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:

IgneousSedimentaryMetamorphic

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 rocksFormed when

lava cooled and solidified on the Earth’s surface or close to the surface

e.g. basalt

I. Volcanic and extrusiveFormed 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-medium-grained

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 pre-existing 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

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)

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

Metamorphic rock Limestone Heat and pressure Various colours Connemara (green) Kilkenny (black) Cork (red) Rathlin Island, Antrim and Carrara, Italy (white)

Quartzite

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)