Chapter 12 Section 2
Types of Volcanoes
I. What controls the explosiveness of a volcanic eruption? Two main factors control whether or not a volcanic eruption is explosive or not. One factor is the amount of water vapor and other gases trapped in the magma. The other factor is the amount of silica present in the magma.
A. Quiet Volcanic Eruptions: During quiet eruptions gases escape easily from the Earth’s crust. Pressure build-up is small. These usually occur at divergent boundaries and at some hot spots.
1. Magma that is low in silica, which is a compound that consists of silicon and oxygen, produces quiet eruptions.
2. Magma that is low in silica is called basaltic magma.
3. Basaltic magma that pours out of volcanic vents and runs down the sides of the volcano is called pahoehoe lava.
4. Lava that flows at lower temperatures is stiff and slow flowing. This lava is called aa lava.
5. Basaltic magma flows from rift zones, which are long, deep cracks in Earth’s surface. Lava escapes easily forming lava fountains.
6. Lava that flows underwater forms pillow lava, which consists of rock structures that are shaped like tubes, balloons, and pillows.
7. Volcanic eruptions of this kind are frequent in Iceland and in Hawaii. Kilauea mainly produces quiet eruptions.
B. Explosive Volcanic Eruptions: These eruptions occur when gases and pressure build up. This is common at convergent plate boundaries, where subduction takes place. The oceanic plate slides under another plate carrying water with it. The trapped water vapor puts great pressure on Earth’s crust creating explosive eruptions.
1. Magma that is high in silica, called silica-rich magma, produces explosive eruptions.
2. As one plate slides under another, Earth’s crust, which is rich in silica, melts. Silica- rich, granitic magma is thick. When trapped with the other gases, pressure builds up, causing an explosive eruption.
a. Mount St. Helen and Soufriere Hills are examples of explosive, silica-rich volcanoes.
C. Some magma is andesitic in composition. This lava is more silica rich than basaltic, so it is still more explosive than basaltic magma.
1. Most of the volcanoes in the Ring of Fire are andesitic. The Andes Mountains in South America are andesitic.
2. One of the biggest eruptions in recorded history was an andesitic volcano named Krakatau.
II. The Three basic Types (forms) of Volcanoes: The type of volcano that forms will depend on the composition of the magma and the amount of pressure built up.
A. Shield Volcano: These are volcanoes that form from quiet eruptions. They consist of basaltic lava and form layers of flat, broad lava with gently sloping sides.
1. Basaltic lava that flows out of large cracks in Earth’s surface are called fissures. Most lava that flows onto Earth’s surface comes from these fissures.
2. The Hawaiian Islands and the Columbia Plateau in the Northwestern U.S. formed from basaltic lava.
3. New seafloor forms from underwater flood basalts at the mid-ocean ridges.
B. Cinder Cone Volcanoes: These volcanoes form from explosive eruptions that throw solid pieces of lava, called tephra, into the air. The tephra falls to the ground forming a steep-sided, loosely packed cinder cone volcano.
1. Tephra is classified by its size. The largest pieces are called blocks and bombs. Smaller pieces are classified as cinders and ash.
2. Paricutin is a famous cinder cone volcano in Mexico that formed in the middle of a cornfield.
C. Composite Volcanoes: These are sometimes called stratovolcanoes. These volcanoes form from alternating quiet and explosive eruptions. Explosive eruptions occur when magma is rich in silica and the amount of trapped gases is great. The tephra from the explosive eruption can then be followed by a quiet eruption where lava flows over the tephra. This process repeats over and over, creating alternating layers of tephra and lava, which form a composite volcano.
1. Composite volcanoes are mainly formed at divergent boundaries where an oceanic plate is sliding under a continental plate.
2. These volcanoes form some of Earth’s grandest mountains.
3. Examples of composite volcanoes would include:
a. Mount St. Helens in the U.S
b. Soufriere Hills in Montserrat
c. Krakatau near Indonesia
d. Mount Vesuvius in Italy
e. Mount Pinatubo in the Philippines