Do you have a granite countertop? Have you ever used a pumice stone? Do you ever
wonder where these materials come from? In this lab you will learn about granite, pumice
and other igneous rocks.
1) Learn some basics of volcanoes
2) Learn how magmas crystallize
3) Learn to classify igneous textures and compositions
4) Learn to identify igneous rocks in hand sample.
Igneous rocks are the youngest rocks on earth, but they can also be some of the oldest rocks.
Igneous rocks form when melted parts of the mantle and earth’s crust solidify into new rocks.
Melting of earth’s crust can be caused by a variety of events ranging from sporadic asteroid
impacts to constant plate tectonic processes. The place most commonly associated with melted
crust, called magma or lava, is at volcanoes.
Plate Tectonics, Volcanoes, and Plutonism
Plate tectonics is the driving force behind all geological processes, one of the most exciting are
the formation of molten rock. The most familiar type of molten rock is
, which is extruded
and cooled onto earth’s surface from volcanoes. Molten rock that cools below earth’s surface is
. Rocks that form from lava and rocks that form from magma look very different
texturally (but more on this later).
Depending on the type of plate tectonic environment that a magma or lava is from effects its
composition. There are 4 types of compositions for igneous rocks:
- high amount of quartz and feldspar, light colored igneous rock (usually pink or
- “middle” composition between felsic and mafic (gray color)
- high amount of pyroxene, amphibole, and Ca-rich feldspar (dark-near black
- high amount of olivine and pyroxene (very dark, commonly green due to
These compositions have a direct relationship to the violence of volcanic eruptions. Felsic and
intermediate lavas are more explosive because they have a higher viscosity (does not flow easily,
thick and sticky, “peanut butter”). Mafic and ultramafic are much less explosive due to lower
lava viscosity (flows easily, thinner, “syrup”).
(such as water and CO2) can also have
an effect on volcanic eruptions because they turn into gasses, which increase the explosiveness