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Planet Earth
Background
HOW ROCKS ARE MADE
By way of information for teachers it will be useful to understand the origins and classification of rocks.
Whether or how much of this background material is taught to primary classes is a matter for individual
teachers to decide.
Based on their sources and methods of formation rocks are usually classified under three headings — Igneous,
Sedimentary and Metamorphic.
Igneous (derived from the Latin word ignis meaning fire)
These rocks are formed by the cooling of very hot molten material called magma which originates deep down
inside the earth. It rises upwards towards the surface of the earth because it is hotter and lighter than the
surrounding rock.
If it reaches the surface of the earth it will erupt as a volcano at which stage it becomes known as lava. Such
an event is known as an extrusion. Due to exposure to air and water it cools fairly quickly and because of this
the crystals which make it up are usually very small and this is described as fine grained. The best local example
of this is the Antrim Plateau and the specific rock is dark or black in colour and is known as basalt.
Alternatively, the magma may not reach the surface of the earth and this leads to it cooling very slowly deep
below the surface — maybe over thousands or millions of years. This type of event is known as an intrusion.
The slow cooling process allows the crystals to become quite large and easily seen. Such a rock is called granite
and it is described as being coarse grained and it is usually light in colour with clearly visible mineral crystals.
The best example in Northern Ireland is the Mourne Mountains in south Co. Down.
The fact that such granite intrusions are found on the surface of the earth today illustrates that, while it did
originally cool slowly at depth, erosion and weathering over millions of years have worn away and removed
the overlying rocks.
Sedimentary
These rocks are formed by the deposition of fragmentary materials which have been eroded and weathered
from other parent rocks. As such they could be regarded as recycled rocks.
They often consist of sand, pebbles and mud removed from the land by erosion, carried by rivers or blown by
the wind, and eventually deposited. These sediments are usually deposited in water in seas and lakes but they
can also accumulate in desert environments. They are often deposited in layers which can be called strata.
As the layers accumulate one on top of another they become buried underneath younger sediments and they
become compressed into solid rock.
The red sandstone at Scrabo, Newtownards, Co. Down is an example of a sedimentary rock which was deposited
in desert conditions.
Some sedimentary rocks are of chemical or organic origin. In some cases the deposits in the sea may consist
almost entirely of the shell fragments of dead sea creatures. These shells are made largely of calcium and when
they accumulate and become compressed they form a rock called limestone. A good example of such limestones can be visited at Marble Arch Caves Global Geopark, Co. Fermanagh.
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Planet Earth
Background
ROCKS
In other cases, the accumulation of large amounts of dead plant material may, over millions of years, turn into
coal which is another type of sedimentary rock
It may be that some dead sea creatures are not fragmented and become buried in their original condition.
When this happens they can become preserved in the developing rock and these are known as fossils.
Metamorphic (Greek: meta meaning change and morphe meaning form)
If sedimentary or igneous rocks become deeply buried they can become subject to very high temperatures and
earth movements can subject them to intense pressure. Such an environment may melt the original rocks and
deform them — hence the term metamorphism which means change in form. Sometimes already buried rocks
may come in contact with hot igneous material and, without becoming molten, they become baked or cooked;
this is known as contact metamorphism.
If a limestone is subject of metamorphism it becomes harder and more crystalline and it then becomes known
as marble. When a shale rock is subjected to metamorphism it becomes compressed into layers at which stage
it is known as slate.
There are few good sizable examples of either marble or slate in Northern Ireland but slates are common in
north Wales and marble can be mined in Connemara, Co. Galway. ROI.
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Planet Earth
Background
ROCKS
Basalt
Basalt is the most common type of rock in the Earth’s crust and it makes up most of the ocean floor. It is dark
grey to black in colour as it is made up of many dark coloured minerals. It is a crystalline rock but typically you
can’t see most of the mineral crystals without using a microscope. This is due to the way in which basalt is
formed.
Much of the Earth’s basalt was made millions of years ago when molten rock (magma) welled up through
long cracks in the earth’s crust and spilled over in rivers of lava (the name given to magma once it reaches the
Earth’s surface) sometimes many miles wide. Since lava coming into contact with the air tends to cool and
solidify quite quickly crystals do not have the opportunity to grow to a size
that can be seen by the human eye.
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Generally, basalt forms as layers of rock with each representing a separate
lava flow. However, the basalt seen at the Giant’s Causeway on the Antrim
coast of Northern Ireland is different; the lava cooled and cracked into huge
columns, most of them with six sides. This happened here because the lava
flowed into a huge river valley meaning that a large volume of lava gathered
and required a much greater time to cool down. When this happened the lava
contracted internally, with joints that extended all the way vertically upwards,
giving rise to the wonderful basalt columns at the Giant’s Causeway.
Smooth flowing lava flows from volcanoes today and cools to form new basalt rock (check out Hawaii). New
basalt rock is also forming on the Atlantic Ocean floor as magma wells up into the crack between the plate
boundaries (cracks in the Earth’s crust) and cools when it comes in contact with ocean water.
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Basalt is a hard, durable rock due to the interlocking nature of its crystalline structure. It is used for a variety
of purposes, but most commonly as an aggregate in construction projects. When crushed, it can be used as
road base.
Glowing lava hardening into black basalt on the island of
Hawaii.
