ROCKS Rocks are conglomerations of minerals, and they form the bulk of the Earth. The main classification of crustal rocks is based on their origin. IGNEOUS.

ROCKS• Rocks are conglomerations of minerals, and they form the bulk of

the Earth. • The main classification of crustal rocks is based on their origin. • IGNEOUS ROCKS form from molten rock called magma. • Uplift and exposure of rocks at the Earth's surface can be affected

by weathering causing the rocks to break down into smaller grains which are transported and deposited as sediments.

• The sediments are lithified (compacted and cemented), and SEDIMENTARY ROCKS are formed.

• Changes in temperature, pressure, and/or rock or fluid chemistry can allow igneous and sedimentary rocks to change physically or chemically to form METAMORPHIC ROCKS.

• At higher temperatures, metamorphic (or any other rock type) rocks may be partially melted, and crystallization of this melt will create igneous rocks.

• Uplift and erosion can expose all rock types at the surface, re-initiating the cycle.

• The approximate volume proportions of these three rock types throughout the Earth’s crust are: igneous 65%: metamorphic 27%; and sedimentary 8%.

The Rock Cycle

Igneous

• Igneous rocks form by direct crystallization of minerals from a molten magma.

• When this magma cools slowly beneath the surface of the Earth, coarse-grained intrusive (plutonic) rocks crystallize: – extrusive (volcanic and pyroclastic) rocks

which crystallize quickly after the magma erupts at the earth's surface are typically very fine-grained. If the melt cools too quickly for crystals to grow, the result is a 'glass'.

Granite

• Granites are coarse-grained rocks with a mottled appearance. Weather-resistant stones are polished for use as decorative facings on buildings.

BasaltBasalt is one of the most common igneous rocks and is major constituent of the upper layer of the ocean floors (usually as pillow lava), and hot spot volcanoes (such as the Hawaiian islands).

It is a finely crystalline basic volcanic rock which is sometimes glassy.

It is recognized primarily by its dark color.

OBSIDIAN

• Obsidian is often referred to as volcanic glass, that has cooled too quickly for crystals to form. It is typically very black, shiny and glassy.

• Obsidian may be red-brown due to the presence of iron.

• It can have very sharp edges and it is used for knives and arrows, which makes it a very important archaeologival material.

SEDIMENTARY ROCKS• About 75% of the Earth’s continental crust is covered by

sedimentary rocks. • Sedimentary rocks form from pre-existing rock particles -

igneous, metamorphic or sedimentary. • The parent rock undergoes weathering by chemical and/or

physical mechanisms into smaller particles. • These particles are transported by ice, air or water. • Deposition takes place as a result of a lowering of energy,

organic biochemical activity or chemical changes. • Once deposited, the sediments, over time, are:

– lithified (turned into rock) through compaction (decrease in rock volume due to the weight of overlying sediment); and

– cementation (chemical precipitation in pore spaces between grains which "glues" the rock together).

LIMESTONES

• Limestone is a sedimentary rock comprising at least 80% calcium carbonate (CaCO3).

• Limestones can be deposited in both marine and fresh water environments, the former being more common, and can take many forms such as those described below.

CHALK

• Chalk is a pure, soft, well-jointed limestone composed primarily of the remains of micro-organisms.

SHELLY LIMESTONE• Fossiliferous

limestones consist of a large number of fossils held together mainly in a calcite cement.

• Carboniferous limestone is an example of a shelly limestone.

OOLITIC LIMESTONE• Oolitic limestone is a carbonate rock made

up mostly of ooliths (or ooids) which are sand-sized carbonate particles,

• Rings are formed around grains of sand or shell fragments that were rolled around on the shallow sea floor, gathering layer after layer of limestone.

CLAYS, MUDSTONES & SHALES

• The finest grained rocks are soft, plastic clays which occur only in young formations.

• The particles are invisible to the naked eye and clay feels smooth when worked.

• Shales are finely laminated or bedded rocks that split into fine layers, whereas mudstones are well consolidated.

• These rocks accumulate in the deepest parts of lakes and seas where the gentlest water movements can transport the smallest particles.

SANDSTONE• Sandstones form a large

proportion of sedimentary rocks around the world.

• They are formed by the cementing together of grains.

• Sandstones can vary enormously in color, texture and composition depending on the nature of the original source material and the environment of deposition.

METAMORPHIC ROCKS• These rocks get their name from the Greek words ‘meta’

and ‘morph’, meaning ‘change of form’. • Metamorphic rocks are produced by the alteration of pre-

formed rocks by pressure, temperature and migrating fluids, often in environments deep in the Earth’s crust.

• Because of the severe conditions which rocks undergo during metamorphism, the original minerals may become unstable and change to maintain equilibrium with the new environment.

• Three main types of metamorphism are recognized. – Regional metamorphism results from mountain-building events

with large scale folding and burial of pre-formed rocks. – Contact metamorphism is produced by direct heating of rocks

around an igneous intrusion, baking the surrounding country rocks.

– Dynamic metamorphism occurs where large scale faulting breaks and deforms rocks next to the fault.

MARBLE • Marble is a metamorphic rock, formed by

the alteration of limestone under high temperature and/or high pressure.

• Marbles formed from pure calcite limestones are white, with a sugary texture.

• Impurities in the limestone may lead to the formation of new minerals, giving the marble a variety of colors.

SLATE • Slate is a metamorphic rock formed from a

mudstone or shale. • Its main characteristic is that it splits very

easily into very thin sheets.• Slate was commonly used a roofing material

on old houses because it splits so readily. • Slate was also used for billiard tables and

blackboards.

QUARTZITE• They are formed mostly of

the mineral quartz. • Quartzite may sometimes

look like marble, but can be distinguished because quartzite cannot be scratched with a knife, unlike marble.

• It is very hard and weather resistant.

• Quartzite is used in the glass and ceramic industries, and builders sometimes use it for flooring and facing.

GNEISS • It is generally a coarse-grained granular

textured rock which can develop from a wide variety of igneous and sedimentary material.

• Gneisses consist of alternating dark and light bands of minerals which can vary in thickness, from millimeters up to a meter and can be highly contorted.

SCHIST

• Schist often has a silvery appearance.

• It tends to split into sheets.

• It is rich in the silvery-white mineral mica.

• This rock type is very widespread in metamorphic regions, such as Scotland, Norway or the Alps.

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