Volcanic Activity
Objectives- Describe what conditions produce a volcanic eruption; explain what causes the different
types of eruptions
A volcanic eruption happens when magma reaches the surface of the earth. Depending on the
amount of gas and the silica content of the magma, the lava and the type of eruption can vary. Silica,
which forms from the elements silicon and oxygen, makes the lava thicker and less runny. When lava is
thick, the volcanic eruption tends to be more explosive. The amount of silica also changes the color and
density of the lava, which then makes different kinds of igneous rocks. As the gas content of the lava
goes up, the eruption becomes more explosive.
The two types of volcanic eruptions are explosive and quiet. As you may have been able to tell
from the first paragraph, the lava that produces explosive eruptions is often high in silica and dissolved
gas. The trapped gas builds up pressure until it gets released with a lot of force. The thick lava breaks
apart into small pieces and quickly hardens into different sizes. The smallest pieces are called ash.
Larger pieces are called cinders and bombs, which are the size of a car. Often explosive eruptions
produce a pyroclastic flow. A pyroclastic flow is an extremely fast moving cloud of volcanic gas and ash.
Mt. St. Helens in Washington produced an explosive eruption the last time it erupted in 1980.
When magma has little silica and gas dissolved in it, the eruption that results is referred to as a
quiet eruption. This is typical of the volcanoes on the Hawaiian Islands and Iceland. Many times, the
lava comes out of long cracks or fissures. Sometimes the lava is very runny and flows out quickly. The
rocks that form from this type of lava often have a wrinkle or ropey look. This type of lava is called
pahoehoe. Other times, the lava flows slowly and hardens into jagged rocks. This kind of lava is called
aa.
Different types of eruptions produce different kind of volcanoes. Quiet eruptions with runny
lava produce shield volcanoes. Shield volcanoes have a broad wide base and gently sloping sides.
Volcanoes that erupt explosively produce cinder cone volcanoes. These are steep sided volcanoes made
up of layers of cinders from previous eruptions. Composite cones form as volcanoes alternate between
eruptions of lava flows with eruption of explosive eruptions with cinder and ash. These volcanoes can
also be steep sided. Mt. St. Helens is a shield cone. When lava flows from long fissures, lava plateaus
build up. Lava flows are high, flat ground built up as lava spreads out. The Columbia Plateau in parts of
Washington, Oregon and Idaho is an example of a lava plateau.
Volcanoes can be classified by how often they are erupting. Ones that that are showing signs of
erupting in the near future or that is erupting are called active. Signs of erupting include changes in
temperature, changes in shape, amount of gas being released or producing earthquakes. Dormant
volcanoes are currently showing signs of erupting, but are expected to in the future. The time spans
between eruptions may be very long. Extinct volcanoes are unlikely to erupt ever again.
Other signs of volcanic activity include hot springs and geysers. A hot spring is a pool of water
heated by nearby magma. When the water seeps down into the earth, it becomes heated and rises
back to the surface. Often hot springs contain minerals that are dissolved in it. In Yellowstone, many of
the hot springs contain colored mineral formations- or worse, smell really bad because of the sulfur.
Geysers are fountains of water and steam that erupt. Old Faithful, also in Yellowstone, is one of the
most geysers in the world.
Magma can produce several landforms. Magma can seep into cracks in rocks, cool and form
igneous rocks. If the crack runs horizontally, the landform is called a sill. If the crack is vertical, a dike
forms. Magma may solidify inside the main tube inside a volcano. If the rock around the main tube
erodes, the main tube can be left behind forming a neck. Devil’s Tower is an example of this. If a large
body of magma solidifies underground, the surrounding rock may erode. The resulting exposed block of
igneous rock is called a batholith. The Sierra Nevada mountains in California formed this way.
1. Identify each of the three types of volcanoes.
a.
b.
c.
2. T/F Magma forms in the lithosphere.
3. What factors determine the force of a volcanic eruption?
4. Circle the letter of each sentence that is true about silica.
a. It is formed from oxygen and nitrogen.
b. It makes magma thicker.
c. It is rarely found in the crust.
d. It produces light colored lava.
5. T/F A volcanic erupts quietly if its magma is thick and sticky.
6. Describe each of the four volcano shapes and describe how each forms.
a. shield volcano
b. cinder cone
c. composite volcano
d. lava plateau