Basalt columns at The Giant’s Causeway. Basalt lava poured
onto the surface here 60 million years ago. A lake of lava filled
a shallow depression in the surface and cracked into these
polygonal columns when it hardened and contracted.
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Planet Earth
Background
ROCKS
Granite
Granite is a light coloured, speckled rock. If you look closely at a piece of granite you will see little flecks of
white, black, grey and sometimes pink. These flecks of colour are the mineral crystals, which in the case of
granite are large enough to be seen with the human eye. Granite is formed like basalt when hot molten rock
cools down and hardens. However granite differs from basalt in a very important way. The magma from
which it is formed never reaches the Earth’s surface. Instead it cools and hardens deep underground and
because it may take thousands of years to cool down the minerals are able to grow into large, easily visible
crystal grains.
The crystals that make up granite interlock together, tightly which is why granite is such a durable rock. It can
be seen covering the walls and floors of many public buildings and polished granite slabs are also used for
kitchen worktops and for headstones.
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The Mourne Mountains in County Down were formed when molten
magma cooled just below the land surface. The granite of which these
mountains are formed is now at the surface due to the erosion of the
overlying rocks.
Mount Rushmore in the USA has the faces of four American
presidents carved into the granite mountainside.
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Planet Earth
Background
ROCKS
Sandstone
Sandstone can come in a variety of colours such as red, brown, pink, tan, yellow and even white or black.
It is composed of numerous individual sand grains, held together by a type of mineral cement and/or
compaction.
Rocks such as sandstone are formed from the broken-down pieces of other rocks, held together as a result of
pressure and/or by minerals deposited between the grains to act as cement. This happens when rocks at the
Earth’s surface are weathered and hence broken into small particles by the forces of wind and water. These
particles (sediments) often end up being transported by water and find their way to the bottom of lakes, oceans,
rivers or streams, where they pile up in layers often hundreds of metres thick. The weight of all the sediment
presses down on the lower layers with tremendous force, compacting it and turning it into rock-like sandstone.
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Sandstone is a popular building material. It is relatively soft, making it easy to carve, but it is generally less
resistant to weathering, as the photo of a weathered sandstone carving in Belfast illustrates, and many other
buildings throughout Northern Ireland have used sandstone as a building stone.
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Layers of sandstone on the shore at Cushendall,
County Antrim.
Old Red sandstone sample from Cushendun,
County Antrim.
Weathered pillar, Mount Charles, Belfast.
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Planet Earth
Background
ROCKS
Shale
Shales commonly come in shades of grey and black. The surface of the rock can feel very slick and shiny
almost like satin. Like sandstone it is composed of compacted grains laid down in layers, the main difference
being that the grains are not easily visible as they are so small (i.e. it is composed of clay-sized rather than
sand-sized particles*). Shale splits very easily into thin layers/brittle sheets.
Black shales contain organic matter that sometimes break down to form natural gas or oil. In this type of
shale tiny particles of organic matter are deposited with the mud/clay from which the shale formed. As the
mud was buried and warmed within the earth some of the organic material was transformed into oil and gas.
This oil and gas is very difficult to extract because it is trapped within tiny pore spaces or absorbed into clay
mineral particles that make up the shale. In the late 1940s it was discovered that pumping water (containing
a mix of sand and chemicals) under pressure into the shale would cause it to fracture, liberating some of
the gas from the pore spaces. This technique is called hydraulic fracturing (fracking) and has caused much
controversy here in Northern Ireland due mainly environmental concerns about the fracking process.
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Planet Earth
Background
ROCKS
Limestone and chalk
White Park Bay in County Antrim.
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Grey limestone.
White limestone or chalk.
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Limestone has a very interesting property that sets it apart from other
rocks. The calcium carbonate of which it is composed reacts with
rainwater that is slightly acidic and this results in the rock dissolving.
Large cave systems, e.g. Marble Arch Caves in Fermanagh, are typically
found in areas of limestone rock, where water has percolated through
cracks joints are the vertical cracks and bedding planes the horizontal
cracks in the rock and over time has dissolved and eroded the rock
creating huge caverns underground. Limestone has many uses. It has
quite a history as a building material, having been used by the ancient
Egyptians in the construction of the pyramids. It is the main ingredient
in cement, which when mixed with sand water and crushed rock, forms
concrete. Crushed limestone is often used underneath road and rail beds.
Farmers will often spread fertiliser that contains ground up limestone on
their crops as it is a source of plant nutrients and helps neutralise soil
acidity.
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Limestone is usually a light colour like white, tan or grey. It is composed
like other rocks of this type (e.g. sandstone and shale) from material
deposited on the ocean floor and compacted over long periods of time
by the weight of the sediments above. However, limestone is composed
of different material. Ocean-dwelling organisms such as oysters, clams,
mussels and coral use calcium carbonate (CaCO3) found in seawater to
create their shells and bones. As these organisms die, their shells and
bones are broken down by waves and settle on the ocean floor where
they are compacted over millions of years, creating limestone from the
sediments and the pressure of the ocean water. The colour of limestone
depends on the other sediments in the mixture besides the mineral
calcite (calcium carbonate) e.g. sand, clay and organic material. Chalk
(or white limestone) is a pure form of limestone made from the shell
remains of microscopic marine organisms.
Maggie’s Hole, Marble Arch Caves.
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