Chapter 11 Main Ideas
11.1 What Is A Volcano?
Volcanoes are places where molten rock escapes to Earth’s surface. Some volcanoes are cone-shaped or hill-shaped mountains, and some eruptions happen along fissures. Eruptions are fed by a magma chamber beneath the volcano. Sometimes a volcano collapses into empty space in the magma chamber beneath, forming a caldera.
What’s the difference between a caldera and a crater?
A crater an opening at the top of a volcano where magma escapes. Craters are tens to hundreds of metres in scale. A caldera is a structure that forms when a magma chamber beneath the volcano is emptied, and the unsupported part of the volcano falls in.
11.2 Materials Produced by Volcanic Eruptions
Volcanoes produce gas, lava flows, and debris called tephra. The characteristics of a lava flows depend on whether the lava is thin and runny (mafic with low gas content) or thick and sticky (felsic with high gas content). Tephra is classified according to size. Ash is less than 2 mm in diameter, lapilli is between 2 mm and 64 mm, and blocks and bombs are larger than 64 mm.
11.3 Types of Volcanoes
Cinder cones are relatively small straight-sided volcanoes that are composed mostly of mafic rock fragments. Composite volcanoes consist of alternating layers of lava flows and tephra. They tend to be intermediate to felsic in composition, and get steeper toward the top. Shield volcanoes are broad, low, hill-like volcanoes that form from layers of low-viscosity mafic lava.
Shield volcanoes are built of thin, runny lava flows. Composite volcanoes are built of lava flows interspersed with volcanic debris (tephra), ranging from ash to blocks and bombs. Cinder cones are almost entirely tephra.
11.4 Types of Volcanic Eruptions
Volcanic eruptions can be classified according to how explosive they are, and how high into the atmosphere they blast material. Hawai’ian eruptions are relatively gentle effusive eruptions of low-viscosity mafic lava, and form shield volcanoes. Strombolian eruptions are more vigorous eruptions of mafic tephra. They blast material hundreds of metres into the air. The tephra falls out of the atmosphere to form a cinder cone. Vulcanian eruptions are explosive eruptions of intermediate to felsic composition lava, producing pyroclastic flows and eruptive columns from 5 to 10 km high. Plinian eruptions are highly explosive eruptions of felsic lava, and can produce eruption columns up to 45 km high. Both Vulcanian and Plinian eruptions are associated with composite volcanoes. Hydrovolcanic eruptions are the explosive result of magma or lava interacting with water, and rapidly changing the water to steam.
What determines whether a volcano will erupt effusively (as in a Hawai’ian eruption) or explosively (as in a Vulcanian or Plinian eruption)? Does the same rule apply to hydrovolcanic eruptions?
The composition of magma will determine how explosive a volcano will be. Effusive eruptions happen when magma is mafic in composition, because basaltic lava flows easily and has less gas to build pressure. Explosive Vulcanian and Plinian eruptions are the result of intermediate to felsic magmas with a high gas content. Pressure from gas combined with the stickiness of the lava causes explosive eruptions. Hydrovolcanic eruptions are different because the explosiveness comes from interacting with water. Lava of any composition can cause an explosion when encountering water because it can turn water to steam almost instantly.
11.5 Plate Tectonics and Volcanism
Volcanism is closely related to plate tectonics. Most volcanoes are associated with convergent plate boundaries (at subduction zones), where magma is formed when water from a subducting plate acts as a flux to lower the melting temperature of the adjacent mantle rock. Volcanic activity also occurs at divergent boundaries and areas of continental rifting. At divergent boundaries magma forms because of decompression melting. Decompression melting also takes place within a mantle plume. In ocean-continent collision zones, and in continental rift zones, magma compositions—and thus the nature of volcanism—can be impacted by conduction melting of surrounding non-mantle rocks.
Plate tectonic setting can influence the composition of lava erupting from a volcano, and thus the type of volcano and type of volcanic eruption. Explain how tectonic setting would influence the type of volcano and eruption for the following:
- A composite volcano undergoes a Plinian eruption along a subduction zone where oceanic crust is colliding with continental crust.
- A hotspot shield volcano in the middle of a plate of oceanic lithosphere undergoes a Hawai’ian eruption.
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11.6 Volcanic Hazards
Most direct volcanic hazards are related to volcanoes that erupt explosively, especially composite volcanoes. Pyroclastic flows, some as hot as 1000 ˚C, can move at hundreds of km/h and will kill anything in the way. Lahars—volcano-related mudflows—can be large enough to destroy entire towns. Lava flows are also destructive, but tend to move slowly enough to permit people to get to safety. Indirect hazards claim far more lives than direct hazards, and include famine related to volcanically-induced climate cooling.
11.7 Monitoring Volcanoes and Predicting Eruptions
Clues that a volcanic eruption might soon occur include earthquakes, a change in the type and amount of gases released, and changes in the shape of the volcano as magma moves within it. Volcanoes are monitored using seismometers to detect earthquakes, volcanic gases are sampled and analyzed, and instruments are used to detect deformation of the volcano. These tools make it possible to assess the hazard posed by a given volcano, and the risk of eruption.
11.8 Volcanoes in British Columbia
British Columbia and the Yukon Territory include examples of volcanoes that form as a result of fluid-induced melting along a subduction zone (the Wrangell and Garibaldi volcanic belts), as a result of decompression where the crust is thinning and stretching (Stikine Volcanic Belt and Wells Gray-Clearwater Volcanic Field), and because of mantle plume activity (Anahim Volcanic Belt).
Key Term Check
What key term from Chapter 11 is each card describing? Turn the card to check your answer.