By Dr. Hans Upadhyay
Professor of Earth Science
Northeastern Illinois University

    A rock is an aggregate of one or more minerals. For example, the rock called granite is made up of two main minerals, quartz and potassium feldspar, with or without minor amounts of other minerals, such as micas.    In terms of their origin, rocks fall into three broad groups: igneous, sedimentary, and metamorphic.


    In Latin, "igneous" means "fiery." Rocks of this group originate from the cooling and solidification of hot ("fiery") molten material called magma. The molten material originates at great depths (up to 200 kilometers), and since this material is less dense than the surrounding rocks, it slowly begins ro rise towards earth's surface, where it erupts as a volcano, and the molten material is called "lava" (as opposed to "magma," which is used to describe the molten rock and dissolved gases deep underground).

    When the magma begins to rise upwards, it moves along various weak zones in the surrounding rocks. Three common weak zones in the rocks are: a) Fractures and cracks in the rocks; b) faults, which are fractures along which the two slabs of rocks have slipped past each other (e.g. California's San Andreas Fault); and c) bedding planes, where two different rock types come into contact. It is noteworthy that the fractures mentioned in example "a" above need not necessarily exist long before the magma tends to rise; the fracture can be developed "instantly" in the surrounding rocks by the tremendous pressure of the rising magma from deep underground.

    The molten material, despite it great depth, begins to cool off even before reaching the surface. However, the cooling at depth is slower than at earth's surface. The slow cooling allows the constituent minerals to grow to fairly large sizes, in many cases up to the size of your thumb. On the other hand, the lava that cools off at earth's surface undergoes very rapid cooling, because it suddenly erupted into relatively cold air (or seawater). For this reason, these rapidly cooled volcanic rocks form very small crystals that are barely visible to the unaided eye. The well known igneous rock called granite, therefore, must have been formed deep underground, because it carries large crystals.

    When the same "granitic magma" reaches earth's surface, it forms a volcanic rock called "rhyolite." This rock has exactly the same minerals, and nearly the same light color as a granite, except that it carries extremely small crystals that cannot be seen without the help of a microscope. On the other hand, if the magma deep underground carries very dark (high iron and magnesium) elements/minerals, it will form a dark igneous rock, but will still carry large crystals. One example of such a dark coarse-grained igneous rock is "gabbro" (commercially known as a "black granite.") If this "black granitic magma" reached the surface, it will form a dark, but very fine-grained volcanic rock called "basalt"(the one which is most common in the islands of Hawaii).

    When the cooling of the lava at a volcanic is super rapid (much more rapid than for a typical volcanic rock), the atoms in the lava do not get any time at all to form crystals, not even small ones; therefore, the lava freezes instantly and the resulting hardened item is a crystal-free rock glass, also referred to as "obsidian" in geology books. This is a very smooth and shiny rock, and resembles an ordinary thick broken glass bottle. If this rock glass has air bubbles in it, it looks like a rock sponge and is known as pumice. This is the only rock that floats on water, and it is used cosmetically as a callous reducer or abrasive file. (There is even a mechanic's soap known as "Lava" that contains ground up pumice as an abrasive.)

The following are some diagnostic properties of igneous rocks:

  1. If coarse grained, they carry crystals that have mostly sharp corners.
  2. General bright in luster.
  3. If it has glass and/or air bubbles, it is definitely igneous.


    "Sedimentum" in Latin means "settling," a reference to solid material settling out of a fluid, generally water. Most, but not all, sedimentary rocks are deposited in this fashion. Unlike igneous rocks, this one involves a near-surface process in which extremely high temperatures do not play any significant role. Examples of sedimentary processes, that eventually form sedimentary rocks, include the transportation and deposition of rock particles by rivers, wind, and glaciers. As piles of sediments accumulate, the layers at the bottom are compressed and compacted by the weight of the overlying layers. Over geologic time, these sediments are cemented together by mineral matter deposited in the voids between particles to form a solid rock.

    Sedimentary rocks are formed in two major ways. The first category includes those in which the rock particles (called "clastic") are physically transported from outside to the site of deposition. Here are three examples of such rock names that are simply based on the size of the particles that constitute the rock:


    Gravel (very coarse grain size) Conglomerate
    Sand (medium, sand-sized) Sandstone
    Clay/silt (very small grains to microscopic) Shale
The second category of sedimentary rocks ("non-clastic") includes those in which the sedimentary material originates from within the site of deposition, that is - it is not brought in from the outside. Some examples of the processes and resulting rock names are tabulated below:
    Chemical reaction and precipitation of calcium carbonate within sea water Limestone
    Accumulation of sea shells (made of calcium carbonate) at the sea bottom Limestone
    Evaporation of restricted (land-locked) shallow oceans Halite (rock salt)
    Rapid burial of plant material in oxygen-poor swamps Coal
Some characteristic features of sedimentary rocks are as follows:
    1. They are generally dull in luster.
    2. They are generally layered due to settling by gravity.
    3. If the rock has rounded grains (e.g. conglomerate or sandstone), it is most probably sedimentary.
    4. If the whole road cut or cliff face is exposed for examination, they may show ripple marks, mud cracks, etc., all of which indicate near-surface deposition.
    5. If a rock has fossils (remains or traces of ancient life), it is definitely sedimentary.


    "Metamorphism" means "to change form." Rocks belonging to this group are produced by changing pre-existing sedimentary or igneous rocks through the effects of tremendous heat and pressure. During metamorphism, the parent rock gets recrystallized without actually melting, which produces new, larger, and brighter crystals.

    Under pressure-dominated or regional metamorphism, these new crystals tend to be aligned in a common direction. Such rocks with aligned crystals are called "foliated," and foliated rocks commonly break when hammered into thin, smooth slabs when broken. Examples of foliated metamorphic rocks include: slate, schist, and gneiss (pronounced "nice"). Slate represents the lowest level of metamorphism, schist intermediate, and gneiss the highest level of metamorphism.

    Under heat-induced or contact metamorphism, the new crystals are oriented in random directions, and when hammered, the rock does not break into thin slabs. This is called a "non-foliated" rock. One good example is marble, which is produced by the metamorphism of a limestone. Heat-dominated metamorphism occurs outside of a magma chamber, but very closed to its outer limits, producing a localized "baked zone," since the metamorphic process needs heat.  Some characteristics of metamorphic rocks are as follows:

  1. They are invariably bright in luster.
  2. Foliated rocks have a layered appearance due to pressure-induced alignment of minerals.
  3. Coarse-grained metamorphic rocks may show crystals that grew during metamorphism.

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