MINERALS AND FUELS

Minerals occur naturally in the rocks of the earth’s surface. They may be either elements or compounds. An element is a single, pure substance. Gold is sometimes found as nuggets of the element. A compound is two or more substances bound together in such a way that chemical processes are necessary to separate them. Iron is usually found as a compound. Most rocks are mixtures of several minerals.

Native elements and ores

Arsenic, copper, iron, gold, silver and sulphur are all elements that may occur by themselves. Geologists call them native elements. Most minerals are compounds of several elements. Geologists have discovered about 20000 minerals, of which only about 100 are common.
The two main groups of minerals are metallic and non-metallic. Metallic minerals contain native elements and ores from which minerals can be extracted. Examples of extracted; bauxite (aluminum); galena (lead and silver); malachite (copper); pitchblende (uranium) and pyrites (iron and sulphur). Non-metallic minerals include graphite, gypsum, halite (rock salt), quartz, talc and diamond.

Identifying minerals

The best way to identify minerals is by chemical analysis in a laboratory, but geologists have devised some simple tests which can be carried out in the field to provide the first clues to the presence of minerals. Laboratory tests are then carried out later on samples. Field tests to identify minerals use colour and streak, lustre, hardness, crystal forms and cleavage.
Colour can sometimes be misleading, but experienced geologists can often make a good guess based on colour. Azurite, for example, is one mineral that is always blue.
The steak of a mineral is the colour produced by scratching the surface to get a small amount of powder, which is often a different colour from the solid mineral.
Lustre is the way the mineral reflects light and with experience it is possible to identify, for example, diamonds, which have a brilliant lustre; quartz, which has a glassy or vitreous lustre; and some other minerals which have a metallic lustre.
Hardness is tested in the field by trying to scratch one mineral with another or with a metal instrument like a knife of file. Geologists use a scale of hardness called Mohs scale to compare and measure hardness in the field. Mohs’ scale lists ten common minerals, from talc, which is very soft, up to diamond, which is extremely hard. A mineral high on the scale will scratch all those below it but not the one above it, so it is possible to grade the hardness of mineral samples in this simple way.
Different minerals have different crystals, and large crystals are easy to identify because each mineral has its own distinctive form. Some minerals, like galena and halite, form a cube; some, like zircon, form twelve-sided crystals; some, like quartz, form long hexagonal crystals.
Cleavage is the way a mineral splits or flakes. Some minerals split easily, others do not. Cleavage runs parallel to the faces of the mineral crystal and is therefore not the same as fracture, or breaking, which may run in any direction in any mineral.

Searching for minerals

Many geologists work as prospectors for mining companies, or governments. They search for water, metals, coal, natural gas, petroleum, uranium and other valuable minerals such as lead, zinc and copper.
The early prospectors lead lonely lives and suffered many hardships in their search for gold and other valuable minerals. Their simple equipment consisted of picks, shovels, and large, flat, sieve-like pans for ‘washing’ earth and minerals.
The modern prospector is usually a geologist trained in chemistry and physics. In the search for valuable metals, minerals, fuels and even water, he will use a wide range of scientific tools. Aerial photographs and pictures transmitted back from satellites are used to show up features of the terrain, or earth’s surface, which cannot easily be seen from the ground. By studying the fossils, prospectors can tell the age of rocks; by detonating explosives in the ground and measuring the shock waves with instruments called seismographs, they can tell the density of the rock. For locating radioactive substances, such as uranium, they use Geiger counters, which respond to the amount of radioactivity in the area by clicking.
Modern prospecting methods are particularly important today in the search for more oil and one of the under-ground structures of particular interest is an anticline. This is an upfold caused by pressure in subterranean layers of rock. Layers of rock are pushed into shapes like arches or domes and caverns can form inside them. Natural gas and petroleum are often found under these domes.

