Classification of Rocks
Rocks are classified based on the process by which they were formed. There are three rock types: Igneous, Sedimentary and Metamorphic.
Igneous Rocks
Igneous rocks, from the Latin for fire ignis, are formed when molten rock cools and hardens. Molten rock is called magma when it is below the surface and is called lava when it is above the earth’s surface. When the magma cools it does so at a slower rate because it is insulated by the surrounding rock. This leads to large grains or crystals being formed; crystallisation. These crystals join together and interlock to form solid rock, usually very hard to break. The crystals are easily seen by the naked eye. Rocks formed beneath the surface are plutonic or intrusive rocks.
When lava cools on the earth’s surface it cools more quickly which leads to smaller crystals being formed. Rocks formed above the surface are extrusive or volcanic rocks.
Igneous rocks may once have been sedimentary rocks or metamorphic rocks before being melted and reformed. Lots of intrusive rocks are now at the surface in Ireland (including Leinster and Galway granites) due to erosion and uplift.
The upper section of the Earth's crust is made up of around 95% igneous rock. There are over 700 hundred types of igneous rocks, and they are generally the hardest and heaviest of all rocks.
Sedimentary Rocks
Sedimentary rocks are formed from small particles/grains which are eroded or weathered off a parent rock. These grains or sediments are composed of minerals, small pieces of plants and other organic matter. These grains are deposited by water, wind or ice over time, usually at the bottom of lakes and oceans. Over time they are compressed and compacted before consolidating into solid layers of rock, cemented together by minerals such as silica or calcium carbonate. This process is called lithification. These strata are often seen in exposed cliffs. Sedimentary rocks cover the majority of the Earth's rocky surface but only make up a small percentage of the Earth’s crust compared to metamorphic and igneous types of rocks. Sedimentary rocks can be further classified as organic (coal, limestone) and non-organic (sandstone, shale). Sedimentary rocks usually contain fossils.
Metamorphic Rocks
Metamorphic rocks were once igneous or sedimentary rocks, but have been changed due to intense heat and/or pressure within the earth’s crust, usually the result of intense tectonic activity. This process, called metamorphism, does not melt the rock but changes the rock by growing new crystals from the original composition of the rock. Uplift and erosion help bring metamorphic rock to the Earth's surface. Often the crystals lie in a particular direction, due to the pressure, and the rock looks like it was squashed.
Some common igneous rocks:
Andesite is an extrusive igneous volcanic rock of intermediate composition, with aphanitic* to porphyritic* texture. In a general sense, it is the intermediate type between basalt and rhyolite, and ranges from 57 to 63% silicon dioxide. The extrusive equivalent of diorite is andesite.
Basalt is made of fine-grained interlocking crystals (about 1mm in size). It is black or dark grey in colour, and often contains vesicles (bubbles of gas that are trapped as the lava cooled).
Dacite has an aphanitic* to porphyritic* texture and is intermediate in composition between Andesite and Rhyolite. The word dacite comes from Dacia, a province of the Roman Empire which lay between the Danube River and Carpathian Mountains (now modern Romania and Moldova) where the rock was first described.
Diorite is a medium-to coarse-grained intrusive igneous rock composed of about two-thirds plagioclase feldspar and one-third dark-coloured minerals, such as hornblende or biotite. The chemical composition of diorite is intermediate, between that of mafic gabbro and felsic granite. The presence of sodium-rich feldspar, oligoclase or andesine, in contrast to calcium-rich plagioclase, labradorite or bytownite, is the main distinction between diorite and gabbro. Diorite has almost the same structural properties as granite but, perhaps because of its darker colour and more limited supply, is rarely used as an ornamental and building material. It is one of the dark grey stones that is sold commercially as black granite.
Dolerite/ Diabase/ Microgabbro is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine grained to aphanitic chilled margins which may contain tachylite.
Gabbro is a coarse-grained, dark-coloured, intrusive igneous rock. It is usually black or dark green in colour and composed mainly of the minerals plagioclase and augite. It is the most abundant rock in the deep oceanic crust.
