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Ro fact-ite
G eo fac t s an d ac t i v i t i es f o r t h e c l ass room
Rocks are formed in
three main ways and are
classified accordingly.
Igneous rocks form from molten material
called magma. Sedimentary rocks form from
sediments deposited out of water or the air.
Metamorphic rocks form from the alteration of
other rocks through temperature and pressure
induced changes in the minerals. However, not
all rocks are easily described without special
laboratory equipment and some can be very
difficult to classify. In addition, the main ways
in which rocks form leaves a lot of room for
variability. This means there is not necessarily
a suite of characters common to all rocks in
each classification. When it comes to identifying
rocks there is no easy solution or reliable
dichotomous key.
Rocks are made of minerals. Most minerals
are natural inorganic crystalline solids but
rocks also contain non-crystalline mineral-like
compounds and organic materials. Most rocks
are composed of several minerals but some
are made of just one. Many rocks are entirely
composed of small mineral grains that cannot
be seen without the aid of a microscope while
in some rocks the minerals are large and
easily seen. Unfortunately, rocks that look very
different from each other because of the types,
size or colour of the minerals may actually be
classified as the same type of rock. Other rocks
that look very similar may have formed through
very different processes and not be classified
the same.
The reason the three way classification of
Igneous, Sedimentary and Metamorphic is
used is because the processes involved in
forming the three rock types usually results in a
characteristic relationship between the mineral
grains that is diagnostic.
Sedimentary rocks form from sediments
deposited from water or air. Each grain in
the rock was originally separate from all the
others. When it stopped moving it settled down,
touching several other grains but leaving pores
spaces between the grains in places where
they cannot touch. Compaction under the
weight of the accumulating sediments may
have distorted soft grains, reduced the porosity
and increased the cohesion between grains.
Minerals precipitating out of watery solution
filling the pores may have formed a cement,
further securing the minerals to each other
to form a solid rock. Common sedimentary
rocks include sandstone, mudstone,
conglomerate and limestone.
Grains in a sedimentary rock can be either
distinct minerals or fragments of rocks. Either
way, they have usually been sourced from the
weathering and erosion of other rocks. Grain
shape can be very angular or very rounded
or anywhere in between. They can also vary
from rod-like to platy to spherical. Some
sedimentary processes form homogeneous
grain assemblages while others result in much
more variable groupings. Grain size and grain
type are used to further classify sedimentary
rocks.
Igneous rocks are formed from cooling
molten rock called magma. Minerals crystallise
and interlock as the melt cools and solid rock
forms. Eventually the entire melt forms a cool
solid rock composed of crystals with no open
spaces and usually showing no preferred grain
alignment. The rock may be entirely composed
of one mineral but is usually made of several
mineral types. The composition and range of
types of minerals is determined by the magma.
The size of the crystals is determined by the
cooling history. In general, slow cooling will
result in large crystals forming while rapid
cooling will produce smaller crystals. Very rapid
cooling may result in non-crystalline glass
forming as part of or even all of the rock. If
some slow cooling is followed by rapid cooling
a fine grained rock with a scattering of larger
crystals, called a porphyry, can result.
Magma can be intruded into other rocks
within the crust or it can be erupted onto the
surface. Intrusive igneous rocks, sometimes
called plutonic rocks, are often coarse grained
while the extrusive igneous rocks erupted or
extruded at a volcano are often fine grained.
When erupted magma flows across the
landscape it is called lava. Lava cools to form
a solid volcanic rock but magma explosively
erupted is blasted into fragments that rain
back down on the landscape, sometimes as
unconsolidated debris but sometimes still hot
enough to weld back into a mass of solid rock.
Explosively formed volcanic rocks are called
pyroclastics.
Volcanic rocks often contain gas bubbles,
some glassy material and fragments of
other rocks caught up in the explosive acts
of formation. Plutonic rocks sometimes also
contain rock fragments that have fallen into
the intruding magma and not melted before it
cooled . They are called xenoliths.
Common igneous rocks include granite,
basalt, andesite and rhyodacite.
Metamorphic rocks form from other rocks,
including other metamorphic rocks. When
rocks are deeply buried, intruded by magma
or are caught up in large tectonic processes
they experience large changes in temperature
and / or pressure. Many minerals respond
to theses changes by changing their crystal
size and crystal type but some assemblages
of minerals undergo more radical changes.
Without actually melting, some mineral
assemblages redistribute the elements within
the starting minerals to form new suites of
minerals that are more stable at the new
pressures and temperatures.
