LC Rock Types and Landscapes

Igneous Rocks 

Granite Characteristics

Irish rocks in the order of their formation over time.

Irish rocks in the order of their formation over time.


Intrusive (plutonic)

Cooled slowly


light coloured


Composed of feldspar, mica, quartz


Wicklow mountains

Connemara (pink granite)

Granite Landscape

Location of Leinster Chain
Wicklow Mountains
Part of the Leinster Mountain chain
Covers approx.  3000 Km2
Composed of very large igneous intrusions
Formed between 450 – 370 million years ago (Caledonian period)
Magma is intruded under the earth’s crust
Doesn’t break through the surface
Cools slowly, produces granite
Existing rocks were metamorphosed to produce schists (Lugnaquilla)
Weathering exposed the granite

Basalt Characteristics


Extrusive (volcanic)

Cooled quickly


Dark grey/Black

Tiny crystals

Rich in iron oxide (feO)


Basalt Landscape

Antrim Plateau

Giant’s Causeway

Types of Lavas

Runny lavas
Runny and flow well
Spread out before cooling
Form along spreading centres (fissures)
as plates diverge
Form basalt plateaus
e.g. Iceland and Antrim

Sticky Lavas

Don’t flow well
Solidify quickly
Form where ocean and continental plates collide
Make explosive, steep volcanic cones

Flood Basalts

Basalt Plateaus (flood basalts)
Form when lava flows out of a fissure or fault in the earth’s crust
Typically such eruptions occur on the sea floor
Can occur on land
Lava cools quickly so crystals are small
Continued eruption results in a lava or basalt plateau
Iceland formed in this way
Iceland sits on the Mid-Atlantic Ridge Rift
A series of fissures traverse the country north to south
Deccan Traps,India

Giant’s Causeway

formed during the early Tertiary period
62 – 65 million years ago
Three lava  outflows occurred
Lower, Middle and Upper Basaltic
Lulls occurred between the outflows
deep  reddish brown layer  (laterite)
which is rich iron and aluminium oxides
weathering of the basalt.
situated in an equatorial  region
hot and humid conditions.
The famous hexagonal columns occur in the middle basalt layer
Produced from rapid cooling of basalt

Sedimentary Rocks

Limestone Characteristics

Organic/Biological sedimentary rock
Tropical seas
Fine to coarse-grained
White to grey
Fossiliferous (rich in fossils)
reacts with acid
calcium carbonate
Soft rock
Easily weathered

Limestone Landscape

The Burren – Formation
The Burren is a unique karst region in Ireland
covers approximately 360 Km2 in north Country Clare.
composed of gently dipping beds of carboniferous limestone.
formed about 350 million years ago
region was submerged beneath warm tropical waters
marine deposition occurred
dead sea creatures fell to the ocean floor.
became fossilised.
region was uplifted
began to weather rapidly.
soil cover formed however
over-cultivation during the Neolithic
exposed the area to increased weathering
result is the present day karstic landscape

Limestone Weathering

Limestone is a calcareous sedimentary rock
weathers differentially.
When rainwater falls combines with small amounts of carbon dioxide
forms a weak carbonic acid.
Through the process of carbonation,
the acid rain changes the calcium carbonate in the limestone into a soluble bicarbonate

CaCO3              +            H2O + CO2           –     Ca(HCO3)2

Calcium Carbonate  +   Carbonic Acid           –       Calcium bicarbonate

Limestone is also a well jointed rock
differential resistance to weathering
explains the pattern of grikes associated with Limestone Pavement
Rainfall picks out weaknesses on the surface
long shallow depressions are exploited by the rainfall as it drains from the surface
Over time these depressions develop into grikes.

Surface Limestone Landforms

Limestone Pavement

Subterranean Limestone Landforms

Continued weathering of limestone also produces underground features
Caves, caverns, Stalactites, Stalagmites, Curtains, etc.

Cycle of karst landscape development
Sandstone Characteristics

Inorganic/clastic sedimentary rock

Composed of sand

Medium to coarse-grained (0.6-2mm)

Red, yellow, grey


grains are visible

Soft-to-hard rock

Sandstone Landscape

Ridge and Valley Province, Munster
Name given to landscape of fold mountains in Munster
Series of fold mountains
380/360 mya
Ireland close to the equator
Sediments were deposited in the shallow sea to the south
Formed Old Red Sandstones of Southern Ireland
Ridge and Valley Province
350/300 mya
Carboniferous Period
Ireland beneath a warm tropical sea
Formation of Carboniferous Limestone
Covered the Old Red Sandstone

300/250 mya
African landmass (Laurentia) collided
with the Eurasian/north American landmass (Gondwana)
Old Red Sandstone in Cork-Kerry are folded
Hercynian/Amorican/Variscan orogeny
Comeragh, Knockmealdowns,
Macgillycuddy Reeks, Galtee Mountains
Ireland on the Equator
Differential erosion
Limestone was worn away
Limestone valleys floors (synclines)
Sandstones Ridges (Anticlines)
This folding had a dramatic effect on the flow of rivers

Shale Characteristics

Inorganic Sedimentary rock

Compressed mud


Soft, easily spilt

Light to dark grey

Metamorphosed into slate

Metamorphic Rocks

Types of metamorphism

Thermal – by heat
Dynamic – by pressure
Regional – by heat and pressure on a large scale

Thermal Metamorphism

Mica schist
Dynamic metamorphism
Pressure along moving faults deforms rocks
Breccias are formed as rocks shatter
Deeper in the crust rocks become ductile
Rock is ground into fine particles
Mylonite forms
It is a laminated rock with lens shaped fragments

Regional metamorphism

Happens on a large scale
Colliding plates
Rocks changed by heat and pressure
Structure and chemical make up of rocks may change

Marble Characteristics

Non-foliated (not layered)metamorphic

Made from limestone

Fine to  coarse-grained

Dense rock


white, green, black red,


Quartzite Characteristics

Metamorphic rock

Made from sandstone



Very hard

Resistant to weathering


Slate Characteristics

Foliated (layered) metamorphic rock



Easily split