Tripura PSC Exam: Identification of Igneous Rocks

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Grain SizeUsual Color
Rock Typefine
darkglassy appearance
lava glassObsidian
many small bubbleslava froth from sticky lava
darkmany large bubbles
lava froth from fluid lavaScoria
fine or mixedlight
contains quartzhigh-silica lava
Felsitefine or mixed
mediumbetween felsite and basalt
medium-silica lavaAndesite
fine or mixeddark
has no quartzlow-silica lava
any colorlarge grains in fine-grained matrix
large grains of feldspar, quartz, pyroxene or olivinePorphyry
wide range of color and grain sizefeldspar and quartz with minor mica, amphibole or pyroxene
lightlike granite but without quartz
feldspar with minor mica, amphibole or pyroxeneSyenite
coarsemedium to dark
little or no quartzlow-calcium plagioclase and dark minerals
medium to darkno quartz; may have olivine
high-calcium plagioclase and dark mineralsGabbro
dense; always has olivineolivine with amphibole and/or pyroxene
mostly pyroxene with olivine and amphibolePyroxenite
denseat least 90 % olivine
Dunitevery coarse
any colorusually in small intrusive bodies
typically graniticPegmatite

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.

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