Granite is made of coarse-grained (5mm or so) interlocking crystals. It is a conglomerate of minerals and rocks, primarily quartz, potassium feldspar, mica, amphiboles, and trace other minerals. The relative proportion of different coloured minerals in a granite is largely due to the original source of molten rock that cooled to form the granite. The minerals that make up the granite give it its unique colour. If the molten rock was abundant in potassium feldspar, the granite is more likely to take on a salmon pink colour. On the other hand, if the molten rock is abundant in quartz and minerals that make up amphibole, you will likely get a black and white speckled granite commonly seen on countertops. Granite must contain at least 20% quartz. Some black granites are in fact gabbro. Its extrusive equivalent is rhyloite.
Minerals that give granite it’s varying colours: Quartz - typically milky white colour. Feldspar - typically off-white colour. Potassium Feldspar - typically salmon pink colour. Biotite - typically black or dark brown colour. Muscovite - typically metallic gold or yellow colour. Amphibole - typically black or dark green colour
Nepheline is the characteristic mineral of alkaline plutonic rocks, particularly nepheline syenites and nepheline gneisses. It occurs in beautiful crystal form with mica, garnet, and sanidine feldspar on Monte Somma, Vesuvius, Italy.
Obsidian/Volcanic Glass, sometimes, when cool, sticky magma erupts, the lava solidifies too rapidly for crystals to form and so volcanic glass is produced. It can have very sharp edges making it useful as a cutting tool or arrowhead.
Peridotite is a dense, coarse-grained igneous rock consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron.
Pumice is formed when gas-rich magma froths up and is rapidly blown out of a volcano. The lava is glassy-looking, and contains so many bubbles that it is very lightweight, floating on water. Pumice is rhyolite with holes.
Rhyolite is an extrusive igneous rock with a very high silica content. It is usually pink or grey in colour with grains so small that they are difficult to observe without a hand lens. Rhyolite is made up of quartz, plagioclase, and sanidine, with minor amounts of hornblende and biotite. It may have any texture from glassy to aphanitic to porphyritic.
Scoria is a highly vesicular (holy, bubbles trapped…), frothy textured, dark coloured volcanic rock that may or may not contain crystals. It is basaltic or andesitic in composition.
Tuff/ Volcanic Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is compacted into a solid rock in a process called consolidation. It is a light, porous rock.
Volcanic Bomb is a mass of molten rock larger than 64 mm in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. They cool into solid fragments before they reach the ground. Because volcanic bombs cool after they leave the volcano, they are extrusive igneous rocks.
*Aphanitic (from the Greek αφανης, "invisible") - igneous rocks that are so fine-grained that their component mineral crystals are not detectable by the unaided eye.
*Porphyritic - igneous rocks that have a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group
Phaneritic - igneous rocks, where the minerals are visible to the unaided eye
The Cache
In order to claim this earthcache, please sen the answers to the following questions via the messaging service.
- Name the type of rock found at the coordinates listed at Stage 1 (listed coordinates)
- Is it sedimentary, igneous or metamorphic?
- Is this rock intrusive/extrusive/plutonic/volcanic?
- Describe the rock: colour, formation, characteristics (rough, smooth, shiny, different colours…)
- What might this rock be used for?
- Name the type of rock found at the coordinates listed at Stage 2. There is no need to climb down to the storm beach. The rock can be observed from the small coastal path.
- Is it sedimentary, igneous or metamorphic?
- Is this rock intrusive/extrusive/plutonic/volcanic?
- Describe the rock colour.
- Where in Ireland is this rock found in abundance?
- In order to prove you visited the island, please give the second word on the information sign adjacent to the'Fáilte go Gabhla sign. A picture of you or your GPS at the Fáilte go Gabhla sign would be nice J. A few bars ♫♪♪♫ of Báidín Fheidhlimí would be even better ♫♪♪♫