Environmental

When you have read this chapter you will have been introduced to:

• the formation and structure of the Earth

• rocks, minerals, and geologic structures

• weathering

• how landforms evolve

• coasts, estuaries, and changing sea levels

• solar energy

• albedo and heat capacity

• the greenhouse effect

• evolution, composition, and structure of the atmosphere

• general circulation of the atmosphere

• ocean currents and gyres

• weather and climate

• ice ages and interglacials

• climate change

• climatic regions and plants

Formation and structure of the Earth

Among the nine planets in the solar system, Earth is the only one which is known to support life. All

the materials we use are taken from the Earth and it supplies us with everything we eat and drink. It

receives energy from the Sun, which drives its climates and biological systems, but materially it is

self-contained, apart from the dust particles and occasional meteorites that reach it from space

(ADAMS, 1977, pp. 35–36). These may amount to 10000 tonnes a year, but most are vaporized by

the heat of friction as they enter the upper atmosphere and we see them as ‘shooting stars’. At the

most fundamental level, the Earth is our environment.

The oldest rocks, found on the Moon, are about 4.6 billion years old and this is generally accepted to

be the approximate age of the Earth and the solar system generally. There are several rival theories

describing the process by which the solar system may have formed.1

The most widely accepted

theory, first proposed in 1644 by René Descartes (1596–1650), proposes that the system formed

from the condensation of a cloud of gas and dust, called the ‘primitive solar nebula’ (PSN). It is now

thought this cloud may have been perturbed by material from a supernova explosion. Fusion processes

within stars convert hydrogen to helium and in larger stars go on to form all the heavier elements up

to iron. Elements heavier than iron can be produced only under the extreme conditions of the supernova

explosion of a very massive star, and the presence of such elements (including zinc, gold, mercury,

and uranium) on Earth indicates a supernova source.

As the cloud condensed, its mass was greatest near the centre. This concentration of matter comprised the

Sun, the planets forming from the remaining material in a disc surrounding the star, and the whole system 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.