explosive eruptions

causes of the explosion

Explosive eruptions are caused by gas-rich, felsic magmas that churn within the magma chamber. When the pressure becomes too great the magma breaks through the rock above the chamber and explodes, just like when a cork is released from a bottle of champagne. Magma, rock, and ash burst upward in an enormous explosion (Figure 1.1).


explosive eruptions

A large explosive eruption creates even more devastation than the force of the atom bomb dropped on Nagasaki at the end of World War II, in which more than 40,000 people died. A large explosive volcanic eruption is 10,000 times as powerful. Explosive eruptions are found at the convergent plate boundaries that line parts of western North America, resulting in the Cascades in the Pacific Northwest and the Aleutians in Alaska.

cascades volcanoes

Prior to the Mount St. Helens eruption in 1980, the Lassen Peak eruption on May 22, 1915, was the most recent Cascades eruption. A column of ash and gas shot 30,000 feet into the air. This triggered a high-speed pyroclastic flow, which melted snow and created a volcanic mudflow known as a lahar. Lassen Peak currently has geothermal activity and could erupt explosively again. Mt. Shasta, the other active volcano in California, erupts every 600 to 800 years. An eruption would most likely create a large pyroclastic flow, and probably a lahar. Of course, Mt. Shasta could explode and collapse like Mt. Mazama in Oregon (Figure 1.4). Crater Lake fills the caldera of the col- lapsed Mt. Mazama, which erupted with 42 times more power than Mount St. He- lens in 1980. The bathymetry of the lake shows volcanic features such as cinder cones.


pyroclastic material

The erupted rock fragments are called tephra. Ash and gas also explode from the volcano. Scorching hot tephra, ash, and gas may speed down the volcanos slopes at 700 km/h (450 mph) as a pyroclastic flow. Pyroclastic means fire rock (Figure 1.2). Left: An explosive eruption from the Mayon Volcano in the Philippines in 1984. Ash flies upward into the sky and pyroclastic flows pour down the mountainside. Right: The end of a pyroclastic flow at Mount St. Helens. Pyroclastic flows knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,000 C (1,800 F). Blowdown of trees near Mount St. Helens shows the direction of the blast and pyro- clastic flow.



volcanic gases

Volcanic gases can form poisonous and invisible clouds in the atmosphere. These gases may contribute to environ- mental problems such as acid rain and ozone destruction. Particles of dust and ash may stay in the atmosphere for years, disrupting weather patterns and blocking sunlight (Figure 1.5). The ash plume from Eyjafjallajkull vol- cano in Iceland disrupted air travel across Europe for six days in April 2010. Click image to the left or use the URL below. URL:


instructional diagrams

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what causes explosive eruptions?

a) dissolved gases that get trapped

b) high silica content in magma

c) pressure builds up

-->  d) all of the above

like a lava flow in hawaii, people can outrun a pyroclastic flow.

a) true

-->  b) false

which of these is a way that volcanoes can affect the environment?

a) acid rain

b) ozone destruction

c) reducing photosynthesis

-->  d) all of the above

what type of volcano is made by only explosive eruptions?

a) cinder cone

b) shield

-->  c) composite

d) flat

most explosive eruptions take place at

-->  a) subduction zones.

b) hotspots.

c) divergent plate boundaries.

d) transform plate boundaries.

what type of plate boundary made the cascades in the pacific northwest?

a) divergent

-->  b) ocean-continent convergent

c) ocean-ocean convergent

d) transform

which of these volcanoes do not erupt or have not erupted explosively?

a) mayon volcano in the philippines

b) mt. vesuvius in italy

c) mt. mazama in oregon

-->  d) mt. kilauea in hawaii

people at pompeii in 79 ad were killed by

-->  a) extremely hot flowing rocks.

b) being run over by mafic lava flows.

c) fires that came after the eruption

d) all of these.

explosive eruptions with scorching hot pyroclastic flows and huge ash clouds are called plinian because of the description of pliny the younger at pompeii.

-->  a) true

b) false

diagram questions

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