FOSSILS

What fossils are

Fossils are the evidence of life on earth many millions of years ago. They are always found in sedimentary rocks, which formed mainly in lakes, seas and swamps. Fossils are the remains of plants and animals, and in soem cases almost the entire bodies of animals have been preserved.
Some fossils are petrified or turned to stone, some are carbonised or turned to carbon, and other are moulds or casts. Animal tracks and the marks made by waves and winds have been fossilised in certain kinds of rock strata and formations.
The rocks containing fossils were once buried deep under other layers of sedimentary rock, but from time to time the earth’s surface moves, such as when mountain ranges are formed, and the old rocks become exposed. Erosion wears away the rocks and we can see the fossils.

How fossils are formed

Usually, when an animal dies in the open, wind and rain will soon crumble even the hardest bones. But if the dead creature is quickly covered over by sediment, it is protected from decay. When there are many fossils of sea creatures in rocks, it is usually because the sea bottom was covered in a thick, soft ooze. Dead creatures sank to the bottom and later became sedimentary rock.
The early creatures of the Pre-Cambrian period were probably soft-bodied and so their remains were not so easily preserved. When creatures developed shells, bones and teeth, their remains could become fossilised much more easily.
Because animal bodies usually decay after death, few have survived in their original state. However, some woolly mammoths have been discovered in the frozen soil of Alaska and Siberia. Here, the climate has been arctic for many centuries and the mammoths were preserved by the cold just as they died. In fact, even though the bodies were 30,000 years old when they were discovered, the meat was still fresh.
Fossils become petrified, that is, turned into stone when the dead creature is buried and the soft parts to decay. Water containing a mineral such as silica seeps into the spaces left by the decayed parts, like was filling a mould. Over many years the silica completely fills the spaces and eventually process takes place with the trees and plants. One famous example is the petrified forest in Arizona.

Looking for fossils

If you want to look for fossils, first find a place where sedimentary rocks are exposedm sucn as a canyon, a sea cliff or a quarry. A good way to start is to pick up and carefully examine small pieces of rock that have fallen down. If the fossil is in a big piece of rock you can chio the whole piece away wuth a chisel or a geologist’s hammer, but the rock around the fossil itself must be removed very carefully with a smaller tool like an awl, for fossils are very fragile.

GEOLOGICAL TIME

The age of the Earth

Until about 300 years ago, most people believed that the earth was formed qiute recently. Some considered that the earth was created in 4004 BC, a date that was calculated from the number of generations of people listed in the Bible. Fossils found in some sedimentary rocks were thought to be creatures that perished in the Flood that is described in the Bible.
In the 1880s, however, scientists began to realize that fossils were the remains of creatures which may have lived thousands or millions of years ago. They also came to understand that fossils represented the story of life on earth. But at the time no one could determine the true age of rocks and fossils. With the discovery of radio-activity and radioisotopes it became possible to fix the age of rocks by a process called radiocarbon dating. Now it is believed that the earth is about 4550 million years old and the oldest rocks about 3500 million years.
By studying rocks, scientists have been able to trace a broad history of the earth. They have identified periods of great volcanic activity, great changes of climate, periods when great mountains were worn down to flat plains.

Geology and paleontology

Geology is the study of the earth, in particular the nature and distribution of the materials that form the crust. It has many branches. The study of rocks is called petrology, the study of minerals is mineralogy. There is also geophysics which applies the laws of physics to the study of the earth, geochemistry which applies chemistry to the study of earth’s crust and structural geology, which studies the arrangement and structure of the rocks of the earth’s surface. All of these branches of geology come under the heading of ‘physical geology’. Historical geology covers the history of the earth, and includes paleontology, the study of rock srata and fossils.
When scientists began the study of sedimentary rocks they discovered that, in some places, they are hundreds of metres deep. It takes a long time for sedimentary material to accumulate on the bottom of seas and lakes today, so this was the first hint that the earth is very, very old. Taking into account that when rocks remain undisturbed the oldest will lie at the bottom, the scientists realized that fossils could be used to establish the relative ages of the rocks. One of the pioneers in this work was William Smith, a British engineer. He discovered that, although some fossils occur in many strata or layers of rock, others, called index fossils in different rocks, geologists could establish that the rocks were of the same age.
Using Smith’s methods, geologists began to classify rocks in order of age and were able to divide the history of the earth into six main eras. These eras were then divided into periods, and the periods were divided into epochs. These are named so that the when they are discussed everyone, wherever they come form, knows which part of the earth’s history is being talked about.

