WHAT ARE ROCKS?
The Earth, the Moon and the other planets are made up of the material we call rock. A rock is a collection of mineral grains and crystals solidified together in a coherent mass. There are many different types of rocks which have different mineral compositions. Minerals can be used to indicate the conditions of formation of the rocks in which they occur. The science of rocks is called petrology and a petrologist (geologist who studies rocks) is interested in the mineralogy of rocks as well as trying to unravel the record of the geological past which they contain. From reading the record of the rocks, it has been possible to learn so much about past climates and geography, past and present compositions and conditions of our planet. Rocks are classified into three main types according to how they formed : igneous, sedimentary, and metamorphic. Igneous rocks are formed by the solidification of molten rock material; sedimentary rocks are produced by the breaking down and accumulation of rock waste at the Earth's surface; while metamorphic rocks are produced through the alteration of igneous and sedimentary rocks by heat and pressure. These processes are schematically represented in Figure 1.
Igneous rocks are the rocks of fire. The word comes from the Latin word 'ignis' meaning fire. They are formed from magma (hot, molten rock from the upper part of the mantle) which migrates upwards into the Earth's crust and either cools and hardens as igneous intrusions below the Earth's surface, forming intrusive igneous rocks, or reaches the Earth's surface and flows out over it as lava, forming extrusive igneous rocks. Most of the islands of Fiji are mainly formed of extrusive and intrusive igneous rocks.
The crust of the Earth is formed mainly of rocks of relatively low density and is about 35 km thick under the continents but averages only about 7 km beneath the oceans; under Viti Levu, the crust is about 15-20 km thick. The mantle is the layer of denser rock beneath the crust. which extends down to a depth of nearly 3000 km.
Igneous activity is of great importance in the formation of economic metallic mineral deposits. At Fiji's largest gold mine, at Vatukoula, the gold is concentrated within extrusive volcanic rocks in the walls of a large volcanic caldera. At Namosi, near Suva, the copper minerals occur within both extrusive and intrusive igneous rocks. Similarly, elsewhere in Fiji, prospectors look for metallic minerals in areas with igneous rocks. Non-metallic mineral deposits such as coal and petroleum, may be destroyed by the high temperatures of igneous intrusions.
Extrusive igneous rocks
All rocks which are formed from magma cooling and solidifying on the Earth's surface are called extrusive igneous rocks, or volcanic rocks. Magma may flow from volcanoes or from fissures (long, narrow openings in the ground) as lava (Figure 1). For instance, Taveuni is composed of a line of volcanoes along a large fissure. Volcanic rocks are widely distributed in Fiji.
Extruded lava cools quickly, forming rocks such as basalt, andesite and dacite which are composed largely of small mineral grains. Basalt is a black rather dense rock which is the most abundant of the extrusive igneous rocks, and is thought to represent the primary magma which comes from the mantle. During violent volcanic eruptions magma may be fragmented when it reaches the surface, and form breccia, ash and pumice. Pumice is light, and commonly floats and can travel long distances. It is frequently found on Fiji's beaches, having drifted with currents from active volcanoes in Tonga.
Intrusive igneous rocks
Magma may also cool and solidify under the Earth's surface into intrusive igneous rocks or plutonic rocks . These rocks cool slowly and hence the mineral grains are coarse or large. They may eventually be exposed at the surface millions of years after they were formed if the overlying rocks are removed by erosion.
Granite is a light-coloured rock that is by far the commonest intrusive igneous rock. It is mineralogically and chemically different from basalt and is thought to be produced either from changing basalt magma or from crustal rocks. Intrusive rocks such as tonalite are found notably at Yavuna near Nadi and monzonite and gabbro in southern Viti Levu. Hot springs often indicate that hot or cooling magma is present not far beneath the Earth's surface and is heating underground water (Figure 1). In Fiji, hot springs are common in the Labasa and Savusavu areas.
The recognition and naming of igneous rocks relies upon assessing the composition (mineralogy), relative amounts and grain size of the constituent minerals, as well as the textural differences. Both extrusive and intrusive igneous rocks have similar mineral compositions but they show differences in texture that depend on their rate of cooling. Slow cooling, common for most intrusions, allows time for the growth of large crystals and gives rise to rocks with large crystals, forming coarse-grained rocks. Rapid cooling, common for all extrusions and some intrusions, does not allow time for the growth of large crystals and therefore produces either glasses (if quenched) or rocks with small crystals, called fine-grained rocks. Additional information is obtained from colour index, structure, and sometimes from relations with surrounding rocks.
Sedimentary rocks are produced by external processes within the Earth, whereas igneous and metamorphic rocks are produced by internal processes within the Earth. The Earth's surface is constantly undergoing weathering and erosion by physical agents such as rain, rivers, wind and moving ice, and chemical decay from percolating waters, and together they break up even the toughest rocks and produce rock waste. Sedimentary rocks are formed from this rock waste (sediment) which is transported, mainly by rivers, wind and ice, deposited, at river mouths, in lakes or in the sea, and then compacted and cemented together. They are usually layered rocks made up of pebbles, sand, silt, clay, broken rock fragments and the remains of living matter that have been deposited over thousands of years (Figure 1).
