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ROCKS
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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. Weatherresistant stones are polished for use as
decorative facings on buildings.
Basalt
Basalt 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.
• Carbonifero
us 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 sandsized 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 preformed 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 silverywhite mineral mica.
• This rock type is very
widespread in metamorphic
regions, such as Scotland,
Norway or the Alps.
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