Geological eras

Rocks formed more than 570 million years ago are called Pre-Cambrian and they contain very few fossils. However, rocks formed in the seas of the Cambrian period, about 570 to 500 million years ago, are quite rich in fossils, providing that some forms of life were abundant at that time.
The word ‘Cambrian’ come from Cambria, the old Roman word for Wales, because it was there that these rocks were first studied. In the Cambrian period, shallow seas spread over large areas, including most of Britain and about one-third of North America. In some parts of North America, movements of the earth’s crust pushed and folded the rocks up into mountains such as the Appalachians, and the seas disappeared.
Life at this time was of the invertebrate kind, that is, animals which have no backbones. Typical animals were the sea-dwelling brachiopods, molluscs, trilobites and an early type of coral. As far as we know there was no plant or animal life on land at this time.
The Palaeozoic era was a long period, lasting about 345 million years, during which plant and animal life began to evolve slowly on earth. The first vertebrates or animals with backbones appeared. These included fish, amphibians and reptiles. The first land plants that we know of also seem to have occured during this period.
An important part of the Palaeozoic era is the Carboniferous period. It lasted about 65 million years during this time the coal that we use today was formed from rich plant life that grew in swamps. Coal-forming plants included trees that grew to 30 metres tall, and giant ferns that are now extinct. Some big deposits of limestone were also formed during this period.
Beginning about 225 million years ago, the Mesozoic era lasted for 160 million years. For convenience, scientists divide it into three periods, the Triassic, the Jurrasic and the Cretaceous. This was a very exciting evolutionary era, for it was then the dinosaurs made their appearance. The dinosaur is one of the largest creatures ever to walk the earth. Other spectacular creatures included the flying reptile called the pterodactyl, various marine reptiles and the first tru bird. Primitive mammals also evolved during this time and flowering plants spread rapidly. In the seas were the spiral-shaped animals called ammonites, and coral.
The word ‘cenozoic’ comes from Greek words meaning ‘new life’, and the Cenozoic era covers the last 65 million years of the earth’s history. Throughout this time most of the living things that exist today evolved. The great mountain ranges of the world, the Alps, the Himalayas and the Andes, were also formed during the Cenozoic era. It was towards the end of this era that man developed.

THE EARTH’S STRUCTURE AND CRUST

The earth’s globe is made up of three layers. The surface layer is called the crust. It’s thickness varies from about 8 km under the oceans to about 30 to 50 km under the continents. The next layer is the mantle, made up of harder, denser, rocks than the crust. It is about 3000 km thick. Next is the core, believed to be liquid outside, then as far as we know, solid right at the centre.

Igneous rocks

The rocks in the crust are classified into three main groups, according to how they were formed: igneous rocks, metamorphic rocks and sedimentary rocks.
‘Igneous’ comes from the Latin word for fire, and igneous rocks are formed from molten material called magma that has cooled and hardened. Magma is molten rock. It probably come from the mantle where solid rock is turned into liquid through a combination of great heat and pressure. When this happens, some of the magma is forced up through cracks in the earth’s crust. An active, or erupting, volcano forces out streams of magma in the form lava. Lava cools very quickly when it reaches the air and becomes one form of igneous rock.
The type of igneous rock formed by cooling lava is called extrusive, because it is forced out or extruded from the interior of the earth. The speed at which the lava cools causes small crystals to form and the rocks are therefore smooth-surface of fine-textured like basalt and obsidian, although pumice is porous becaise of the gas bubbles.
Another type of igneous rock is called intrusive and here the magma cools slowly beneath the earth’s surface. This sometimes forms large masses called batholiths. These rocks are coarse-textured with large crystals resulting from slow cooling. Typical intrusive igneous rocks are granite, gabbro and syenite.
Minerals found in both extrusive and intrusive igneous rocks include felspar, hornblende, mica, quartz and pyroxene. Some of the oldest rocks on earth are igneous, but so are some of the newest. Every time you hear of a volcano erupting and pouring out lava, you know that when the lava cools, new igneous rocks will be formed.

