Minerals
and Rocks
What is the earth made of? Gases make the atmosphere,
water makes the oceans and solids ( minerals and rocks )
make the earth. Most people think of oxygen in tems
of the air we breathe. Yet most rocks are composed largely of oxygen,
which is the most abundant element (takes up 93% of the volume of an average
rock) in the earth’s crust. Silica is a term for oxygen plus silicon.
Because silicone is the second most abundant element in the crust, most
minerals contain silica.
The common mineral quartz
(SiO2
)
is pure silica.
The third
common element in the earth’s crust is aluminum. Iron and calcite come
later on.
Most solids are crystalline.
A crystalline substance is one in wchich the
atoms are arranged in a regularly repeating, orderly pattern.
To be a mineral in the
geologic sense of the term, a substance must satisfy five conditions:
1.
a crystalline solid,
2.
naturally occurring,
3.
inorganic,
4.
having a definite chemical composition, and
5.
showing characteristic physical properties.
A mineralogist is a scientist who studies the
chemistry and crystallographic structure of minerals. I will be
particularly concentrated on the rock-forming minerals here
because they make up most of the rocks of the earth’s crust. From the
several thousand identifiable minerals only a few hundred are classified as
rock-forming minerals. All are silicates. Olivine
, Quartz
, Feldspar
group (potassium- and plagioclas-feldspar), proxenes (augite), amphiboles
(hornblende), mica
group (biotite and muscovite) are also known as
“Bowen’s reaction series minerals” continously
produced from magma. There are non-silicate minerals
include native elements, which are minerals composed of only one element.
Gold
and graphite (used in pencils) are examples of native element. Other
non-silicates are calcite (calcium carbonate) minerals. Limestone and
marble are rocks composed mainly of calcite. Most of ore minerals (or
economic minerals) are not silicates. Among the ore minerals are oxides
(magnetite and hematite) and sulfides (pyrite: fool’s gold, chalcopyrite,
galena), halides
(table salt).
Rock is naturally formed,
consolidated material composed of grains of one or more minerals.
Granite forms from magma solidifying within the earth’s crust. It is
made up mostly of the minerals feldspar and
quartz.
Rocks
are made of minerals. The three
major rock types are igneous, metamorphic, and sedimentary . Igneous rock
form when magma solidifies. If the magma is brought to the surface by a
volcanic eruption, it may solidfy into an extrusive
igneous rock .
Magma may solidfy very slowly beneath the surface.
The resulting intrusive
igneous rock may be exposed to erosion and undergo weathering.
The debris (eroded particles) produced is transported (by wind, water) and
eventually deposited (usually on a delta, lake or sea) as sediment
. If the unconsolidated (loose) sediment becomes lithified (cemented or consolidated into a rock), it
becomes a sedimentary
rock . As the rock is buried by additional layers of sediment
and sedimentary rock, heat and pressure increase. Tectonic forces may
also increase the temperature and pressure. If the temperature and
pressure become high enough the rock is no longer in equilibrium and recyrstallizes. The new rock that forms is
called a metmorphic rock . If the
temperature gets very high, the rock melts and becomes magma again, completing
the cycle.
Rocks associations of some major plate tectonic environments
Divergent
Oceanic mid-ocean ridge
basalts (MORB),
gabbro,
peridotite,
ophiolite
suite rocks
Back-arc basin
basalts
calc-alkaline
basalts
Continental Rift
alkali basalts
kimberlites
carbonatites
Convergent
Ocean-continent
calc-alkaline basalts,
andesite,
dacite,
rhyolite,
granite,
granodiorite
Ocean-ocean
basalts
Continent-continent
andesites
dacite,
rhyolite,
granite
Intrusive activity and the origin of igneous rocks
Igneous rocks that formed at great depth are called plutonic rocks.
These are characteristically coarse (large)-grained, reflecting slow
cooling of magma. Most plutons are
irregular in shape, unlike dikes and sills. Large plutonic bodies are
called batholith. Dike and sill are common intrusive
structures. A dike is tabular discordant (cut layers vertically),
intrusive body. A sill is also tabular but concordant (parallel, does
not cut layers) intrusive body. Most common coarse-grained plutonic
rocks are:
granitic-magma
originated
granite (quartz, pink/white
feldspars)
basaltic-magma
gabbro (no
quartz, dark gray feldspars)
ultramafic rocks
peridotite
(olivine, pyroxene)
andesitic-magma
diorite (white/dark gray feldspars)
Granite predominates in the continental crust. Younger batholiths
occur mostly within younger mountain belts. Diorite is largely
restricted to a narrow zone along convergent plate boundary. Gabbro predominates in the oceanic crust, and peridotite in the lower crust.
Volcanism and extrusive rocks
Volcanic eruptions also provide important information on the working of
the earth’s interior. Understanding volcanism provides a background for
theories relating to mauntain building, development
and evolution of continental and oceanic crust, and how the crust is deformed.
Our observations of volcanic activity fit nicely into plate tectonic
theory. The kinds of eruptions take place along spreading centers
(mid-oceanic ridges) are different from those associated with converging plate boundaries
(subduction zone related volcanic arcs). For
example,
Magma does not always reach the earth’s surface before solidifying, but
when it does it is called lava. Lava may erupt quitely
or violently. Volcanic activity, or volcanism, is not restricted to the
eruption of lava. Rock fragments (pumice with vesicles, tuffs, glasses,
volcanic bomb) and escaping gases may come out during the volcanic eruption.
Volcanic glass, obsidian is a volcanic rock.
