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Formation and Characteristics
• Rocks are formed on Earth as igneous, sedimentary, or
metamorphic rocks.
• Igneous rocks form when rocks are heated to the melting point
which forms magma.
• Sedimentary rocks are formed from the cementing together of
sediments, or from the compaction (squeezing together) of
sediments, or from the recrystallization of new mineral grains which
are larger than the original crystals.
• Metamorphic rocks form from heat and pressure changing the
original or parent rock into a completely new rock.
The word "metamorphic" comes from Greek and means "To Change Form".
• Metamorphic rocks form from HEAT
and PRESSURE changing the original
or parent rock into a completely new
• The parent rock can be either
sedimentary, igneous, or even another
metamorphic rock.
Solid rock can be changed into a new rock by stresses
that cause an
increase in
that cause
an increase
from the core
• Temperature increases can be caused by layers of
sediments being buried deeper and deeper under the
surface of the Earth.
• As we descend into the earth the temperature
increases about 25ºC for every kilometre that we
descend. The deeper the layers are buried the hotter
the temperatures become.
• The great weight of these layers also causes an
increase in pressure, which in turn, causes an
increase in temperature.
• Metamorphism can take millions of years as in the
slow cooling of magma buried deep under the surface
of the Earth.
The descending of rock layers at subduction zones
causes metamorphism in two ways:
 the shearing effect of the plates sliding past each other causes
the rocks coming in contact with the descending rocks to
change. Some of the descending rock will melt because of this
 When rock melts it is then considered igneous not metamorphic,
but the rock next to the melted rock can be changed by the heat
and become a metamorphic rock.
 The diagram above shows you where metamorphic rock
(yellow) can be produced at a subduction zone.
There are 3 factors that cause an increase in pressure
which also causes the formation of metamorphic
rocks. These factors are:
• The huge weight of overlying layers of sediments.
• Stresses caused by plates colliding in the process of
mountain building.
• Stresses caused by plates sliding past each other, such as
the shearing stresses at the San Andreas fault zone in
California, or under another, such as the Pacific & IndoAustralian plates beneath New Zealand.
Metamormorphism can be instantaneous as in the
shearing of rocks at plate boundaries.
There are 2 main ways that metamorphic rocks
can form. These are: Contact and Regional
• Contact Metamorphism occurs when magma
comes in contact with an already existing body
of rock.
• When this happens the existing rocks
temperature rises and also becomes infiltrated
with fluid from the magma.
• The area affected by the contact of magma is
usually small, from 1 to 10 kilometres.
• Contact metamorphism produces non-foliated
(rocks without any cleavage) rocks such as
marble, quartzite, and hornfels.
• Regional Metamorphism occurs over a
much larger area than contact metamorphism.
• This metamorphism usually produces rocks
such as gneiss and schist.
• Regional metamorphism is caused by large
geologic processes such as mountainbuilding.
• These rocks when exposed to the surface
show the unbelievable pressure that cause
the rocks to be bent and broken by the
mountain building process.
• Metamorphic rocks are almost always harder than
sedimentary rocks. They are generally as hard and
sometimes harder than igneous rocks.
• They form the roots of many mountain chains and are
exposed to the surface after the softer outer layers of
rocks are eroded away.
• Many metamorphic rocks are
found in mountainous regions
today and are a good indicator
that ancient mountains were
present in areas that are now
low hill or even flat plains.
Investigate the rocks of the
South Island.
• Foliate comes from the Latin word that means
sheets, as in the sheets of paper in a book.
• Metamorphic rocks are divided into two
categories- foliates and non-foliates.
• Foliates are composed of large amounts of
micas and chlorites. These minerals have very
distinct cleavage.
• Foliated metamorphic rocks will split along
cleavage lines that are parallel to the minerals
that make up the rock. Slate, as an example, will
split into thin sheets.
Silt and clay can become deposited and
compressed into the sedimentary rock shale.
The layers of shale can become buried deeper
and deeper by the process of deposition.
• Deposition is the laying down of rock forming material
by any natural agent (wind, water, glaciers) over time.
• Because these layers are buried, temperatures and
pressures become greater and greater until the shale is
changed into slate.
• Slate is a fine-grained metamorphic rock with perfect
cleavage that allows it to split into thin sheets.
• Slate usually has a light to dark brown streak.
• Slate is produced by low grade metamorphism,
which is caused by relatively low temperatures and
• Slate has been used by man in a variety of ways over the
• One use for slate was in the making of headstones or grave
markers. Slate is not very hard and can be carved easily. The
problem with slate though is its perfect cleavage. The slate
headstones would crack and split along these cleavage
planes as water would seep into the cracks and freeze which
would lead to expansion. This freeze-thaw, freeze-thaw over
time would split the headstone. Today headstones are made
of a variety of rocks, with granite and marble being two of the
most widely used rocks.
• Slate was also used for chalk
boards. The black colour was good
as a background and the rock
cleaned easily with water. Today, it
has been replaced by materials
that do not have the disadvantage
of slate, i.e. weight and the splitting
and cracking over time.
• Schist is a medium grade metamorphic rock.
This means that it has been subjected to more
heat and pressure than slate, which is a low
grade metamorphic rock.
• Schist is a more coarse grained rock than slate.
The individual grains of minerals can be seen by
the naked eye. Many of the original minerals
have been altered into flakes. Because it has
been squeezed harder than slate it is often
found folded and crumpled.
• Schists are usually named by the main minerals that
they are formed from. Bitotite mica schist, hornblende
schist, garnet mica schist, and talc schist are some
examples of this.