Sedimentary rocks

Sedimentary rocks are formed by the slow but never-ending process of erosion or weathering which breaks down the hardest rocks to small particles. Over millions of years these particles of rock are carried from their original site by the wind, rivers, glaciers and ice sheets to be deposited somewhere else. Eventually, layers of rock sediment are built up and become compacted and cemented together to form new rocks. Sedimentary rocks form a small part of the earth’s crust, but they form a large part of the surface of the earth, because it is only on the surface that weathering and erosion takes place. Sedimentary rocks are important because they sometimes contain fossils (the remains of plants and animals) which give clues to what life was like on earth hundreds of thousands of years ago.

Metamorphic rocks

Metamorphism is a word that means change from one thing to another. Metamorphic rocks are rocks that were originally either igneous or sedimentary and have been changed by heat or pressure or a combination of both. Metamorphism is caused by the upwelling of molten magma, or by great movements of the earth that squeeze and crumple rocks into mountain ranges.
When magma forces itself to between cracks int he earth’s crust, it is extremely hot and its heat bakes and hardens rocks around it. Sometimes the heat releases substances from the rocks and this also changes their composition. When mountains are formed, pressure crushes and grinds the rocks. When heat and pressure are combined, other changes takes place. This is how we get marble, which was originally sandstone. Slate is another example, formed from compressed clay and shale. As slate is in layers which can be split easily, it can be used for school blackboards and roofing tiles. Today it is mainly used for flooring tiles.

THE EARTH AND THE SOLAR SYSTEM

Our Earth is one of the planets of the solar system, which is made up of the sun in the centre, nine planets including the Earth, and many smaller bodies. The smallest bodies that rotate around the sun are called asteroids, and fragments of these are meteors. Collections of dust and gas form comets.
All the objects in the solar system, including the Earth, are under the gravitational pull of the sun which holds them orbiting, or turning, around it.

The formation of the solar system

There are several ideas or theories about how the solar system was formed. One theory is that the solar system was created at the same time as the sun, which is a star, when it began to form from a giant cloud of gas. This gas drew together to form a core around which the force of gravity collected an encircling disc of left-over gas and particles of dust.
The particles which made up this disc collided with each other and caused the formation of other small solid bodies. These attracted more material fromt he disc until there was enough to form the planets. The other objects in yhe solar system, such as meteors, are the ‘rubbish’ left over fromt that time.
Our solar system is part of the universe, some of ehich we see when we look out at the night sky. No one knows just how big the universe is, but we believe it stretches farther than we can see with the best telescopes. Int he universe there are many systems like our solar system.
This theory therefore suggests that planets are formed as a result of star formation. In some cases, there are two stars in a system and, as far as we know, these have no planets, but there are plenty of single stars which we believe have planets rather like our own.

Galaxies and light years

A system of stars and planets like the one containing our solar system is called a galaxy. There are many galaxies in the universe.
There are great distances between the planets, and even the greater distances between the stars. These distances are measured in light years. One light year is the distance light travels i one year. Since light travels about 300,000 km a second, a light year is a vast distance. The sun is eight light minutes away, which means the light from the sun takes eight minutes to reach us. Looking into space we are looking backward into time. We see the sun as it was eight minutes ago and the nearest galaxy to ours, Andromeda, as it was millions of years ago.