Rocks are aggregates of minerals. Geologists divide rocks into three groups: igneous, metamorphic, and sedimentary. Igneous rocks crystallize from magma. Metamorphic rocks form by the deformation and/or recrystallization of pre-existing rock by changes in temperature, pressure, and/or chemistry. Sedimentary rocks form by weathering and erosion of preexisting rock to make sediment, which is lithified into rock.
Most rocks in Hawaii are igneous. If magma solidifies within the crust, the rock is called an intrusive igneous rock. Intrusive igneous rocks have been exposed by erosion along rift zones of some of the older volcanoes in Hawaii. Intrusive rocks help to built the volcanoes from the inside. This photo shows the Uwekahuna laccolith that is exposed in the wall of Kilauea caldera. A laccolith is a pocket of magma that became trapped in the rocks layers before it reached the surface. Laccoliths have flat bottoms and convex tops. Photo by Steve Mattox, July 1990. If the magma erupts at the surface, it is called an extrusive igneous, or volcanic rock. Extrusive igneous rocks are more abundant and comprise most of the islands.Volcanic rocks are classified by the size, abundance, and type of crystals. If no crystals are visible, geologists call the rock aphanitic. If crystals are abundant, the rock is called porphyritic. The most common mineral in rocks from Kilauea and Mauna Loa Volcanoes is olivine. Pyroxene and plagioclase are also common, but their crystals are much smaller and may be difficult to see without a microscope. These three minerals help to identify a rock-type called tholeiitic basalt. Olivine is also common in rocks from Hualalai, Mauna Kea, and Kohala. However, unlike the tholeiitic basalts from Kilauea and Mauna Loa, the pyroxene and plagioclase crystals are conspicuous. These rocks are called alkali basalt. A classification scheme for Hawaiian rocks is presented in figure 13.6.
Classification scheme for igneous rocks. Intrusive igneous rocks are coarse-grained because they cooled slowly inside the Earth. Extrusive (volcanic) igneous rocks are fine-grained because they cooled quickly at the Earth’s surface.
Gabbro and basalt contain abundant ferromagnesian minerals and lesser amounts of plagioclase feldspar. Note the basalt has gas bubbles (called vesicles by geologists).
Volcanic rocks from continental volcanoes can also be identified by their mineralogy. For example, rocks from stratovolcanoes in the western United States, like Mount Shasta (shown above), have pyroxene, Ca-rich plagioclase, and hornblende. This mineral assemblage indicates that the rock is an andesite.
Diorite and andesite contain abundant plagioclase feldspar and lesser amounts of ferromagnesian minerals. Dacite is similar to andesite except it has less plagioclase and more quartz.
Granite and rhyolite contain quartz, orthoclase and plagioclase feldspar, and minor amounts of ferromagnesian minerals.
Obsidian is a black or dark-colored volcanic glass, usually of rhyolite composition, characterized by conchoidal fracture. It is sometimes banded.
Snowflake obsidian contain white or gray spherulites. The spherulites are radiating fibrous crystals of alkali feldspar or silica.