Extrusive rocks are fine-grained or glassy textured rocks. There
are two types of basalt flows, both types have Hawaiian names: “pahoehoe (pronounced pah-hoy-hoy)”
and “aa (produced ah-ah)”.
First type is very liquid, and characterized by ropy surface. The
second is more viscous and form pillow basalts. Tuffs are extrusive
rocks composed of ashes and dusts. When a lava
solidifies while gas escapes holes are trapped in the rock, creating a
distinctive vesicle texture. A vesicle is a cavity caused by gas in
lava. Vesicular basalt and scoria are common mafic
extrusives. Pumice is also felsic
type of vesicular extrusive rock.
Mafic -silica poor with magnesium, iron rocks are
basalts which are dark colored minerals (olivine, augite,
dark feldspars, no quartz). Felsic
rocks are other extreme, high silica content rocks are rhyolites
which are light in color because of low iron and magnesium content minerals
(quartz, pink feldspars). Intermediate extrusive rocks are andesites, usually medium to dark gray in color.
The most obvious landform created by volcanism is a volcano, a hill or
mountain like feature formed by the extrusion of lava or ejection of rock
fragments from a vent. The Agri, Nemrut, Suphan, Erciyas are typical volcanoes in
Sediments and Sedimentary rocks
Most sedimentary rocks form from loose grains of sediment.
Sediment is solid particles that originate from:
1. wheathering and
erosion of pre-existing rocks,
2. chemical precipitation from solution,
including secretion by organisms in water.
These particles usually collect in layers on the earth’s surface.
Gravel, pebble, sand, silt and clay are sediment particles defined by grain
size.
The composition of sediment is controlled by the rates of chemical
weathering, and erosion. During transportation grains become rounded and
sorted. Sedimentary rocks may be clastic,
organic, and chemical. Clastic sedimentary rocks from mostly by compaction and cementation of
grains. Conglomerates form from coarse, rounded sediment grains
that have been transported only a short distance by rivers, wind or waves.
Sandstone forms from sand deposited by rivers, waves or turbidity
currents. Shale is fine-grained clastic
sedimentary rock that form in river lake delta or
muddy layers in the seafloor. Fossils are traces of an organism’s
hard parts or tracks preserved in rock (mostly shale and limestones).
Limestone consists of calcite, formed either as a chemical precipitate
in a reef or, by cementation of shell and coral fragments or oolites. Coal is an organic sedimentary rock which
consists of parts of plants. Evaporites, such
as rock salt and gypsum form as water evoporates.
Precipitation of calcite in the form of travertine terraces around a hot
spring also forms chemical sedimentary rocks.
Sedimentary rocks are usually found in beds separated by bedding planes
because the original sediments are deposited in horizontal layers.
Cross-bedding forms where sediment is deposited on sloping surface in sand
dune, delta, or river bar. Graded bed forms as coarse particles fall
from suspension before fine particles, perhaps in a turbidy
current (you know it on the passive margins). The environments of
deposition of a sedimentary rock include river chanell,
delta, lake, beach, dune, marine shelf, and deep sea floor. Sea level is
not stable. In the geologic past sea level has risen and fallen many
times. If sea level rises or the land sinks (subsides), large areas of
land will be flooded and the rock deposition will migrate across the land.
This a transgression of the sea as moves across the land, and the result
is the seaward edge is older than the landward edge. In a regression the
sea moves off the land. A drop in sea level can occur. Older
rocks overlay younger rocks.
Metamorphism, metamorphic rocks
You know from the rock cycle that rocks change when their physical
environment changes radically. What happens to rocks that are deeply
buried but are not enough to melt? They become metamorphosed.
Metamorphism is the solid-state transformation of pre-existing rock into
texturally or mineralogically distinct new rock as a
result of high temperature, high pressure, or both. The new rock is
called metamorphic rock. Marble from limestone, slate from shale,
schist, phyllite and gneiss are metamorphic
rocks. However, when rocks have been formed entirely by precipitation of
ions derived from hydrothermal solutions, we rather call them hydrothermal
rocks. Hydrothermal rocks are most commonly found in veins. These
veins contain quartz, calcite or other ore minerals (gold, pyrite, lead, zinc, silver,
and other metals). Hot veins also form by convection groundwater heated
by the pluton.
Pressure is very important influence on the texture of a metamorphic
rock. It forces the minerals become parallel to one another. The
parallel alignment of textural and structural features of rock is called
foliation. Marble dos not show foliation, but slate has slaty cleavage, schist has schistosity.
Terms to Remember
aa
andesite
basalt
batholith
bedding
bomb
caldera
cementation
clastics
coarse-grained rock
compaction
conglomerate
crater
crystal
deposition
dike
evaporite
extrusive rock
felsic rock
fine-grained rock
fossil
gneiss
gravel
hydrothermal rock
lava
limestone
marble
mafic rock
mineral
obsidian
pahoehoe
phyllite
pillow
pluton
pumice
rock
rock cycle
rounding
sand
sandstone
schist
schistose
sediment
shale
silicates
silslate
slaty cleavage
sorting
tuff
turbidites
ultramafic rock
vent
vesicle
Questions for review
1. What are the three most common elements in the earth’s crust?
2. Is ice a mineral? What happens to the atoms in water when it freezes?
3. Where these rocks can be found? gabbro,
diorite, granite, peridotite.
4. Why are extrusive rocks fine-grained?
5. List the clastic sediment particles in
order of decreasing grain size.
6. How would you distinguish
granite and gneiss
marble and limestone ?