Download rocks and minerals and the rock

Rocks & Minerals
(Adapted from: Geology and Geological Features: A Photographic Slide Collection compiled by B.F. Kean,
1993)
Minerals are naturally formed crystals that are com posed of one or m ore chem ical elem ents. They are
disting uished from other natural solid m ate rials by their crysta lline structure. S om e natu ral solid m ate rials
form ed in ro ck s are not m inerals bec aus e they lack a crystalline stru cture , i.e., they are am orph ous , eg.,
obsidian or volcanic glass. Most minerals are formed by inorganic processes; however, a few are formed
by organic proc ess es (living orga nism s), eg ., calcite in cora l.
Single-element m inerals include diamond (from carbon) and the native (i.e., elemental) metals gold,
copper and silver. Other minerals, such as chalcopyrite, sphalerite and mica, are chemical compounds
con sisting of se vera l elem ents .
Rock s are m ade up of m inerals, the exce ption being glassy rocks (eg., som e rhyolites). Most rock s are
polymineralic (i.e., contain more than one mineral); however, a few are monom ineralic, eg., limestone and
m arble.
Rock-forming processes, some rapid and others requiring millions of years, are as active today as in the
geo logica l past. Rocks are divided into three main classes:
IGNEOUS ROCKS are formed as m agma (m olten rock) cools and solidifies. Magma is produced far
below the Earth's surface by heat generated mainly from radioactive disintegration of uranium, thorium
and potassium. The two main types of igneous rock are extrusive and intrusive. Extrusive rocks, or lava,
form when m agm as rea ch the surface of the Earth through volcanic fiss ures or vents. They cool ra pidly
and are fine-grained or glassy, eg., basalt and obsidian. Explosive eruptions produce rocks composed of
rock dust or fragments (pyroclasts). Intrusive rocks form when m agma cools slowly at depth, eg., granite,
allowing the constituent minerals to grow much larger. These m agmas intrude fissures and other zones of
weakness in the Earth's crust, crystallize, and are eventually uplifted and exposed by erosion. Some
magm as carry fragments of rocks and minerals from deep within the Earth, and this allows us to study
samples from regions too deep to be reached by drilling. The table below shows the relationship between
different igneous rocks and how they form.
Extrusive Igneous
Rocks (volcanic)
Intrusive Igneous
Rocks
Main
Minerals
Basalt
Andesite
Rhyolite
Gabbro stock;
diabase dyke
Diorite stock;
or dyke
Granite batholith
or stock;
pegmatite dyke
Feldspar
pyroxene
olivine
Feldspar
pyroxene
olivine
Quartz
feldspar
mica
olivine
(dark coloured)
MAGMA
SiO2 (light coloured)
MgO
SEDIMENTARY ROCKS are fo rm ed b y the ac cum ulation and cem enting of loose s edim ent (e g.,
sandstone), the deposition of chemical compounds held in solution in water (eg., limestone), or by the
acc um ulation of an imal or plant debris (e g., coal).
W ind, rive r an d ocean currents are the m ain m eth ods of tra nsport. T his transported m ate rial settles to
form distinct layers, which are compressed by overlying layers and cemented to form solid rock. Rock
layers c an b e distinguished from eac h other by difference s in grain size, colou r and com pos ition.
Sedimentary rocks may have a coarse-grained, gravel-like appearance or be extremely fine grained, and
may be hard or soft. The principal varieties are sandstone, limestone and shale. Many sedimentary rocks
conta in fo ssils and som e, such as coral re efs, a re com posed entirely of such organic re m ains. T he table
below illustrates how s edim entary rocks form .
1. Surface rock
Conglomerate
erosion
Sandstone
Shale
2. Chemicals dissolved in sea water
precipitation
Limestone, Dolostone
accumulation
3. Animal (shell, coral) or plant remains
Coal seams
4. Salts dissolved in sea water
evaporation
Salt, Potash, Gypsum
METAMORPHIC ROCKS are formed when rocks of any class are subjected to heat and pressure at
depth. This causes new minerals to form and other minerals to recrystallize. During the process, material
from the rock may be added or lost. Marked changes in temperature (T) and pressure (P) occur, which
may produce completely new types of rocks. In addition to the formation of new minerals, existing
minerals may be realigned into parallel bands and new textures may be formed. At sufficiently high
temperatures, the rock may undergo partial melting to form m agma, which may then become the source
of an igneous roc k. Meta m orphic ro ck s include, gneiss, s chist, s late , quartzite and m arble, and the table
below show s how they form .
Sandstone
Shale
Limestone
T,P
T,P
T,P
high T, P
flaky cleavage
high T, P
partial melting,
mineral bands
Quartzite
Slate
Marble
Schist
Gneiss
Igneous, sedimentary or
other metamorphic rocks
high T, P
extensive melting
Migmatite
The Geological Cycle
Soone r or later m ost ro ck s are exp ose d to the proc ess es o f erosion. The res ulting produ cts a ccu m ulate to
form sed imenta ry rock s. Thes e m ay be b uried to great depths and con verted by he at and pre ssu re into
m etam orphic rocks . At still greater depths, they m ay be m elted to form m agm a, which if crystallized form s
igneous rocks; if uplifted and exposed, these renew the cycle. The cycle may, however, be interrupted and
follow any of the paths shown in the diagram below.
MA
MEL
TIN
G
N
IO
AT
CR
YS
TA
L
H
RP
MO S
TA CK
ME RO
IZ
MAG
MET
AM
ORPHISM
IGNEOUS RO
CKS
IC
I
ROMEN
CK TA
S RY
N
O
SI
RO IO
(E AT
T
ING R
ER SPO
TH
WEA TRAN
AND
SE
D
N
SE
DIM
EN
TS
ND
GA
CEMENTIN NG
I
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E
D
HAR
Click here for a “Schematic cross-section depicting geological concepts and processes”.
PYROCLASTICS
VOLCANIC EJECTA
VARIABLE CLAST
SIZE
VOLCANIC
VENT
STOCK
VOLCANIC
ROCKS
SILL
LAVA
DUST CLOUD
UNC
O
CUTS ACROSS LAYERS
COOLS QUICKLY
SMALL CRYSTALS
DYKE
(erosio
n)
COOLS RAPIDLY
FINE GRAINED
PARALLELS LAYERS
COOLS QUICKLY
SMALL CRYSTALS
GRANITE
BATHOLITH
COOLS SLOWLY
LARGE CRYSTALS
N FO
RM
ITY
SEDIMENTS
SEDIMENTARY ROCKS
METAMORPHIC
ROCKS
U
N
C
ONFO
RMIT Y
FA
UL
T
TILL
UNCONSOLIDATED
SURFACE MATERIAL
UNSORTED
(erosion)
OLDER
METAMORPHIC
ROCKS