VOLCANIC ROCKS
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.
Basalt.
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.
Dacite.
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.
Rhyolite.
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.
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