Metamorphic rocks formed from magmatic
intrusions heating country rock (the cool
rock around the magma) are called thermal
or contact metamorphic rocks. Metamorphic
rocks resulting from wide spread temperature
and pressure changes caused by tectonic
processes are called regional metamorphic
rocks.
Metamorphic rocks are also usually composed
of interlocking crystals but they often display
visible alignment of some crystals in one
orientation. This is due to the rod-like and platelike new minerals that form aligning themselves
with respect to the pressure field applied to the
rocks. Some minerals may also separate into
distinct layers, especially where the temperature
and pressure changes have been extreme.
Common metamorphic rocks include marble,
slate, schist and gneiss.
Acknowledgement: Greg McNamara, Education and Outreach, Geological Society of Australia
http://www.gsa.org.au
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G eo fac t s an d ac t i v i t i es f o r t h e c l ass room
Rocks are the
foundation of everything!
1. Rivers have been washing gravel, sand and mud down
into Sydney harbour for thousands of years. Deep in
the ancient deposits these materials have been packed
down by the weight of overlying layers and pore spaces
have been in-filled by cement-like carbonate minerals.
Are the rocks that are forming Igneous, Sedimentary or
Metamorphic?
Explain your reasoning.
2. Ancient limestone rocks of far north Queensland were
intruded by magma sometime in the Permian. Heat from
the intrusion triggered a chemical reaction between the
calcium carbonate mineral, calcite [CaCO3], and quartz
[SiO2] in some parts of the limestone where they are
found together. This reaction formed a new mineral,
wollastonite [CaSiO3]. Would you classify the rock
containing the wollastonite as an Igneous, Sedimentary
or Metamorphic rock?
Explain your answer.
3. Huge areas of western Victoria are covered in flood
basalts. Lava flowed from numerous vents and volcanoes,
often filling in previous river channels, ‘flooding’ the
landscape with this new rock type. Are these basalts
Igneous, Sedimentary or Metamorphic rocks?
Justify your answer.
4. Magma rising through the rocks in southern Victoria came
into contact with ground water near the surface. This
resulted in a steam explosion that created a deep hole
in the landscape and a massive blanket of fragmentary
material around the crater. This place is now known
as Tower Hill. Would the blanket of material be best
classified as a lava or as a pyroclastic rock?
Explain your reasoning.
5. The Great Barrier Reef is a rocky mound built in shallow
ocean waters by corals. Corals are marine animals that
build a skeleton out of the mineral calcium carbonate
[CaCO3]. The rocky mound of the GBR is composed of the
intact and fragmentary remains of coral skeletons and the
skeletons of other organisms held together by carbonate
muds and secondary minerals growing in the pore spaces.
This type of rock is called a limestone because it is mostly
composed of calcium carbonate and there are many
examples in Australia of ancient limestones that formed in
exactly this way.
Explain why these limestones do not fit the usual
definition of a sedimentary rock yet are classified as
sedimentary rocks.
6. When a quartz sandstone is metamorphosed the quartz
grains recrystallise but do not change composition.
The resultant quartzite has quartz grains that are now
interlocking but roughly the same size as the grains in the
parent sandstone. When a mudstone is metamorphosed
the clay minerals undergo dehydration reactions and a
number of new minerals can be formed including mica
and garnet. Large, easily visible garnet crystals with
wrappings of visible flakes of mica are characteristic of
many high grade metamorphic rocks.
Broken Hill is world famous. It is where BHP was formed
to mine the wealth of one of the largest lead-silver-zinc
deposits ever found. Some of the rocks at Broken Hill
started as layers of mud and layers of sand on the ocean
floor. When these layers were first lithified were they
Igneous, Sedimentary or Metamorphic rocks?
Justify your answer.
7. Some of the rocks at Broken Hill started as lava flows
on the ocean floor. Were they Igneous, Sedimentary or
Metamorphic rocks at first?
Justify your answer.
8. The ore body at Broken Hill was found associated with
layers of rock that frequently consisted of interbedded
quartzites and garnet schists. Are they Igneous,
Sedimentary or Metamorphic rocks and why?
9. Broken Hill also has numerous pegmatite outcrops.
What type of rock is a pegmatite and what are its
characteristics [you will need to look further than this factite for the answer]?
Acknowledgement: Greg McNamara, Education and Outreach, Geological Society of Australia
http://www.gsa.org.au