IDENTIFYING ROCKS
There are three basic types of rocks that planet Earth is composed of: igneous,
sedimentary, and metamorphic. Follow these simple guidelines to determine the three
types of rocks when looking at various types of rocks.
Igneous Rocks
Igneous rocks form as a result of the cooling of magma. When magma cools and
solidifies underground, the resulting igneous rock is plutonic. These types of rocks
may be surfaced through weathering processes or the movement of plate tectonics.
When magma cools and solidifies during or after being moved to the surface through
volcanic processes, the resulting igneous rock is volcanic.
Igneous rocks are classified based on the size and composition of their constituent
mineral crystals. As a general guide, the longer the igneous rocks take to cool and
crystallize, the larger the mineral crystals will become.
You can tell igneous rocks at
first glance if the minerals of the rock are crystallized and fused together with no
surrounding cement. Some well-known types of igneous rocks include granite,
rhyolite, andesite, diorite, obsidian, gabbro, and basalt.
Sedimentary Rocks
Sedimentary rocks take their compositional elements from other sources that have
been weathered to sediment, re-deposited, and re-solidified through compression and
cementation. Sedimentary rocks generally have a layered appearance—as sediment is
pressed and solidified by the overlying crust; the particles align themselves horizontal
to the pressure.
You can tell sedimentary rocks at first glance if the constituent elements are “glued”
together by a surrounding cement, or if the elements are of varying size, shape, and
composition as opposed to simple mineral crystals.
Metamorphic Rocks
Metamorphic rocks are types of existing rocks that have been altered in form as a
result of changes in heat and pressure. In mountainous regions, for example, where
rock matter is pressed and altered by plate tectonic activity, metamorphic rocks are
very common. If you should stumble upon a metamorphic rock outcrop in the field,
outside of any mountain range, chances are good that you’re standing at the location
of an ancient mountain range that has been weathered away over millions of years.
Common types of metamorphic rocks, such as marble, schist, and gneiss, bear the
appearance of having been twisted, folded, or swirled as though the rocks had
become pliable or altered. Partial meltdown, pressure, and re-crystallization that occur
deep within Earth’s crust at regions of plate tectonic activity are responsible for these
characteristics.
Every rock on Earth may be classified as one of these three types of rocks. Igneous
rocks account for the majority of rock outcrops on the planet while the secondary
sedimentary rocks are only slightly less common. Metamorphic types of rocks are less
abundant and are limited to regions of mountainous and volcanic plate tectonic
activity.