Metamorphic rocks are formed when igneous rock or sedimentary rock are changed by great heat or pressure or both. This heat is generated from magma moving close to the surface of the earth and coming into contact with igneous or sedimentary rock.
Thermal metamorphism is when rocks are changed as a result of great heat. Thermal metamorphism occurs when heat from volcanic eruptions changes the rock with which it comes into contact. In Ireland, quartzite in the Sugar Loaf Mountain, Co. Wicklow was formed as a result of thermal metamorphism. As the hot molten magma came in contact with the surrounding sandstone rock, intense heat caused it to change to quartzite. This quartzite was exposed as the overlying rock was eroded as a result of weathering and erosion. Quartzite can also be found in Croagh Patrick, Co. Mayo. Marble is also formed as a result of thermal metamorphism of limestone at convergent plate boundaries.
Regional metamorphism is caused when heat and pressure occur over a large area. This occurs as a result of plate tectonics at convergent plate margins. The majority of Ireland’s metamorphic rocks were formed in this way 450 million years ago during the Caledonian fold mountain building period when the North American and European plates collided. The collision of the plates caused the rocks to buckle and magma rose into the buckled rock.
Mudstone in County Wicklow was changed to schist in this way. Granite in Belmullet, Co.
Mayo was changed to gneiss.
Dynamic metamorphism occurs when sections of the crust slip past each other in opposite directions at conservative plate boundaries. Shale is put under huge pressure at converging plate boundaries. As a result of this pressure the shale is compressed forming slate.
Metamorphism changes the characteristics of a rock. Metamorphic rocks are harder than their original rock, e.g. marble is harder than limestone. Metamorphism also changes the colour of the rocks due to the effect of heat, e.g. limestone is grey but marble may be black, green or white. As a result of metamorphism, mineral crystals may be rearranged to form foliated metamorphic rocks such as slate. The minerals are flattened into parallel layers
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In this answer I will discuss the formation of limestone. Limestone is an example of an organically formed sedimentary rock. The sediments from which limestone was formed came from the remains of once living organisms. Limestone is a stratified rock, laid down far from the shore on the bed of a warm, clear sea out of reach of land sediments. It was formed from skeletal remains (teeth, bones, shells) of marine creatures such as fish, sea urchins and corals. These accumulated over millions of years and were compacted and cemented.
Ireland’s limestone was formed about 350 million years ago when Ireland lay under a warm, shallow, tropical sea. This type of limestone formed during a period referred to as the
Carboniferous period. Today, the remains of these sea creatures can be seen by the naked eye as fossils. Limestone consists almost entirely of calcium carbonate. In Ireland limestone can be found in the central plain. The Burren in Co. Clare is an example of a karst landscape.
Metamorphic rocks are formed when the texture, appearance and chemical composition of either igneous or sedimentary rocks are changed by either great heat or pressure or both.
Limestone is changed into the metamorphic rock, marble, as result of regional metamorphism. Regional metamorphism occurs when heat and pressure occur over a large area. This occurs as a result of plate tectonics at convergent plate margins. The majority of
Ireland’s metamorphic rocks were formed in this way 450 million years ago during the
Caledonian fold mountain building period when the North American and European plates collided. The collision of the plates caused the rocks to buckle and magma rose into the

buckled rock. As a result marble is formed from limestone. Metamorphism changes the characteristics of the rock. Limestone is grey in colour but Connemara marble is green and
Cork marble is red.
In this answer I will explain the formation of the igneous rocks, granite and basalt. Igneous rocks are formed when magma cools and solidifies on or beneath the earth’s surface.
Intrusive or plutonic igneous rocks are formed beneath the surface of the earth, deep within the crust. Magma is formed at convergent plate boundaries where descending plates are subducted into the hot mantle. Plumes of magma rise and insert themselves into the rocks above. Here, in the heart of fold mountains, the magma cools slowly, surrounded as it is by a mass of insulating rock. As a result, large crystals form, e.g. glassy quartz. Granite is an example of an intrusive igneous rock formed at convergent plate boundaries. Granite is grey in colour, containing quartz, feldspar and other minerals. Granite is found in the Wicklow and
Donegal mountains. These mountains were formed during the Caledonian folding era around
450 million years ago when the North American and Eurasian plates collided. The molten magma intruded into the folds in the Wicklow mountains and cooled slowly to form masses of granite, e.g. Leinster batholith. This rock appeared on the surface when the overlying rock was weathered and eroded.
Extrusive or volcanic rock forms when lava pours out onto the surface of the earth, cools and solidifies quickly. Basalt is an example of an extrusive igneous rock that forms at divergent plate boundaries. Basalt is dark brown or black in colour. The lava cools so quickly that crystals of mica, feldspar and quartz do not have time to develop and therefore, are not visible. The lava reaches the surface through a volcanic crater/vent or a fissure. Basalt can be found in the Antrim Derry Plateau (the Giant’s Causeway). This basalt plateau was formed over 65 million years ago when the Eurasian and American plates began to separate and lava poured through the fissure that was created on the earth’s crust. Successive flows cooled quickly to form layers of basalt rock
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Metamorphic rocks are formed when igneous rocks or sedimentary rocks are changed as a result of great heat or pressure or both.
Thermal metamorphism is when rocks are changed as a result of great heat. Thermal metamorphism occurs when heat from volcanic eruptions changes the rock it comes in contact with. In Ireland, quartzite in the Sugar Loaf Mountain, Co. Wicklow was formed as a result of thermal metamorphism. As the hot molten magma came in contact with the surrounding sandstone rock, intense heat caused it to change to quartzite. This quartzite was exposed as the overlying rock was eroded as a result of weathering and erosion. Quartzite can also be found in Croagh Patrick, Co. Mayo.
Regional metamorphism occurs when heat and pressure occur over a large area. This occurs as a result of plate tectonics at convergent plate margins. The majority of Ireland’s metamorphic rocks were formed in this way 450 million years ago during the Caledonian fold mountain building period when the North American and European plates collided. The collision of the plates caused the rocks to buckle and magma rose into the buckled rock.
Mudstone in County Wicklow was changed to schist in this way. Granite in Belmullet Co.
Mayo was changed to gneiss. Marble is formed from limestone. Metamorphism changes the characteristics of the rock. Limestone is grey in colour but Connemara marble is green and
Cork marble is red.