Student Information Sheet
Igneous Rocks
Basalt is an extrusive igneous rock, so it is rather fine-grained. Basalt is usually dark gray or black,
with a fairly smooth surface. There are no visible layers.
Granite is an intrusive igneous rock. It is coarse-grained with large crystals. Granite can be white to
light gray, with black specks. Granite is often pinkish in hue. The crystals are not arranged in layers.
Obsidian is an extrusive igneous rock that looks like black glass. It is very smooth, often with semicircular
rings in places where it is fractured. The color may be grayish or greenish at times.
Pumice is an extrusive igneous rock, so it is rather fine-grained with no crystals. It is not a smooth
rock, however, as gas bubbles trapped within the rock during cooling give it many air pockets. Pumice
is very light and is usually gray in color (but can be white or pink).
Sedimentary Rocks
Sandstone is a very common sedimentary rock. It is composed of layers that are brown, pinkish, white,
or reddish in color. Sometimes sandstone is not well compacted, and often sand grains can be scraped
from the rock.
Shale is a fine-grained sedimentary rock made from clay. It can be many different colors, but is usually
black, gray, or brown.
Limestone is a sedimentary rock that is formed chemically from the mixing of calcite and sediment.
It can have many different grain sizes, and is usually white or light colored. It often contains fossils.
Because it contains calcium carbonate, limestone fizzes when vinegar is dropped on it. Acids like
vinegar will dissolve it over time.
Metamorphic Rocks
Gneiss is a coarse-grained metamorphic rock. It has layers that look like bands running through it.
Gneiss is formed when great pressure is applied to granite or schist.
Marble can vary in grain size. Crystals can usually be seen in marble. The color is usually light, often
white, and often has streaks of other colors within it. Marble is formed when limestone is subjected
to pressure. Like limestone, marble often fizzes when vinegar is applied.
Slate is a fine-grained, dark colored metamorphic rock, formed when shale is put under pressure. Slate
is layered and easily splits into flat pieces.
Ola Ka Honua: Volcanoes Alive 192 ©2001, 2007 UAF Geophysical Institute
