Geology 12 – Dalesandro
Assignment #6 – Igneous Rocks
Instructions: Use the attached information on igneous rocks (and the internet, if
necessary) to answer the questions below. Don’t forget to include
your name and the name of your partner if you have one.
Due: Thurs, Oct. 22nd.
Value: 15 points.
Questions:
1) What is the difference between igneous rocks that are fine-grained as
opposed to those that are coarse-grained?
2) Define intrusive and extrusive igneous rocks.
3) Create a chart that organizes the igneous rocks below according to grain size
and whether the rock formed above or below ground.
4) What minerals were in the magma that andesite formed from? Describe this
rock’s appearance.
5) What minerals is basalt made out of? Describe this rock’s appearance.
6) Which minerals make up the igneous rock diorite? Describe this rock’s
appearance.
7) What is gabbro made of (which minerals)? Describe this rock’s appearance.
8) What is the difference between granite and pegmatite?
9) How does obsidian form? Describe what it looks like.
10) Describe peridotite. What minerals make up this rock?
11) What are vessicles? List three igneous rocks that are vesicular (have
vessicles).
12) “Geology Joe” claims welded tuff should not be classified as an igneous
rock. Why would he say that, and do you agree or disagree with Geo Joe?
13) Which type of igneous rock is your favorite? Explain why.
Igneous Rock Types
Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such
as hornblende, pyroxene and biotite. Because it is fine-grained, it cooled from magma rather rapidy.
Extrusive means it cooled above ground.
Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene.
It cooled from magma rapidly above ground. Basalt is heavier than other igneous rocks.
Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende
and sometimes quartz. Since it is coarse-grained, it cooled slowly from magma. Intrusive means it formed
deep underground.
Gabbro
Gabbro is a coarse-grained, dark colored, intrusive igneous rock that contains feldspar, augite and
sometimes olivine. It cooled slowly deep underground.
Granite
Granite is a coarse-grained, light colored, intrusive igneous rock that contains mainly quartz and feldspar
minerals. It is one of the most common igneous rocks and cooled from magma slowly deep underground.
Obsidian
Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It
cools so rapidly (above ground) that crystals do not have time to form.
Pegmatitie
Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock. It forms near the edges of a
magma chamber during the final phases of magma chamber crystallization. It often contains rare minerals
that are not found in other parts of the magma chamber. Can you spot pyrite (fool’s gold) crystals in the
specimen above?
Peridotite
Peridotite is a reatively coarse-grained intrusive igneous rock that is composed almost entirely of olivine. It
may contain small amounts of amphibole, feldspar, quartz or pyroxene. It cooled fairly slowly, at a medium
disance below the surface.
Pumice
Pumice is a light-colored vesicular igneous rock. It forms through very rapid cooling and solidification of
magma. The “holes” (called vesicles) are the result of large gas bubbles trapped in the magma at the time of
solidification.
Rhyolite
Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar
minerals. It cooled very quickly at the surface, and the magma contained many small gas bubbles, now
seen as vessicles.
Scoria
Scoria is a dark-colored, vesicular, extrusive igneous rock. The vesicles are a result of trapped gas within
the magma at the time of solidification. It often forms from the frothy crust at the top of a lava flow. It can
also form when molten material is ejected from a volcano’s vent and solidifies while still airborne.
Tuff (Welded)
Welded Tuff is a rock that is composed of materials that were ejected from a volcano, fell to Earth, and then
lithified into a rock. It is usually composed mainly of volcanic ash and sometimes contains larger size
particles such as sand and gravel.