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Types of Rock
A ROCK IS A NATURALLY occurring and coherent aggregate of
one or more minerals - although there are few rocks made from other
substances, such as the decayde vegetation of which coal is composed.
There are three major classes of rocks - igneous,sedimentary, and
metamorphic - and each of these three classes is further subdivided
into groups and types, principally based on differences in their mineral
composition and texture.
IGNEOUS ROCKS
The magmas (molten rocks – Picture 1) from which igneous rocks are formed
either solidify underground, creating intrusive rocks such as granite ( 2 ) known as “plutonic“ rocks - or flow onto the surface of the land or ocean bed,
forming extrusive rocks such as basalt.Igneous rocks vary in texture, depending
on how rapidly the magma cooled. Rapid cooling creates tiny crystals and
freezes the liquide to glass, while slower cooling forms coares - grained rocks.
The oceanic crust is primarly composed of igneous rocks, includig volcanic
basalts ( 3 ) and the underlying gabbros.
(1)
(2)
(3)
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SEDIMENTARY ROCKS
Sedimentary rocks are generally made of deposits laid down on the Earth´s
surface by wather, wind, or ice. These deposits are almost always laid down in
layers( sedimentary layers- pic. 1) or strata. The stratifaction survives
compaction and cementation, and is distinguishing characteristic of sedimentary
rocks. They are also characterized by the presence of fossils( pic.2), which are
rarely found in any other type of rock. Some sedimentary rocks are formed by
eroded fragments of other rocks in some way cemented together.They are
classified by particle size, particle shape,and mineralogy. Some sedimentary
rocks are of chemical origin, having been deposited in solid form from solution,
and some are of biochemical origin, composed predominantly of calcium
carbonate. There are also organic deposits, such as coal, derived from the
breakdown of the organic tissue of dead plants and animals.
(1)
(2)
METAMORPHIC ROCKS
Metamorphic rocks are formed when existing rocks are subjected to extreme
temperatures or pressures (or both) that alter their mineralogical
composition,textuure, and internal structure. Quartzite, for example, is
metamorphosed sandstone, and slate is metamorphosed mudstone or shale. Such
changes typically occur deep within the crust, and may result from the
deformation caused by plate tectonics. But metamorphic rocks can also form
virtually at the surface of the Earth under the impact of meteorites or near
igneous intrusions, which create zones of high temperature around them.
Metamorphic rocks include schists and gneisses, whose component minerals
tend to segregate into separate bands, giving a layered appearance.
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Tasks:
A/ Answer the questions.
1.
2.
3.
4.
5.
6.
7.
Define the way how igneous, sedimentary and metamorphic rocks are formed.
Name some igneous rocks.
What is the oceanic crust mainly composed of?
In what type of rock can we usually find fossils?
What sedimentary layer is the oldest?
What is the name of metamorphosed sandstone?
How can metamorphic rocks forma t the surface of the Earth?
B/ Decide wheter given rock is igneous, sedimentary or metamorphic.
1. Granite2. Chalk3. Limestone4. Marble5. Basalt6. Gneiss7. Sandstone8. Dolomite9. Obsidian10. Rock salt-