Garnet-mica schist –
microscopic view
In this specimen of coarse-grained biotite
schist you can see clearly the flakes of
dark mica, lined up parallel to each other,
and separated by light-coloured material,
which is mostly millimetre-sized quartz
crystals. In the photo above you see the
thin edges of the mica flakes.
PARENT ROCK sedimentary
• Gneiss is an old German
word meaning bright or
sparkling – it is pronounced
• It is a high grade metamorphic
rock. This means that it has
been subjected to more heat
and pressure than schist.
• Gneiss is coarser than schist
and has distinct banding. This
banding has alternating layers
that are composed of different
• The minerals that compose
gneiss are the same as granite.
• Feldspar is the most important
mineral that makes up gneiss
along with mica and quartz.
Gneiss can be formed from a
sedimentary rock such as
sandstone or shale, or it can be
formed from the metamorphism
of the igneous rock granite.
Gneiss can be used by man as
paving and building stone.
• Nearly all traces of the original structures (including
fossils) and fabric (such as layering and ripple marks)
are wiped out as the minerals migrate and re-crystallise.
The streaks are composed of minerals, like hornblende,
that do not occur in sedimentary rocks. See the photos
on slide 21 for some of the variety to be found in gneiss.
• Thicker veins of large-grained minerals form in it, unlike
the more evenly layered appearance of schist.
• With still more metamorphism,
gneisses can turn to migmatite
and then totally re-crystallise
into granite.
• Non-Foliates are metamorphic rocks
that have no cleavage at all.
• Quartzite and marble are two examples
of non-foliates .
• Quartzite is composed of sandstone that
has been metamorphosed.
• Quartzite is much harder than the parent
rock sandstone.
• It forms from sandstone that has come into
contact with deeply buried magmas.
Quartz sandstone
• This is a pure sandstone
composed of rounded grains of
quartz, cemented by further
quartz between the grains. This
makes the rock very hard and
resistant to weathering and
erosion, rather like a
metamorphic quartzite formed by
the action of heat and pressure.
• Marble is much harder than its parent rock. This allows it
to take a polish which makes it a good material for use as
a building material, making sink tops, bathtubs, and a
carving stone for artists. Today, headstones are made
from marble and granite because both of these rocks
weather very slowly and carve well with sharp edges.
• Quartzite looks similar to its parent rock.
• The best way to tell quartzite from
sandstone is to break the rocks.
• Sandstone will shatter into many individual
grains of sand while quartzite will break
across the grains.
• Marble is metamorphosed limestone or
• Both limestone and dolomite have a large
concentration of calcium carbonate (CaCO3).
• Marble has many different sizes of crystals.
• Marble has many colour variances due to the
impurities present at formation. Some of the
different colours of marble are white, red,
black, mottled and banded, grey, pink, and
• Limestone is usually not
made of sediment as we
think of it—not clay or
sand, derived from rocks—
but instead is built from the
tiny calcite skeletons of
microscopic organisms in
shallow seas.
• Dolomite looks like
limestone, but unlike
limestone it does not
bubble when treated with
weak acid. The mineral
responsible is also called
• This fresh boulder shows
that much of limestone is like
this, with no discernible
structure or texture. It looks
strong, but the quarry this
came from was set in this
place not for building stone
but because the rock was
pure and readily burnt for
lime (CaO, calcium oxide) for
making cement. There must
have been a good wood
supply near to make that
operation efficient.
• Close up, this piece of Hoyt
Limestone shows no more detail
or structure than the boulder in
the previous picture.
• Its crystals are small and tightly
interlocked, due to the effects of
pressure and mild heat that
solidifies rock.
• The colour of limestone tends to
lie in the range of white to dark
gray owing to the presence of
organic matter, though it also
takes warmer hues from clay
• Marble is what happens to fairly pure
limestone or dolomite rock after
metamorphism. Heat and pressure cause
the grains of calcite (in limestone) or
dolomite (in dolomite rock) to combine into
larger crystals.
• In this hand specimen of marble, the crystals are large. For
fine marble of the sort used in buildings and sculpture, the
crystals are small.
• Like other metamorphic rocks, marble has no fossils, and any
layering that appears in it probably does not correspond to the
original bedding of the precursor limestone. And like
limestone, marble tends to dissolve in acidic fluids. It is quite
durable in dry climates, as in the Mediterranean countries
where ancient marble structures survive.
MARBLE with ruby
• Pure limestones, made of calcium
carbonate, are converted into
crystalline calcite marble (also
calcium carbonate) during
• However, if the limestone contains
impurities, other minerals may form
in the marble.
• This rock had aluminium as its principal impurity (probably in
the form of clay particles), and high grade metamorphism has
produced large crystals of corundum (aluminium oxide).
• There must also have been tiny amounts of the metal
chromium, which colours the corundum red - this is the
gemstone ruby.
• The coarse interlocking texture of the calcite crystals can also
be made out.
with serpentine
• This rock formed from a dolomitic limestone (calcium
magnesium carbonate) with some silica impurities.
• High grade metamorphism formed the mineral olivine
(magnesium silicate).
• Along bands and fractures, where water got in while the
rock was still hot, this has been converted to the
distinctive yellow-green colour of serpentine (hydrated
magnesium silicate).
• Serpentine-rich marbles are commonly quarried, cut into
slabs, and used for decorative purposes.
• Metamorphic rocks form from HEAT and
PRESSURE changing the original or parent
rock into a completely new rock.
• The parent rock can be either sedimentary,
igneous, or even another metamorphic rock.
• Slate, schist, gneiss, and marble are all
examples of metamorphic rocks.