Geology Reference
In-Depth Information
tals) one atom at a time over a long period. The result is that all the crystals are
large enough for you to see, and they are all approximately the same size. The
crystals may be different colors or shapes depending on the composition of the
magma and the minerals that form during cooling.
Aphanitic
(E,I): An igneous rock with
aphanitic
texture is created when lava cools
very quickly — so quickly that the minerals don't have time to grow and are very
tiny. You can see the individual mineral crystals in an aphanitic rock only through
a microscope.
Porphyritic
(I,E): A
porphyritic
igneous rock is created by slow cooling followed by
quick cooling of a magma. A magma begins to cool slowly, and then its environ-
ment changes (perhaps it is pushed up closer to the earth's surface or erupts as
lava) so that it suddenly cools much more quickly. The resulting rock will be com-
posed of some large crystals (that grew during slow cooling) mixed in with smaller
crystals (that cooled quickly). Large crystals trapped within a mix, or
matrix,
of
smaller crystals is a porphyritic texture. In a porphyritic igneous rock, the large
crystals are called
phenocrysts.
Pegmatite
(I): Some intrusive igneous rocks form near (but still below) the surface
under conditions of low temperatures, with high amounts of water mixed into the
magma. In a pegmatite-forming magma, the water helps the ions (see Chapter 5)
move around and form large crystals. The result is a rock composed of
very
large
crystals with no matrix of smaller crystals around them; it's just a mash of giant
crystals. This type of igneous rock is called a
pegmatite.
Glassy
(E): An igneous rock has a
glassy
texture if it cools extremely fast from a
lava flow — so fast that the atoms don't have time to form crystals at all, resulting
in a smooth, glassy surface. This texture is most common in lavas that have a high
amount of silica in them. (Silica is the same molecule that makes the glass in your
windows.)
Vesicular
(E): A rock full of holes (like a sponge) is described as
vesicular.
As the
rock cools, gas bubbles are trapped in it. Eventually, the gas escapes, leaving
holes called
vesicles
in the rock. This characteristic is most common in rocks that
form from volcanic eruptions.
Pyroclastic
(E): When a volcano erupts, other materials such as fragments of rocks
from the volcano walls and ash may erupt along with the lava. Rocks that form
from these erupted materials are called
pyroclastic.
Pyroclastic rocks may look like
sedimentary rocks (which I describe in a later section of this chapter), but they
are still igneous — composed of volcanic rock materials. If the fragments are small
(less than 2 millimeters), the rock is called
tuff.
If the fragments are large (more
than 2 millimeters), the rock formed is called
volcanic breccia.