Sedimentary rocks can be recognised by their texture (including grain size), structure, mineralogy, field relationships with other rocks, and, to some extent, colour. They can be classified according to the source of the materials that formed them as clastic, chemical or organic.
Clastic sedimentary rocks are formed from rock fragments derived from the erosion of pre-existing rocks and mechanically transported by wind, water or ice. They are further subdivided according to grain size (see below for broad subdivision). Examples of this type of rock in Fiji include sandstone, siltstone and mudstone.
Chemical sedimentary rocks are formed by chemical action, e.g. precipitated from material dissolved in seawater or fresh water, residue left after leaching, or by chemical replacement of one mineral by another. They are further subdivided according to chemical composition (see below). Examples in Fiji are widespread limestones at Qalimare in Sigatoka and on many of the smaller outer islands. Bauxite deposits at Wainunu, Vanua Levu are chemical sedimentary rocks left as residue after leaching.
Organic sedimentary rocks are composed mostly of the remains of once-living organisms, whether plant or animal. They are further subdivided according to their chemical composition (see below).
In Fiji we find organic-rich limestones (at Cuvu near Sigatoka), and peat bogs which represent the first stage of coal formation. Phosphate deposits in Lau and the island nation Nauru are organic sedimentary rocks derived from the excreta of sea-birds.
Metamorphic rocks are produced when igneous and sedimentary rocks are subjected to high temperatures and pressures. The word metamorphism literally means changing form or shape therefore metamorphism refers to the process by which the composition or structure of the existing rocks is changed by the action of high heat or pressure or by liquids or gases percolating through them (metasomatism). Examples of metamorphic rocks are marble and slate. There are two types of metamorphism : regional and contact metamorphism.
Regional metamorphism is used to describe the widespread effects of metamorphism that accompany mountain building. It is caused by high pressure and heat due to stresses produced by faulting and folding when mountains are formed. Its effects may extend for several hundred kilometres. Rocks in southern Viti Levu were subjected to regional metamorphism during a stage of mountain building about 8 million years ago.
Contact metamorphism is restricted to the vicinity of igneous intrusions and is caused by heat given out by magma which bakes and changes rocks close to the hot magma (like dough is changed to bread in a hot oven). The temperature of magma is high, though variable, and usually in the range 700 to 1100oC. The magma heats up the rocks surrounding the intrusion, causing recrystallisation and growth of new crystals. There may be a zone of baked altered rocks around a hot magma intrusion called the contact metamorphic aureole (Figure 1). The size of the aureole depends on the temperature of the magma and the size of the intrusion. Rocks in southern Viti Levu were subjected to contact metamorphism about 12 to 10 million years ago.
Because of the wide variety of possible rocks from which they are formed and the range of temperatures and pressures to which they may have been subjected, metamorphic rocks display a wide range of texture, structure and mineralogy. Examples of metamorphic rocks are schist. gneiss, slate, quartzite and marble.
Changes that may occur during metamorphism include the formation of new minerals, e.g. garnet, and recrystallisation of the original rock. Limestone changes to marble (found at Wainivesi in Tailevu) and organic carbon compounds can change to coal, oil or diamond. Metamorphism may also concentrate valuable constituents so as to form an economic ore deposit.
ROCKS, MINERALS AND SOILS
Rocks, minerals and soils are part of the Earth's crust. Natural forces which cause great changes to the Earth's crust include : volcanoes, earthquakes, erosion and weathering. These changes are part of the life cycle of rocks. The life cycle of rocks can cover millions of years. During that time rock materials are melted, cooled, squeezed and pushed, hardened, weathered by wind and rain, carried along by rivers and streams, and eventually re-worked into another rock.
The Rock Cycle
Rocks, minerals and soils are constantly being recycled in a complex system of natural geological processes known as the rock cycle (Figure 2). Igneous rocks formed from rising magma are gradually weathered into small particles. These particles are transported and redeposited by wind, water or ice, and they eventually form sedimentary rocks. If these rocks are buried deep within the Earth they may be metamorphosed. Or eventually they may be remelted to form new igneous rocks.
Soil is made from rock that has been so broken down by weathering and erosion that it will support plant life. Most soils contain a certain amount of humus, dark organic material produced by the decomposition of plant and animal materials. Soil forms a thin layer over weathered rock and bedrock. Eventually over time, new rocks will form from the remains of soils. Soil is therefore a stage in the life history of rocks.
The Ecological Cycle
Rocks are not created once and for ever. Rocks and soil are all part of an ecological cycle of which humans are also an important part. The natural actions which created the rocks we see today are going on all the time and new supplies of rocks and minerals are always being formed.
USES OF ROCK