(a) Volcanoes. When molten rock and hot gases, which have originated deep within the earth, rise to
the point where they reach the earth's surface, they form an opening, or vent, through which they are extruded as
lava. As the eruption continues, the molten material accumulates around the conduit, often creating a conical hill
or mountain before solidifying. The combination of a vent and its associated cone is known as a volcano.
Volcanoes vary greatly in size, ranging from very small hills to some of the loftiest mountains on earth.
(b) Lava Flows. If molten material extruded at the earth's surface is too fluid to accumulate around
the opening, a volcanic cone may not develop. Instead, the lava may outpour laterally, creating extensive sheets
of extrusive rock called lava flows. A distinctive characteristic of lava flows is columnar jointing, a type of
jointing in which five- or six-sided vertical columns a few inches in diameter are formed as the molten rock
shrinks during cooling. Successive, intermittent outpouring of lava flows in a specific area may result in layered
deposits thousands of feet in thickness.
(2) Extrusive Rock Types. Extrusive igneous rocks may reach the earth's surface as either a lava flow
or a pyroclatic ejection of fragments. In the case of a lava flow, the resulting extrusive igneous rocks develop one
of three types of aphanitic texture--stony, glassy, or frothy. Pyroclastic materials exhibit their own unique
(a) Stony Texture. Extrusive igneous rocks with a stony texture are dull in appearance and contain
very few vesicles, indicating a lack of trapped air bubbles. There are two major types of extrusive igneous rocks
with a stony texture-felsite and basalt.
Felsite is a light-colored, fine-grained, extrusive igneous rock with a stony texture. The
chemical composition of rhyolite, a specific type of felsite, is identical to that of granite; the only difference
between the two is the size of individual component grains. Felsite displays relatively light photo tones on aerial
photographs, because of its light color.
Basalt is a dark-colored, fine-gained, extrusive igneous rock with a stony texture. It is the
aphanitic chemical equivalent of gabbro-diorite. Basalt is the most common and widespread type of extrusive
igneous rock. Many volcanic islands, such as the Hawaiian Islands and Iceland, are composed primarily of basalt.
Basalt displays dull, dark gray to black photo tones on aerial photographs because of its naturally dark color.
Scattered light spots are common, indicating variations in soil cover. One distinctive feature of basaltic lava
flows is a mottled pattern referred to as "snake skin" or "lizard skin." In addition, a "dried blood" pattern may
exist on air photos where isolated patches of fluid basalt have broken through overlying basaltic crust.
(b) Glassy Texture. The only extrusive igneous rock with a glassy texture is obsidian. Obsidian is a
dark, silvery-gray to black rock that underwent extremely rapid cooling. Extrusions of obsidian are generally not
extensive enough to form readily identifiable photo characteristics.
(c) Frothy Texture. Frothy rocks contain numerous vesicles formed as gas bubbles were escaping
from molten igneous rock material. There are two types of extrusive igneous rocks that exhibit a frothy texture-
pumice and scoria.