Understanding Crustal Rocks

Understanding Crustal Rocks

What is SIAL and SIMA?

  • The earth is made up of several concentric layers. The outer layer is the earth’s crust-the lithosphere which comprises two distinct parts.
  • The upper part consists of granitic rocks and forms the continents. Its main mineral constituents are silica and alumina so it is collectively referred to as the sial. It has an average density of 2.7.
  • The lower part is a continuous zone of denser basaltic rocks forming the ocean floors, comprising mainly silica, iron and magnesium. It is therefore called sima and has an average density of 3.0. The sial and the sima together form the earth’s crust which varies in thickness from only 3-4 miles beneath the oceans to as much as 30 miles under some parts of the continents. Since the sial is lighter than the sima, the continents can be said to be ‘floating’ on a sea of denser sima.

The Classification of Rocks

The earth’s crust is made up of various types of rocks, differing from one another in texture, structure, colour, permeability, mode of occurrence and degree of resistance to denudation.

Igneous Rocks

Igneous rocks are formed by the cooling and solidification of molten rock (magma) from beneath the earth’s crust. They are normally crystalline in structure. They do not occur in strata (layers) nor do they contain fossils. Igneous rocks may be subdivided based on mineral composition. When they contain a high proportion of silica, they are said to be acid. Acid igneous rocks, such as granite, are less dense and are lighter in colour than basic rocks These contain a of basic oxides, e.g. greater proportion 2. of iron, aluminium or magnesium and are thus denser and darker in colour in terms of origin there are two main classes of igneous rocks.

  • Plutonic rocks: These are igneous rocks, formed at some depth in the earth’s crust. They have cooled and solidified slowly so that large, easily recognised crystals have been able to form. These intrusive rocks such as granite, diorite, and gabbro, are exposed at the surface by the processes of denudation and erosion.
  • Volcanic rocks: These are molten rocks poured out of volcanoes as lavas. They solidify rapidly on the earth’s surface and the crystals are small.

Sedimentary Rocks

Sedimentary rocks are formed from sediment accumulated over long periods, usually under water. They are distinguished from the other rock types in their characteristic layer formation and are termed stratified rocks. The strata may vary in thickness from a few inches to many feet. The rocks may be coarse or fine-grained, soft or hard. The materials that form sedimentary rocks may be brought by streams, glaciers, winds or even animals. They are non-crystalline and often contain fossils of animals, plants and other micro-organisms. Sedimentary rocks are thus the most varied in their formation of all rocks. Sedimentary rocks are classified according to their age and different kinds of rocks formed during the same period are grouped. It is more useful to know the characteristics of the various kinds of rocks.
Sedimentary rocks may be classified under three major categories by their origin and composition.

  • Mechanically formed sedimentary rocks: These rocks have been formed from the accumulation of materials derived from other rocks which have been cemented together, Sandstones are probably the most familiar sedimentary rocks They are made from sand grains, often quartz fragments derived from granites. Their texture, composition and colour vary tremendously. Many types of sandstones have been quarried for building purposes or for making grindstones. A coarser type of sandstone is known as grit. When larger pebbles are firmly cemented to form a rock, it is called conglobe rate when the pebbles are rounded, or breccia when the fragments are angular. The finer sedimentary materials form clay, widely used for brickmaking, shale or mudstone. Sand and gravel may occur in uncemented form.
  • Organically formed sedimentary rocks: These rocks are formed from the remains of living organisms such as corals or shellfish, whose fleshy parts have been decomposed, leaving behind the hard shells The most common rocks formed in this way are of the calcareous type. They include limestones and chalk. The carbonaceous rocks are also organically formed but from vegetative matter swamps and forests. The pressure of overlying sediments has compressed the plant remains into compact masses of carbon which eventually become peat, lignite or coal, all of which bear great economic value.
  • Chemically formed sedimentary rocks: Such rocks are precipitated chemically from solutions of one kind or another. (Rock salts are derived from strata which once formed the beds of seas or lakes. Gypsum or calcium sulphate is obtained from the evaporation of salt lakes, such as the Dead Sea, which have a very high salinity. In similar ways, potash and nitrates may be formed.

Metamorphic Rocks

All rocks whether igneous or sedimentary may become metamorphic or changed rocks under great heat and pressure. Their original character and appearance may be greatly altered by such forces, particularly during intense earth movements. In this manner, clay may be metamorphosed into slate, limestone into marble, sandstone into quartzite granite into gneiss, shale into schist and coal into graphite.