Sandro Lanfranco

Department of Environmental Science, GF Abela Junior College, University of Malta, Msida

The solar system (including the Earth) formed approximately 4.6 billion years ago through the condensation of an interstellar cloud of gas. This cloud, referred to as the solar nebula, started to contract and therefore heated up. The central parts of the cloud formed the Sun whilst the peripheral regions of the cloud gave rise to the Earth and the other planets and materials( asteroids, comets, debris) of the Solar System. At this early stage in its history, the Earth was very different from its present form as it was composed of an unordered accumulation of fragments with no sharply defined internal structure. Such fragments were continually accumulating through meteoritic impacts. This continuous meteoritic bombardment, coupled with heat derived from radioactive decay within the Earth, raised the internal temperature of the planet to such an extent that it became molten. Since the Earth was now in fluid form the different chemical compounds making it up could now flow according to their differences in density. The heavier materials, such as iron, gravitated towards the central regions of the Earth whilst the lighter molten rock floated above this. The central iron-rich region was to form the core whilst the lighter rock gave rise to the mantle. Once this process of differentiation was complete, the Earth started to cool. The reasons for this are several and probably include the decrease in intensity of the heavy asteroid bombardment. The outermost region of the Earth was the first to cool and has since formed a thin crust. The interior of the Earth is however still molten. Temperature and pressure both increase with depth within the Earth.

The differentiation of Earth has generated three distinct layers, the central core, the mantle and the crust. The core (radius: 3471km) is almost entirely composed of iron and nickel and is subdivided into two regions: a liquid outer core (thickness 2260 km) and a solid inner core (thickness 1220 km). The high temperatures within the outer core maintain a fluid state, whilst the very high pressure in the inner core promotes a solid state. The mantle (thickness 2283 km) is composed of lighter rocky materials mainly composed of the elements magnesium, silicon, iron and oxygen. The crust is the rocky outer layer of the Earth, extending to a mean depth of 35 km beneath the continents and to a mean depth of 7 km beneath the ocean floor. Two main forms of crust are distinguished: oceanic crust and continental crust. Oceanic crust is chiefly composed of basalt whilst continental crust is made up of granite. In general, continental crust is much more ancient that oceanic crust, and may be as old as 4500 million years while ocean crust is seldom older than 200 million years. The minerals present in the crust are largely made up of calcium, aluminium, magnesium, sodium, potassium, iron, silicon and oxygen. The solid, rigid outer part of the Earth, comprising the crust and part of the upper mantle is referred to as the lithosphere.

The lithosphere is not a single, homogenous block of material, but is dissected into a number of interlocking plates of varying dimensions. Six large plates (called major plates) and several smaller plates (minor plates) are recognised. Plates may be composed of ocean crust or of an assemblage of both continental and oceanic crust. Plates are not static but are in continuous motion, driven by convection currents within the upper mantle. Since plates are interlocking and in continuous motion, their margins tend to interact in three ways:

• By moving away from each other, forming a divergent boundary (also called a constructive boundary)
• By moving towards each other, forming a convergent boundary (also called a destructive boundary)
• By sliding alongside each other forming a transform boundary (also called a conservative boundary)

Divergent boundaries occur at localities (called mid-ocean ridges) where plates are moving away from each other. When two plates separate, the space created between them is filled by magma welling up from the mantle. Such magma solidifies to form basalt, filling the gap between the separating plates. New rock is therefore formed, explaining the derivation of the term "constructive boundary"

Convergent boundaries occur at collision zones where plates are moving towards one another. This boundary is distinguished by several features including volcanism, earthquake activity, mountain-building and formation of ocean trenches. In cases where collision between an ocean plate and a continental plate occurs, the sequence of events is predictable. The two plates push against one another until the denser plate (in this case, the ocean plate) is forced downward into the mantle. This downward subduction of the continental plate generates earthquake activity. As the ocean plate sinks into the mantle, water escapes from the subducting slab causing part of the mantle to melt and form magma. This magma travels vertically upward and encounters the overlying continental crust forming a volcanic arc. When two plates collide, the sediments situated between them are upthrust to form a mountain range, and orogeny is therefore a significant characteristic of convergent boundaries. The subducting slab enters the mantle at an angle, giving rise to a deep trench between the two plates. One such trench (depth 5100m) is present in the Tyrrhenian Basin of the Mediterranean Sea. The deepest such trench is the Marianas Trench (depth 11000m), located close to the Philippines. Convergent boundaries are also marked by distinct earthquake belts (or seismic belts) characterised by frequent earthquake activity. The most prominent such belt occurs along the Rim of the Pacific Ocean, where the Pacific Plate is colliding with the Eurasian Plate, Philippine Plate and South American Plate. Another seismic belt occurs in the Mediterranean region and is caused by collision of the African Plate with the Eurasian Plate.

Rocks are the fundamental units from which the lithosphere is constructed and rocks, in their own turn are composed of minerals. A mineral is a solid substance with a specific chemical composition. Halite (NaCl), Calcite (CaCO₃ ) and Quartz (SiO₂) are all minerals. Rocks, on the other hand, are composed of aggregations of minerals. Limestone, sandstone, shale, slate, granite and basalt are all rocks. Rocks may be classified into three basic classes – igneous, sedimentary or metamorphic- depending on their mode of formation. Rocks that have solidified from molten magma are igneous. Such rocks that cooled below the surface of the Earth are termed intrusive and are characterised by large crystals – a consequence of their slow rate of cooling. Magma that cools on the surface of the Earth forms extrusive rock. These rocks are characterised by small crystals since they cooled rapidly. Basalt and granite are examples of igneous rock. Other rocks are composed of fragments of pre-existing rock. Such rocks are sedimentary. They usually from when loose, unconsolidated accumulations of sediments (such as sand, silt, clay, boulders, pebbles etc.) are cemented (or lithified) into a unified structure. Such lithification may occur in various environments, but generally proceeds most readily in aquatic environments. The presence of water accelerates the dissolution of various chemicals that may be present within the rock giving rise to a fluid that percolates between the grains of sediments. This fluid may eventually harden, acting as a chemical cement which bind the grains together forming a rock. Limestone, sandstone and shale are all examples of sedimentary rock. A third class of rock, metamorphic rock, forms when pre-existing rock is structurally modified by heat or pressure or both. For instance, mudstone, which is a sedimentary rock, metamorphoses into slate when heat and pressure are applied to it. As more heat and pressure are applied, slate (which is itself metamorphic) changes further and metamorphoses into schist. Schist may in turn metamorphose into gneiss if more heat and pressure are applied. If even more heat and pressure are applied, gneiss will start to melt and form migmatite. If further heart and pressure are applied migmatite melts completely and forms a magma from which igneous rock would arise upon cooling. The heat and pressure required for metamorphism are usually encountered in fault planes or at great depths within the Earth. To quote the previous example, a piece of rock that is recognisable as mudstone on the surface of the Earth will be metamorphosed into gneiss if transported to a depth of 20km below the surface. At this depth, the temperature and pressure are sufficient to reorder the crystalline structure of the original rock forming a metamorphic rock which bears little resemblance to mudstone.

Many minerals are rich sources of materials required by industry. These include copper, gold and silver. Most minerals are however not found on their native state but in combination with other chemicals as part of a chemical compound. Hematite (Fe₂O₃), for example is a mineral from which iron can be extracted, while bauxite is an important source of aluminium. Other economically important minerals are non-metallic. These include common salt (NaCl), limestone and fossil fuels.

Types of Rock