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Rock Types & Formation
5th year Geography
Learning Outcome
• Identify the three main rock groups.
• Locate examples of each rock type on a map
of Ireland.
• Understand the formation & main
characteristics of two types of rock from each
group.
• Answer an essay style exam question,
explaining in detail the formation of one rock
type.
Rocks
• Rocks vary in their:
 Origin
 Formation
 Characteristics
Rock Groups
•Three major rock groups exist:
 Igneous
 Sedimentary
 Metamorphic
Igneous
1. Magma rises, cools and solidifies to form
igneous rocks
2. Magma reaches the surface, it forms volcanic
rock
3. Magma cools and hardens within the crust, it
forms plutonic rock
2 Types of Igneous Rock
I. Plutonic or
intrusive rocks
 Formed when
lava cooled and
solidified on the
Earth’s surface
or close to the
surface
 e.g. basalt
I. Volcanic and
extrusive
 Formed when
magma cooled
down and
solidified inside
the
Earth’s crust
 e.g. granite
Igneous rocks contain crystals
 Size tells length of time it took rock to cool
 Large crystals cooled slowly within the
Earth’s crust
 Small crystals, cooled quickly on the Earth’s
surface
Granite
Granite
 Coarse-grained
 Formed when magma cooled deep within
the Earth’s crust
 Contains minerals of feldspar, quartz and
mica
 Varies in colour – white, grey, pink or black
 Crystals are large, the rock cooled slowly
 e.g. Wicklow Mountains
Basalt
Basalt
 Fine-to-mediumgrained
 Ranges in colour –
black to dark grey
 Formed when lava
cooled quickly
on the Earth’s
surface
 Small crystals
 e.g. Antrim-Derry
Plateau
 Giant’s Causeway
 Six-sided columns,
hexagonal in
shape
Giants Causeway
Sedimentary
 Rock is destroyed where denudation
occurs on the surface
 Weathering
 Erosion
 Break down into sediments
 Sediments are then deposited and
compressed
Lithification
• A process where sediments compact under
pressure and over time become solid rock.
• Eg; Loose particles of sand become sandstone.
Sedimentary rock
 Formed over millions of years
 Rock fragments and the remains of
animals and plants compressed
under pressure to form solid rock
 e.g. limestone and sandstone
Two types of sedimentary rock
I. Inorganic
sedimentary rock
 Inorganic
sedimentary rock
formed from the
broken down
remains of preexisting rock
I. Organic
sedimentary rock
 Organic
sedimentary rock
formed from the
remains of
animal and plant
life
Limestone
 Organic sedimentary rock
 Most common type of rock in Ireland
 Cemented and compressed remains of fish and
other sea creatures
 Over millions of years compressed and formed
slowly into solid rock
 Carboniferous period
 Contains calcium carbonate
 Varies in colour - greyish white, black, dark grey
 e.g. the Burren in Co. Clare
Limestone
Sandstone
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Inorganic sedimentary rock
Coarse
Usually brown or red
Formed approximately 400 million years ago
Sediments laid down in layers
Each successive layer compressed the next layer
Cemented to form sandstone
e.g. Comeragh Mountains, Co. Waterford
Sandstone
•
Metamorphic
Modified due to either great heat
or pressure or both
Metamorphic Rock
 Sedimentary or igneous rocks changed by
either great heat or pressure (or both)
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Formation can depend on;
Magma
Folding
Appearance, texture and chemical
composition change
Types of metamorphism
i.
Thermal
 heat from
magma
 rocks change in
composition
ii.
Regional
 great heat and
pressure over a
large area
 fold mountain
 colliding plates
 rising magma
Marble
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Metamorphic rock
Limestone
Heat and pressure
Various colours
Connemara (green)
Kilkenny (black)
Cork (red)
Rathlin Island, Antrim and Carrara, Italy (white)
Quartzite
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Metamorphic
Sandstone
Heat and pressure
Mountain building
Sandstone came into contact with magma
Light-coloured rock, e.g. Croagh Patrick, Co.
Mayo
Croagh Patrick
Exam Question
• Name one type of igneous / sedimentary rock.
Name one Irish location where this type of
rock was formed and explain in detail how it
was formed. (30m)

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