gravitational attraction, and as their masses increased they gathered more particles
and continued to grow. At some point it is believed that a collision between the proto-Earth and a very
large body disrupted the planet, the material re-forming as two bodies rather than one: the Earth-Moon
system. This explains why the Earth and Moon are considered to be of the same age and, therefore, why
lunar rocks 4.6 billion years old are held to be of about the age of the Earth and Moon.
The material of Earth became arranged in discrete layers, like the skins of an onion. If accretion was
a slow process compared to the rate at which the PSN cooled, the densest material may have arrived
first, followed by progressively less dense material, in which case the layered structure has existed
from the start and would not have been altered by melting due to the gravitational energy released as
heat by successive impacts. This model is called ‘heterogeneous accretion’. If material arrived quickly
in relation to the rate of PSN cooling, then it would have comprised the whole range of densities. As
the planet cooled from the subsequent melting, denser material would have gravitated to the centre
and progressively less dense material settled in layers above it. This model is called ‘homogeneous
accretion’ (ALLABY AND ALLABY, 1999).
As it exists today, the Earth has a mean radius of 6371 km, equatorial circumference of 40077 km,
polar circumference of 40009 km, total mass of 5976×10^24 g, and mean density of 5.517 g. cm-3. Of
its surface area, 149×10^6 km^2 (29.22 per cent) is land, 15.6×10^6 km^2 glaciers and ice sheets, and
361×10^6 km^2 oceans and seas (HOLMES, 1965, ch. II). Land and oceans are not distributed evenly.
There is much more land in the northern hemisphere than in the southern, but at the poles the positions
are reversed: Antarctica is a large continent, but there is little land within the Arctic Circle.
At its centre, the Earth has a solid inner core, 1370 km in radius, made from iron with some nickel
(see Figure 2.1). This is surrounded by an outer core, about 2000 km thick, also of iron with nickel,
but liquid, although of very high density. Movement in the outer core acts like a self-excit-ing dynamo
and generates the Earth’s magnetic field, which deflects charged particles reaching the Earth from
space. Outside the outer core, the mantle, made from dense but somewhat plastic rock, is about 2900
km thick, and at the surface there is a thin crust of solid rock, about 6 km thick beneath the oceans
and 35 km thick (but less dense) beneath the continents.
Miners observed long ago that the deeper their galleries the warmer they found it to work in them.
Surface rocks are cool, but below the surface the temperature increases with depth. This is called the
‘geothermal gradient’. A little of the Earth’s internal heat remains from the time of the planet’s
formation, but almost all of it is due to the decay of the radioactive elements that are distributed
widely throughout the mantle and crustal rocks. The value of the geothermal gradient varies widely
from place to place, mostly between 20 and 40°C for every kilometre of depth, but in some places,
Figure 2.1 Structure of the Earth (not to scale)