Lecture 2
ROCKS
Rock Cycle
The rock cycle can best be described as follows:
1.
1.
magma cools and solidifies (crystallizes) forming igneous rock;
2.
2.
igneous rock weathers, become transported, and deposited as sediments;
3.
3.
sediments through cementation
and compaction lithify to form sedimentary rock;
4.
4.
sedimentary rock through pressure and temperature become metamorphosed
forming metamorphic rocks;
5.
5.
metamorphic rock becomes melted forming magma once again;
6.
6.
igneous rock can also be metamorphosed without becoming sediments;
7.
7.
metamorphic and sedimentary rock can be weathered, transported and
deposited creating sediments.
Igneous rocks form from magma that cools and
crystallizes at or near the surface and at depth.
Two types of igneous rocks are extrusive and
intrusive.
1.
1.
Extrusive rocks form when lava, which is similar to magma, but has lost
most of its gases, makes its way to the surface and solidifies.
2.
2.
Intrusive rocks form when magma not reaching the surface cools and
solidifies at depth.
The rate of cooling and crystallization affect the
size of mineral crystals.
Slow cooling results in the formation of large
crystals.
Fast cooling results in the formation of small
intergrown crystals.
If the molten material is quenched instantly,
usually during violent volcanic eruptions, glass is produced. Glass has no
crystalline structure.
Igneous rocks are classified based on texture and
mineral
constituents.
Texture describes the overall appearance of an
igneous rock based on the size and arrangement of the interlocking crystals. It
aids in describing the cooling and crystallization history of the rock.
The various types of textures can best be
described as:
·
·
fine grained texture (crystal too small to be seen with the unaided
eye),
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·
coarse grained texture (appear to be a mass of intergrown crystals
roughly equal in size and large enough to be seen with the naked eye),
·
·
porphyritic texture (large crystal embedded in a matrix of smaller
crystals, indicative of more than one cooling episode),
·
·
glassy texture (results when volcanic eruptions eject molten material
into the atmosphere, which becomes quenched very quickly).
The mineral composition of igneous rocks can be
described as follows:
The mineral makeup of an igneous rock depends on
the chemical composition of magma from which it crystallizes.
N. L. Bowen discovered that as magma cools in
the laboratory, certain minerals crystallize first at very high temperatures.
At successively lower temperatures other minerals crystallize. This process is Bowens reaction
series.
Examples of Igneous Rocks:
The weathering process
produces sediments, which then become lithified to form sedimentary rocks.
Mechanical/physical weathering is the process of
breaking rocks into small pieces, each piece retains the characteristics of the
original rock material. Some examples of mechanical/physical weathering are
frost wedging, unloading, and biological activity.
Chemical weathering alters the internal structure
of minerals by removing and/or adding elements; during this process the original
rock is altered into substances that are stable in the surface environment.
Compaction and cementation of
sedimentary rocks.
Lithification is the process of
turning sediments into rocks.
Compaction and cementation transform sediment
into solid sedimentary rock.
·
·
Compaction occurs when over time the weight of overlying material
compresses the deeper sediments. As grains are pressed closer and closer
together the pore space is greatly reduced.
·
·
Cementation occurs when cementing materials are carried in solution by
ground water. Through time the cement precipitates onto the sediment grains and
fills the pores and joins the particles. Some types of cements include:
calcite, silica and iron oxide.
Detrital sedimentary rocks are formed from
material called detritus. Detritus consists of particles of weathered rock
material. The following particle size
scale is used to distinguish among detrital sedimentary rocks:
·
·
gravel (between 256 mm and 2mm),
·
·
sand (1/16-2 mm),
·
·
silt (1/256-1/16 mm);
·
·
clay (less than 1/256 mm).
Some examples of detrital sedimentary rocks are
(these rocks are composed of various particle shapes):
·
·
conglomerates are composed of rounded shape gravel,
·
·
breccias are composed of angular shaped gravel,
·
·
sandstones are composed of sand size particles,
·
·
siltstones are composed of silt size particles,
·
·
shale, mudstone, claystone are composed of clay size particles
·
·
arkose is a feldspar rich sandstone or conglomerate
Chemical sedimentary rocks are formed when dissolved
material are precipitated as solids.
