Igneous rocks make up most of the Earth's crust by volume and have a crystalline appearance (interlocking crystal mineral grains). They tend to be massive - that is they don't have any planar or linear features in them (with the exception of some volcanic rocks). Because of the massive nature and the interlocking crystals, igneous rocks are very hard and people who study them tend to be called hard-rock geologists.
In this lab, we will be concentrating on those igneous rocks that cooled very quickly above the Earth's surface because of volcanic eruptions. Geologist believe that volcanism is an important process not only in the formation of the Earth's land surface but also on other planets and moons as well.
There are many types of volcanoes to become familiar with:
Intrusive igenous rocks are formed when magma, while trying to reach the surface, intrudes into the overlying rock and cools. They are a source of heat which can cause the surround country rock to become metamorphosed (see Contact Metamorphism below). Igneous intrusions are classified by their size and orientation within the country rock. Here are some examples:
An important aspect of science involves dividing things into groups based on their dominant characteristics. Geologists who study igneous rocks [those derived by solidifying (crystallizing) from a melt (magma) use only two main characteristics to classify them.
For an important reason, some igneous rocks contain abundant iron, and magnesium bearing minerals, which are dark in color. We call these MAFIC rocks (for magnesium and iron (Fe). Other igneous rocks tend to contain mostly feldspar and quartz, which are light colored minerals. We call these FELSIC rocks (for feldpsar and silica). Rocks with both mineral types are called INTERMEDIATE. As you can see if you look at the display cabinet 11 just outside the lab door, mafic rocks tend to be very dark in appearance, and felsic rocks tend to be very light in appearance. Because the mafic minerals are also very dense, mafic rocks tend to be more dense than felsic rocks.
Looking at this classification diagram, we see that feldspar is one of the most abundant minerals found in common igneous rocks. Note that one of the very first questions you should ask when identifying an igneous rock is which of the two feldspars is more common in this rock? Determining this has an important bearing on the name that you give the rock (i.e., granite versus diorite).
Grain size is controlled largely by how fast or slowly the melt cools when it solidifies. Fine-grained rocks are aphanitic, whereas coarse-grained rocks are phaneritic in texture. Display cabinet 11 also shows a variety of other textures that you should acquaint yourself with (such as vesicular, porphyritic, glassy, amygdaloidal, fragmental, and pegmatitic).
There is also a great display cabinet upstairs outside room 331 showing many spectacular samples of igneous textures and structures that form in igneous rocks (such as columnar jointing, pillows, etc.).
It is often very useful to look at thin sections (very thin slices of rock mounted on slides) under a microscope to identify minerals within a rock, especially when mineral crystals are very small.
For more information on Volcanoes see these important web sites:
