'Rocks' Geography Worksheet printable pdf download

ROCKS

The rocks of the Earth’s crust are grouped into three major classes: igneous, sedimentary, and

metamorphic rocks.

Igneous rocks form when molten material from the Earth’s interior cools and solidifies in the crust. Magma

cooled slowly below the surface forms coarse-textured intrusive (plutonic) igneous rocks, e.g. gabbro,

granite, diorite. Lava cooled rapidly at the surface forms fine-textured extrusive (volcanic) igneous rocks,

e.g. basalt, andesite, rhyolite, phonolite (znělec). Igneous rocks consist mainly of silicate minerals

containing silicon, oxygen, and metallic elements. The type of present metallic elements determines the

mineral density.

In petrology an acid igneous rock is one that has 10 per cent or more free quartz, e.g. granite and rhyolite.

A basic igneous rock is one that contains no quartz and has feldspars with more calcium than sodium, e.g.

basalt and gabbro.

Felsic = a word for the light-colored minerals like feldspars, quartz, etc., in an igneous rock.

Igneous rocks consisting mainly of felsic minerals are referred to as leucocratic, while those consisting

mainly of dark minerals are referred to as melanocratic.

Mafic = a word for the ferromagnesian minerals like olivine, biotite, etc. in an igneous rock.

(from Longman Illustrated Dictionary of Geology)

Sedimentary rock can form in one of the three ways:

a) Most sedimentary rock, like sandstone, is made up of particles of other rocks and minerals that have

been deposited in one place, usually by moving water, ice or wind. As layer builds upon layer, the bottom

layers become pressed together by the weight of the layers above them. Elements in the water form

cement that joins (compacts) the particles of sediments together to form a solid (lithify).

b) Some sedimentary rock, like limestone, is formed by the remains of organisms that were once alive.

c) Other sedimentary rock (for example, rock salt and other kinds of limestone) is formed from minerals

that were once dissolved in water, i.e. they are chemically precipitated. Newly-formed mineral crystals

form on or settle to the bottom of the water and build up layers of sediment.

So, if the rock is sedimentary, you know that the rock most likely formed beneath a body of water or from

desert or seacoast sands.

A layer of sedimentary rock that is marked off above and below by surfaces that can be seen and is made

up of material that is the same in all its parts is called a bed. A surface that is parallel to the original surface

on which the sediment was deposited is referred to as a bedding-plane.

Metamorphic rock is formed when minerals and rocks are changed by very great heat from a nearby flow

of magma or lava or from heat and pressure very deep below the surface of the earth.

Sometimes the atoms of minerals change position, and new minerals are formed. When this happens,

certain mineral grains will sometimes grow larger. Other times minerals lose or gain atoms. In response to

pressure, the minerals in most metamorphic rocks tend to form layers. In the rocks that have been under

great pressure the layers may be noticeably bent or folded.

Considered as four pairs, each rock on the right is a metamorphic form of the rock on its left.

granite (A)= žula

- gneiss [!nais] (B) = ortorula

sandstone (C) = pískovec

- quartzite (D) = kvarcit

limestone (E) = vápenec

- marble (F) = mramor

shale (G) = jílovitá břidlice

- schist (H) = krystalická břidlice

ROCKS

'Rocks' Geography Worksheet

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