[Rhyolite | Granite | Andesite | Diorite | Basalt | Gabbro | Syenite | Serpentinite | Peridotite | Pyroclastics | Pegmatite] |
[Conglomerate | Sandstone | Shale | Mudstone | Siltstone | Limestone | Chert | Coal] |
[Hornfels | Slate | Phyllite | Schist | Quartzite | Gneiss | Marble | Migmatite] |
|
|
This guide on the common rocks of Malaysia is aimed to help people with little or no previous knowledge of geology, particularly students secondary school to identify common rocks of Malaysia. There are far more rock types in Malaysia than what is covered in this pictorial guide, and this guide will be constantly updated to include more rocks. Future updates will include photomicrograph of rocks and common minerals. This guide is modified from the poster Common Rocks of Malaysia published by the Geological Society of Malaysia. |
|
|
|
|
|
Last updated: 30 April 2005
Send your comments to the
webm@ster; or to the original site
editor here
Introduction
The earth is made of rocks, which are aggregates of minerals. Rocks can be categorized into 3 groups: IGNEOUS ROCKS, SEDIMENTARY ROCKS, and METAMORPHIC ROCKS. Igneous rocks are crystallized from molten rock materials. Sedimentary rocks are produced by the accumulation material produced by the breakdown of the other rocks at the earth's surface. Metamorphic rocks are formed through the alteration of igneous and sedimentary rocks by heat and pressure.
|
|
Igneous rocks are rocks that have crystallized from molten rock material called magma. The magma may reaches the surface of the earth, and flow over it as lava. The rocks formed in this process is called extrusive ingeous rocks or volcanic rocks. Examples of volcanic rocks are basalt and rhyolite. Intrusive igneous rocks such as granite and gabbro are crystallized in the earth's crust. The identification and naming of igneous rocks is based on the relative amounts of constituent minerals, grain size, texture and colour index. Common textures of igneous rocks Differences in the degree of crystallinity, and in the size of the crystals determine the texture of an igneous rock. Both of these factors are controlled primarily by rate of cooling, though the chemical composition of the magma and its content of volatile materials play important roles. Basic Textures Pyroclastic (Greek, "broken by fire"). Composed of slivers of volcanic glass, bits of frothy pumice, phenocrysts, and broken fragments of volcanic rock deposited together. The glass slivers and pumice may be largely altered to clay. Pyroclastic rocks are the products of volcanic explosions or of pyroclastic flows. Glassy. Composed almost entirely of massive or streaky volcanic glass. Small phenocrysts of feldspar or other minerals may be scattered throughout the glass. The glass may be frothy, filled with minute bubbles, forming a pumiceous glassy texture. Aphanitic (Greek, "invisible"). Composed chiefly of tiny crystals (less than 0.5 millimeter in diameter), with or without a glassy residue between the crystals. The crystals are mere specks, large enough to be seen but too small to identify without the aid of the microscope. Their presence gives the rock a stony or dull luster in contrast to the glassy (vitreous) luster of rocks with glassy texture. Most lava flows have aphanitic texture; in some, flow has aligned the tiny mineral grains, giving a streaky or flow-banded appearance. Granular (Latin, "a grain"). Composed of crystals that are of approximately equal size and large enough to be seen and identified without the aid of lens or microscope. In different rocks the average size may vary from about 0.5 millimeter to more than 1 centimeter in diameter, but the common granular rocks such as granite have grains averaging from 3 to 5 millimeters in size. Modifying Texture Porphyritic. Composed of larger crystals set in a finer-grained groundmass, giving a spotted appearance. Because porphyritic texture is most common in small intrusive bodies or in lavas, it has been attributed to a change in the rate of cooling while the magma was crystallizing. The large crystals, called phenocrysts, are within a groundmass of aphanitic crystals. Such a lava has a porphyritic aphanitic texture. The adjective porphyritic is used to modify the prevailing texture of the groundmass. Rocks with porphyritic granular texture large crystals in a granular groundmass of finer grain are common in intrusive bodies. Porphyritic glassy texture appears in some lava flows, and in the pumice fragments of pyroclastic rocks. Rarely, conditions other than a change in the rate of cooling may produce porphyritic rocks. |
|
|
Sedimentary rocks are produced by the accumulation of material derived from the breakdown preexisting rocks, together with material of organic origin, on the earth's surface. The break down of rocks through weathering, erosion and transportation of rocks by physical agents such as river, wind and rain is helped by chemical decay. The material produced known as sediment is transported mainly by river, is deposited at river mouth, in lakes and in the sea. Sedimentary rocks produced by this mechanical process in called clastic rocks. Sedimentary rocks are also produced by accumulation of organic debris and by precipitation from solution of dissolved salts. Common textures of sedimentary rocks Differences in the nature of the constituent particles, and in how they are bound together determine the texture of a sedimentary rock. Clastic (Greek, "broken"). Composed of broken and worn fragments of pre-existing minerals, rock particles, or shells that have been cemented together. Further distinctions can be made on the size of the particles, and on the amount of rounding by wear of the individual fragments. Organic. Composed of accumulations of organic debris (shells, plant remains, bones, etc.) in which the individual organic particles are so well preserved (not notably broken and worn) that organic features dominate the texture of the rock. Crystalline. Composed of crystals precipitated from solution and therefore tightly interlocked by mutual interpenetration during growth. The rock owes its coherence to this interlocking of crystals, instead of to the presence of a cement as in the clastic and organic textures.
|
|
Metamorphic rocks are formed by alteration (metamorphism) of igneous and sedimentary rocks. Metamorphism occur in solid state when a rock is subjected to heat and pressure. The type of metamorphic rock produced depend on the type original rock material, and the temperature and pressure conditions. Common textures of metamorphic rock Differences in the orientation, or alignment, of the crystals and in the size of the crystals determine the texture of a metamorphic rock. There are two general textural groups: foliated textures (Latin, "leafy"), in which platy or leaflike minerals such as mica or chlorite are nearly all aligned parallel to one another so that the rock splits readily along the well-oriented, nearly parallel cleavages of its constituent mineral particles, and nonfoliated, composed either of equidimensional minerals or of randomly oriented platy minerals, so that the rock breaks into angular particles. Basic Textures Gneissose (Greek, "banded rock"). Coarsely foliated; individual folia are 1 millimeter or more, even several centimeters, thick. The folia may be straight, pancake-like, or wavy and crenulated. They commonly differ in composition; feldspars, for example, may alternate with dark minerals. Mineral grains are coarse, easily identified. Schistose (Greek, "easily cleaved"). Finely foliated, forming thin parallel bands along which the rock splits readily. Individual minerals are distinctly visible. The minerals are mainly platy or rodlike chiefly mica, chlorite, and amphibole. Equidimensional minerals like feldspar, garnet, and pyroxene may be present but are not abundant. Slaty (from Old High German, "to split"). Very fine foliation, producing almost rigidly parallel planes of easy splitting due to the nearly perfect parallelism of microscopic and ultramicroscopic crystals of platy minerals, chiefly mica. Granoblastic (Greek, "sprouting grains"). Unfoliated or only faintly foliated. Composed of mutually interpenetrating mineral grains that have crystallized simultaneously. Minerals are large enough to be easily identified without the microscope, and are chiefly equidimensional kinds such as feldspar, quartz, garnet, and pyroxene. Corresponds roughly to the granular texture of igneous rocks. Hornfelsic (German, "hornlike rock"). Unfoliated. Mineral grains commonly microscopic or ultramicroscopic, though a few may be visible. Breaks into sharply angular pieces with curved fracture surfaces. | |
