The Rock Cycle and Rocks for Leaving Cert Geography

Tip: I’ve found that the most valuable part of this chapter was definitely the rock cycle diagram, which you can find below. This will be especially true if you’re a visual learner (like me). Pay close attention to the diagrams.

Explain the rock cycle with the aid of a diagram

Types of rock


Name: Basalt 

Description: Black, hard, has tiny holes in it, extrusive rock 

Formation: Comes from a volcanic eruption. Lava cools and hardens when it comes into contact with the sea/air. Comes through fissure eruption. The basic, runny lava cools quickly and small crystals form. Antrim-Derry plateau formed 65 million years ago over 14 million years, with several flows 5-40 metres thick. 

Location: Antrim-Derry Plateau. Giant’s Causeway (contracts to form hexagonal columns) 

Use: building roads as it’s a dull rock 

Name: Granite 

Description: Large crystals, can range from pink to grey, fire-lighter like qualities, intrusive rock, coarse 

Formation: Magma cools slowly forming large crystals. High in silica- acid lava. Contains mica, feldspar and quartz (which give it its colour). Leinster Batholith formed 400 million years ago during the Caledonian period. It’s an intrusive rock. 

Location: Blackstairs Mnts. Dublin/Wicklow Mnts. Leinster Batholith. 

Use: Good for building. Worktops, headstones. 


Name: Limestone 

Description: ranges from white to black (depending on impurities). It’s permeable. 
Formation: Remains of sea-creatures i.e bones, shells, teeth of fish etc. Piled up over millions of years. Cementation occurs with Calcium Carbonate. They are compressed and lithified into strata. Organic sedimentary rock. Formed 350MYA in Ireland, below a tropical sea. 

Location: The Burren, Co. Clare 

Use: Cement (after it’s been ground down), roadworks, fertiliser, statues, headstones 

Name: Sandstone 

Description: Reddish-brown colour. Easily weathered. 

Formation: Grains of sand compressed, cementation occurs (silica) and also lithification. Has strata. Inorganic. Deposition in rivers/ the sea piled up over millions of years. 380MYA, it formed in Ireland when there was a hot desert climate. 

Location: Munster Ridge and Valley Province 

Use: building 


Name: Marble 

Description: smooth when polished, white, green, red or black. 
Formation: Limestone/chalk changed into a metamorphic rock by intense heat (thermal metamorphism). The colour of marble depends on the intensity of the heat and the purity of its calcium. White marble is pure marble, and is very rare. 

Location: Kilkenny 

Use: Buildings, fireplaces, ornaments, gravestones 

Name: Quartzite 

Description: very hard, grey-white, resistant to erosion 

Formation: sandstone changed by heat and pressure, usually during periods of mountain folding. The sandstone is changed by the heat and pressure from the magma. Quartzite is hard and not easily eroded. Usually found on caps of mountains. 

Location: Co. Mayo- Croagh Patrick, Co. Wicklow- Great Sugar Loaf

Use: Watches, roads 

The Rock Cyclle

Q: Explain the rock cycle with the aid of a diagram


Rocks are constantly being destroyed, reformed and relayed inside and outside of the earth’s crust. If a rock egins as igneous eg Granite, weathering and erosion can result in the breaking down of the rock ‘in situe’. The rocks can be broken down by chemical weathering (eg Carbonation) or by mechanical weathering (eg exfoliation). The rocks, now broken down, now become sediments and are transported and deposited by rivers, the sea, ice and wind.

These sediments (i.e the remains of sea creatures, non-living things and vegetation) build up over millions of years. Compression occurs. As does cementation by calcium carbonate (in limestone) and silica (in sandstone). Layers called strata build up, which is called lithification. This creates sedimentary rock eg Limestone, sandstone.

If sedimentary rock is faced with high heat/pressure (eg at a convergent or subducting plate boundary), it can change form. Limestone/chalk changes to marble, and sandstone turns to quartzite. These are called metamorphic rocks.

If metamorphic rocks melt (eg during subduction), they become lava or magma. When the lava/magma hardens and cools, it forms an igneous rock, thus completing the rock cycle.

There are variations to the basic rock cycle. An igneous rock may melt due to heat and becomes lava/magma. When it cools and hardens, it will again become an igneous rock. If they are faced with extreme heat/pressure, they can become metamorphic rocks.

If a sedimentary rock weathers and erodes, it may become sediment. This sediment builds up, cementation, lithification and compression occur, and sedimentary rock is formed again. When faced with high heat, it can become igneous rock.

Metamorphic rock can weather and erode into sediments, build up, cement, lithify and compress to become sedimentary rock.

The Rock Cycle and Rocks for Leaving Cert Geography

Popular Posts