Recognizing Rocks: Igneous, Sedimentary, and Metamorphic Types - Geology - page 85

Dome: A volcanic dome is created when erupting materials cover the vent, creating
a dome-shaped feature that grows as gas and magma continue to fill it, until the
pressure within forces another eruption.
Cone: A volcanic cone is a mountain-like structure created over thousands of years
as the volcanic lava, gas, ash, and pyroclastics (rock fragments) spill out onto the
surface.
Distinguishing three types of volcanoes
You may be familiar with the image of a volcano as a tall mountain with smoke exiting
from the very top. However, volcanoes can take different shapes depending on the ma-
terials that create them. Here I describe three of the most common types of volcanoes.
Shield volcanoes
Shield volcanoes usually form from basalt lava that erupts through a vent in the ocean
floor (though they can form on continents as well). Shield volcanoes are the largest vol-
cano type created by erupting magma. For example, Mauna Loa, Hawaii, spans more
than 50 kilometers (31 miles) across.
Figure 7-2 illustrates the major features of a shield volcano. Over time the erupting
basalt builds up, creating islands that are low and spread out over a wide area forming a
shield shape. As the shield grows, lava flows from the original vent, as well as other
vents, creating flank eruptions (or fissures) ) along the slopes of the shield. Because the
magma of a shield volcano is created from mantle rocks, the resulting igneous rocks
(such as basalt) are primarily composed of iron-rich or mafic minerals.
Figure 7-2:
Features of a
shield volcano.
Stratovolcanoes
The type of volcano you probably imagine when you think about a volcano is a stratovol-
cano, dome volcano, or composite cone. Most of these type of volcanoes are found