These precipitated solids form as a result of chemical weathering
dissolving out sediment and placing the material in solution. The precipitation
process may be due to physical processes or indirectly through life processes
(biochemical origin).
·
·
limestone, which is the most abundant chemical sedimentary rock composed
mainly of calcium carbonate, and 90% is biochemical in origin,
·
·
coquina, which is composed of loosely cemented shells and shell
fragments, chalk, which is composed of soft, porous rock material made up
almost entirely of the hard parts of microscopic organisms,
·
·
travertine which is composed of inorganic calcium carbonate due to
chemical changes or high water temperatures increase the concentration of
calcium bicarbonate to the point that it precipitates,
·
·
chert, flint and jasper are composed of microcrystalline quartz, which
form due to dissolved silica precipitating out of solution,
·
·
evaporites such as rock salt, and gypsum are left behind when ancient
seas have been evaporated away.
Sedimentary rock features and
structures.
·
·
Strata or beds are the single most characteristic feature of sedimentary
rocks. These are layers of rock material.
·
·
Bedding planes are flat surfaces along which rocks tend to separate or
break. Each bedding plane marks the end of one episode of sedimentation and the
beginning of another.
·
·
Fossils are traces or remains of prehistoric life.
·
·
Ripple marks are evidence of flowing water processes
Metamorphic Rocks
Metamorphism means to change form; therefore,
metamorphism changes existing rocks in size, shape, texture, and even minerals
they contain.
The agents creating metamorphism are:
o
o
heat: the most important metamorphism agent of the three, it provides
the energy to drive chemical reactions that recrystallize minerals, and
temperature increases with depth,
o
o
pressure: like temperature, pressure increases with depth; buried rocks
are subjected to the force exerted by the load above; also during mountain
building (orogeny) rocks are subjected to directional forces (i.e., stress),
which bends and folds rock material, and
o
o
chemically active fluids: water contained in the pore spaces of rock
acts as a catalyst by aiding the migration of ions; in some cases the minerals
recrystallize into more stable crystal shapes or ion exchange among minerals
forces the formation of completely different minerals.
The degree of metamorphism can be described as
follows:
o
o
low-grade metamorphism: the most common type, rocks are only slightly
changed, rocks maintain identifiable characteristics (e.g., layering) from the
original parent rock, and
o
o
high-grade metamorphism: the transformation/metamorphism is so complete
that the identity of the parent rock can not be determined (e.g., bedding
planes, layers and fossils that existed in parent rock no longer exist in the
metamorphic rock created).
Some metamorphic settings are :
o
o
regional metamorphism: occurs during mountain building where great
quantities of rock are subjected to intense stresses and high temperatures
associated with large scale deformation; therefore, extensive areas are
metamorphosed, and
o
o
contact metamorphism: the metamorphism is caused by the high
temperatures of molten material surrounding rock and in effect "bake"
the surrounded rock.
Metamorphic rock textures.
The degree of metamorphism is reflected in the
rock's texture and mineralogy.
o
o
Foliated texture is due to the alignment of minerals as a result of
metamorphism. The alignment of minerals usually gives the rock a layered or
banded appearance.
o
o
Non-foliated texture is due to only one mineral that forms
equidimensional crystals; thus, show no foliation.
Classifying metamorphic rocks.
Some examples of foliated rocks are:
o
o
slate: very fine grained foliated rock composed of minute mica flakes,
has excellent cleavage (splits easily into flat slabs), which makes it an
useful rock for roof, floor tile; chalkboards),
o
o
schist: strongly foliated, formed by regional metamorphism; they are
platy and can be readily split into thin flakes or slabs, parent material was
shale, and
o
o
Gneiss (pronounced nice): banded metamorphic rocks that contain mostly elongated
and granular minerals, exhibits strong segregation of light and dark silicate
minerals which gives them the banded appearance
Some examples of non-foliated rocks are:
o
o
marble: is a coarse, crystalline rock whose parent rock was limestone;
it is composed of large interlocking calcite crystals which form from the
recrystallization of smaller grains in the parent rock, and
o
o
quartzite: is a very hard metamorphic rock, often formed from quartz
sandstone parent rock material, forms under moderate to high metamorphism which
causes the sand grains in the sandstone to fuse.