Chapter 3—Identification of Igneous Rocks
We move now from our 2-week study of minerals to the study of rocks. Rocks
are composed of grains of one or more minerals.
We’ll first study igneous rocks. Igneous rocks are those that form as magma or
lava cools and solidifies. Magma is molten rock underground; lava is molten rock that is
at the earth’s surface. The material that makes up the molten rock comes from deep
within the earth’s crust or from the uppermost regions of the mantle.
Igneous Rocks will be classified using three things: texture, color, and
composition. Texture refers to the grain size, whether the grains are coarse- or finegrained; composition refers to the suite of minerals that make up the rock. The color of
igneous rocks is affected by the composition. Dark-colored rocks contain a certain suite
of minerals, while light-colored rocks contain a different suite of minerals.
You should be familiar with the terms intrusive and extrusive. Intrusive rocks are
formed from magma that intrudes into existing crustal rocks, cooling slowly and
solidifying beneath the surface; these rocks are coarse-grained. Extrusive rocks (volcanic
rocks fall under this category) are formed from lava that extrudes onto the earth’s surface
and cools quickly; these rocks are fine-grained.
TEXTURE
Texture refers to the grain size of the minerals in igneous rock. The size of the
minerals depends on how much time it takes the molten rock that forms them to cool.
Therefore, intrusive rocks are coarse-grained, since they have cooled slowly
underground; extrusive rocks are fine-grained, since they have cooled rapidly above
ground.
There are basically three textures you need to know: Phaneritic, aphanitic, and
porphyritic. Phaneritic rocks have visible mineral grains; many intrusive rocks are
phaneritic in texture. Aphanitic rocks have no or few visible mineral grains; most
extrusive (volcanic) rocks are aphanitic in texture. Porphyritic rocks have 2 distinct grain
sizes. The rock will have either a phaneritic or aphanitic “ground mass” (meaning most
of the grains are this texture) and larger included grains called “phenocrysts.” Refer to
figure 3.1 on page 31 of the lab manual for a visual explanation of texture.
One other texture you should be familiar with is “vesicular.” Vesicular rocks are
extrusive (volcanic) and they have vesicles (gas cavities) preserved in their structure.
Basically the lava cooled quickly while gas was trapped in it, and the gas bubbles were
preserved. We will examine two vesicular rocks in lab: scoria and pumice.
COMPOSITION
Magma and lava are composed of the eight (8) most abundant elements in the
earth’s crust: silicon, oxygen, aluminum, iron, sodium, potassium, calcium, and
magnesium; nearly every other naturally occurring element can be found in them as well,
but only in minute amounts.
The amount of oxygen in magma far exceeds the amount of other elements
present, so the minerals that form in the liquid state are primarily oxides such as SiO2,
Al2O3, Fe2O3, etc. There are seven minerals which generally occur in igneous rocks, but
not all igneous rocks contain all of them. These minerals are: Olivine, augite,
hornblende, biotite, plagioclase feldspar, potassium feldspar, and quartz. (Now do you
see why we studied these minerals first?)
Minerals at the beginning of this list contain abundant calcium, magnesium, and
iron, while being deficient in silica (SiO4). Minerals at the end of the list contain more
sodium and potassium, and are rich in silica. Minerals with very little silica, like olivine
and calcium-rich plagioclase feldspar, crystallize first from magma at higher
temperatures. Minerals with abundant silica, like quartz, crystallize last from magma at
lower temperatures. The crystallization sequence is known as a ‘reaction series,’ and is
best shown in picture. Please refer to Figure 3-2 in the lab manual, page 34.
The iron-, calcium- and magnesium-rich minerals that crystallize first from
magma or lava impart a dark color to the rocks that they form. These dark-colored rocks
are termed “mafic” rocks. The sodium-, potassium-, and silica-rich minerals that
crystallize last from magma or lava impart a light color to the rocks that they form.
These light-colored rocks are termed “felsic” rocks. Rocks that contain both mafic and
felsic minerals are termed “intermediate” rocks, and contain both dark- and light-colored
minerals.
FORMATION OF IGNEOUS ROCKS
The lithosphere (crust) of the earth is broken into pieces, or “plates.” Igneous activity is
mostly confined to boundaries between these plates. Where plates diverge (spread apart),
lava floods out and cools quickly to form dark-colored or “mafic” rocks with “aphanitic”
textures. Where plates converge (come together), one plate is forced beneath the other in
a subduction zone. This can occur between 2 continental plates, one continental and one
oceanic plate, or 2 oceanic plates. Where oceanic and continental plates meet, the
“mafic” oceanic crust mixes with the “felsic” continental crust to form rocks of
“intermediate” composition. A subducting plate melts as its temperature increases with
depth and the rising magma may cool slowly underground to form a pluton of igneous
rock (a “phaneritic” texture will result) or it may mix with the surrounding rock and
eventually extrude at a volcano to form extrusive rock. A full treatment of the formation
of igneous rocks is beyond the scope of this set of notes and is found on pages 35-37 of
the lab manual. Special attention should be payed to Figure 3.3, page 37 in the lab
manual and Figure 3.5, page 40 in the lab manual. A treatment of this subject will also
be given in class during the igneous rocks lab. A working knowledge of the terms on
pages 40-41 in the lab manual is essential to understanding what is going on with igneous
rocks.