NET, IAS, State-SET (KSET, WBSET, MPSET, etc.), GATE, CUET, Olympiads etc.: Identification of Igneous Rocks
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Grain Size | Usual Color |
Other | Composition |
Rock Type | fine |
dark | glassy appearance |
lava glass | Obsidian |
fine | light |
many small bubbles | lava froth from sticky lava |
Pumice | fine |
dark | many large bubbles |
lava froth from fluid lava | Scoria |
fine or mixed | light |
contains quartz | high-silica lava |
Felsite | fine or mixed |
medium | between felsite and basalt |
medium-silica lava | Andesite |
fine or mixed | dark |
has no quartz | low-silica lava |
Basalt | mixed |
any color | large grains in fine-grained matrix |
large grains of feldspar, quartz, pyroxene or olivine | Porphyry |
coarse | light |
wide range of color and grain size | feldspar and quartz with minor mica, amphibole or pyroxene |
Granite | coarse |
light | like granite but without quartz |
feldspar with minor mica, amphibole or pyroxene | Syenite |
coarse | medium to dark |
little or no quartz | low-calcium plagioclase and dark minerals |
Diorite | coarse |
medium to dark | no quartz; may have olivine |
high-calcium plagioclase and dark minerals | Gabbro |
coarse | dark |
dense; always has olivine | olivine with amphibole and/or pyroxene |
Peridotite | coarse |
dark | dense |
mostly pyroxene with olivine and amphibole | Pyroxenite |
coarse | green |
dense | at least 90% olivine |
Dunite | very coarse |
any color | usually in small intrusive bodies |
typically granitic | Pegmatite |
Chemical Sedimentary Rocks
These same ancient shallow seas sometimes allowed large areas to become isolated and begin drying up. In that setting, as the seawater grows more concentrated, minerals begin to come out of solution (precipitate) , starting with calcite, then gypsum, then halite. The resulting rocks are certain limestones or dolomites, gypsum rock, and rock salt respectively. These rocks, called the evaporite sequence, are also part of the sedimentary clan. In some cases chert can also form by precipitation. This usually happens below the sediment surface, where different fluids can circulate and interact chemically.
Diagenesis: Underground Changes
All kinds of sedimentary rocks are subject to further changes during their stay underground. Fluids may penetrate them and change their chemistry; low temperatures and moderate pressures may change some of the minerals into other minerals. These processes, which are gentle and do not deform the rocks, are called diagenesis as opposed to metamorphosis (although there is no well-defined boundary between the two) .
The most important types of diagenesis involve the formation of dolomite mineralization in limestones, the formation of petroleum and of higher grades of coal and the formation of many types of ore bodies. The industrially important zeolite minerals also form by diagenetic processes.
Sedimentary Rocks Are Stories
The beauty of sedimentary rocks is that their strata are full of clues to what the past world was like. Those clues might be fossils, marks left by water currents, mudcracks or more subtle features seen under the microscope or in the lab.
From these clues we know that most sedimentary rocks are of marine origin, usually forming in shallow seas. But some sedimentary rocks formed on land: Clastic rocks made on the bottoms of large freshwater lakes or as accumulations of desert sand, organic rocks in peat bogs or lake beds, and evaporites in playas. These are called continental or terrigenous (land-formed) sedimentary rocks.
Sedimentary rocks are rich in geologic history of a special kind. While igneous and metamorphic rocks also have stories, they involve the deep Earth and require intensive work to decipher. But in sedimentary rocks you can recognize, in very direct ways, what the world was like in the geologic past.