Competitive Exams: Identification of Igneous Rocks

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.