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TYPES OF ROCKS (Igneous or magmatic rocks (it is possible to identify…

- - - - they are formed when magma hardens close to the surface in cracks in the Earth's crust after a period of slow cooling deep inside the Earth. This allows some parts of the magma time to crystallise, while others do not. Porphyry and pegmatite are examples of them.
    - - Volcanic rocks are formed when magma reaches the Earth's surface without cooling. Outside, the magma cools and hardens rapidly. The resulting rock has either very small crystals or no crystals at all. Basalt, gabbro, obsidian and pumite are volcanic rocks.
    - - such as granite and syenite, are formed by the slow cooling of magma under the Earth's surface. This can take thousands or even millions of years, which allows for larger crystal formation.
- - - - Clastic rocks are formed from fragments of other rocks. They are classified depending on the size of the rock particles inside them.
        
        Conglomerate is made up of rounded rock fragments cemented together. The pebbles have been rounded by the action of moving water. The size of these rock fragments is over 2 mm in diameter.
        
        Breccia is very similar to conglomerate but the fragments that make it up are angular. The size of these rock fragments is over 2 mm in diameter.
        
        Sandstone is made of sand grains cemented together into solid stone. The grain size of sandstone is 0.02 to 2 mm in diameter. Sandstone is mostly made up of quartz.
        
        Clays and mudrocks are made of very fine particles (diameter under 0.02 mm).
    - - Biochemical sedimentary rocks form from the gradual accumulation of the remains of living things, such as shells or dead plant material. They include shelly limestone, coralline limestone, coquina, coal and crude oil.
    - - Chemical sedimentary rocks are formed by chemical processes. Depending on their composition, they are classified as carbonates or evaporites.
        
        Carbonates, such as limestone, are formed by the precipitation of calcium carbonate dissolved in water.
        
        Evaporites are formed when the water from a salt solution evaporates, leaving solid salt behind. Examples: gypsum and halite or salt rock.
- - - - Foliated metamorphic rocks appear banded or layered Examples of rocks in this group, from the least metamorphosed to the most, are slate, schist and gneiss.
        
        GRADES
        
        Schist is a coarse grained metamorphic rock that is fissile, which means that it can split into thin layers.
        
        Gneiss is a dense foliated rock that has light and dark-coloured banding. Gneiss isn't fissile. Many types of gneiss are formed from granite.
        
        Slate is made up of clay minerals. Slate is foliated and easily splits into thin, flat, parallel layers.
    - - Non-foliated metamorphic rocks usually contain just one mineral. They are uniform in texture. Marble and quartzite belong to this group