Rocks
Definition: rock is a naturally occurring aggregate of minerals and/or mineraloids.
Classification:
 mineral and chemical composition;
 the texture of the constituent particles
 processes that formed them.
Three types of rocks:
 igneous
 sedimentary
 metamorphic
1. Igneous rocks: formed from molten magma (latin ignis = "fire")
 plutonic rock (named after Pluto, the Roman god of the underworld) : magma cools and crystallises slowly
within the Earth's crust
 volcanic rock (named after Vulcan, the Roman name for the god of fire): magma reaches the surface either
as lava or fragmental ejecta.
The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite. When exposed by
erosion, these cores (called batholiths) may occupy huge areas of the surface.
Variables used for the classification of igneous rocks:
- texture (crystal size) - depends upon the cooling history:
large crystals enough to be seen by the naked eye intrusive;
crystals too small to be seen (or no crystalization)extrusive.
mineral composition of the rock.
Two main chemical parameters:
 Chemical - silica versus iron - magnesium content:
 acid igneous rocks present a high silica content (example, granite)
 intermediate igneous rocks (example, diorite or andesite)
 basic igneous rocks have low silica and high iron - magnesium content (example, basalt)
 alkalic igneous rocks have low silica and low iron-magnesium (example syenite)
 Mineralogic contents - felsic versus mafic:
 felsic rock, with predominance of quartz, alkali feldspar and/or feldspathoids: the felsic
minerals; these rocks (e.g., granite) are usually light coloured, and have low density.
 mafic rock, with predominance of mafic minerals pyroxenes, olivines and calcic
plagioclase; these rocks (example, basalt) are usually dark coloured, and have higher
density than felsic rocks.
 ultramafic rock, with more that 90% of mafic minerals (e.g., dunite)
Mode of occurrence Acid
Intrusive
Granite
Extrusive
Rhyolite
Composition
Intermediate Basic
Diorite
Gabbro
Andesite
Basalt
Ultrabasic
Peridotite
2. Sedimentary rocks
Two ways of forming:
from overburden pressure as particles of sediment are deposited out of air, ice, or
- chemical precipitates (water flows carrying the particles in suspension), eg. carbonate-rich
sediments.
Classification:
-
-
-
Clastic sedimentary rocks:
- formed from overburden pressure as particles of sediment are deposited out of air, ice
- composed largely of quartz with other common minerals including feldspars, amphiboles, clay
minerals, and sometimes more exotic igneous and metamorphic minerals.
Biogenic sedimentary rocks: materials generated by living organisms, and include carbonate minerals
created by organisms, such as corals, molluscs, and foraminifera, which cover the ocean floor with
layers of calcite which can later form limestone; other eg: chalk
Precipitate sedimentary rocks: chemical precipitates (water flows carrying the particles in suspension),
eg. carbonate-rich sediments; eg. limestone, gypsum, halite (rock salt)
Clastic sedimentary rocks may be regarded as falling along a scale of grain size:
shale: finest (particles < 0.004 mm)
siltstone: intermediate (particles between 0.004 to 0.06 mm)
sandstone: coarser (grains 0.06 to 0.2 mm)
conglomerates and breccias: coarsest (grains 2 to 256 mm).
Sedimentary rocks are economically important in that they can be used as construction material.
3. Metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to
different temperature and pressure conditions than those in which the original rock was formed. These temperatures
and pressures are always higher than those at the earth's surface, and must be sufficiently high so as to change the
original minerals into other mineral types or else into other forms of the same minerals (e.g. by recrystallisation).
Foliation
The layering within metamorphic rocks is called foliation (derived from the Latin word folia, meaning "leaves"),
and it occurs when a strong compressive force is applied from one direction to a recrystallizing rock. This causes the
platy or elongated crystals of minerals, such as mica and chlorite, to grow with their long axes perpendicular to the
direction of the force. This results in a banded, or foliated, rock, with the bands showing the colours of the minerals
that formed them.
Textures:
Foliated: a product of differential stress that deforms the rock in one plane, sometimes creating a plane of
cleavage: for example, slate is a foliated metamorphic rock, originating from shale.
Non-foliated: do not have planar patterns of stress; rocks that were subjected to uniform pressure from all
sides, or those which lack minerals with distinctive growth habits, will not be foliated.
Examples:
Slate: very fine-grained, foliated metamorphic rock
Phyllite: coarse
Schist: coarser
Gneiss: very coarse-grained
Marble: generally not foliated, which allows its use as a material for sculpture and architecture.
Metamorphic rock textures
The five basic metamorphic textures with typical rock types are:
 Slaty: slate and phyllite; the foliation is called 'slaty cleavage
 Schistose: schist; the foliation is called 'schistocity'
 Gneissose: gneiss; the foliation is called 'gneisocity
 Grandoblastic: granulite, some marbles and quartzite
 Hornfelsic: hornfels and skarn