18.1 Volcanoes
Zones of Volcanism – volcanoes are fueled by magma. Volcanism describes all the processes associated
with the discharge of magma, hot fluids, and gases.
As we are discussing this section, there are
about 20 volcanoes erupting. In a given
year, volcanoes will erupt in about 60
different places on Earth. The distribution
of volcanoes is not random on Earth. Most
form at plate boundaries, with the majority
of them at convergent and divergent
boundaries. Here magma rises towards
Earth’s surface. Only about 5% of magma
erupts far from plate boundaries.
Two major volcano belts are formed from
convergent boundaries. The larger is the
Circum-Pacific Belt or the Ring of Fire. It
outlines the Pacific Ocean. It stretches along
the western coast of North and South
America, across the Aleutian Islands, and
down the eastern coast of Asia. The smaller belt is called the Mediterranean belt. It includes Mount Etna
and Mount Vesuvius in Italy. It’s outlines correlates the boundaries between the Eurasia, African and
Arabian plates.
Convergent volcanism – As oceanic plates descends
below another plate into the mantle, magma forms. The
magma moves upward because it is less dense than the
surrounding solid material. The magma mixes with
rock, minerals, and sediments as it rises. Most
volcanoes on land are a result of oceanic-continental
subduction. These volcanoes are characterized by
explosive eruptions.
Divergent volcanoes – These are where the plates move apart. In the ocean at ocean ridges, the magma
rises and fills the gap. Here, the lava takes the form of giant pillows and is called pillow lava. Unlike at
convergent boundaries, the volcanism here is non-explosive, with giant flows of large amounts of lava.
About 2/3 of Earth’s volcanism occurs underwater at divergent boundaries at ocean ridges.
Hot Spots – Some
volcanoes form far from
plate boundaries over hot
spots. Hot spots are
unusually hot regions of
Earth’s mantle where
high-temperature plumes
of magma rises to the
surface. The Hawaiian
Islands are over a plume
of magma. As the rising
magma melted through
the crust, it formed
volcanoes. The hot spot
formed by the magma
plume remained
stationary while the Pacific Plate moved slowly northwest. Over time, the hot spot left a trail of volcanic
islands on the Floor of the Pacific Ocean. The oldest volcanoes in the Hawaiian chain are no longer active
because they are not longer over the hot spot. Even older volcanoes aren’t even above sea level. Kilauea
is the current active volcano on the big island of Hawaii because it is currently over the hot spot. Another
new volcano, Loihi, is forming on the seafloor southeast of the big island and might eventually rise above
the oceans surface.
If a hot spot occurs beneath continental crust, they can lead to the formation of flood basalts. Flood
basalts form when lava flows out of long cracks in Earth’s crust. The cracks are called fissures. Over
hundreds of thousands of years, these fissure eruptions can lead to the formation of plateaus. The
volume of basalt erupting from fissures can be tremendous. The Columbia River basalt in the state of
Washington could fill Lake Superior 15 times. There is an even bigger basalt in India that could cover the
island of Manhattan with a layer of lava 6,213 miles thick or all of New York with a layer 2.5 miles thick.
The huge flow in India happened about 65 mya and this is what scientists believe lead to the extinction of
the dinosaurs.
Anatomy of a Volcano – Lava reaches the Earth’s surface through a tube-like structure called a conduit,
and emerges through an opening called a vent. As the lava cools and solidifies, it will begin to layer and
form a volcano. At the top of the volcano, around the vent, is a bowl-shaped depression called a crater.
The crater is connected to the magma chamber by the conduit. Volcanic craters are usually less than 1
km (.62 mi) in diameter. A caldera is a large depression that is formed after the magma chamber empties
completely of magma. When this happens, the summit or side of the volcano will collapse into the empty
chamber, leaving an expansive, circular depression. Sometimes water will fill this space forming a lake.
Crater Lake in Oregon is an example.
Types of volcanoes – Volcanoes are categorized by two things: the type of material that forms the
volcano and the type of eruption that occur. There are three main types of volcanoes.
Shield volcanoes
 broad , gently sloping sides
 nearly circular base
 form when layers of lava accumulate during quiet nonexplosive eruptions
 largest type of volcano
Cinder cone volcano
 formed when small pieces of material are ejected, fall back to
Earth and pile up around the vent
 steep sides
 generally small, most less than 500m (1,640ft) high
 form on or very near larger volcanoes
 explosive eruptions
Composite volcano
 formed of layers of hardened chunks of lava from violent
eruptions alternating with layers of lava that oozed
downslope before solidifying
 generally cone-shaped with concave slopes
 larger than cinder cones
 very explosive eruptions,
 very dangerous to humans and the environment
 tall, majestic mountains