effusive eruptions

effusive eruptions

Mafic magma creates gentler effusive eruptions. Although the pressure builds enough for the magma to erupt, it does not erupt with the same explosive force as felsic magma. Magma pushes toward the surface through fissures. Eventually, the magma reaches the surface and erupts through a vent (Figure 1.1). Effusive eruptions are common in Hawaii, where lavas are mafic. In effusive eruptions, lava flows readily, producing rivers of molten rock. A Quicktime movie with thermal camera of a lava stream within the vent of a Hawaiian volcano is seen here:

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types of lava

Low-viscosity lava flows down mountainsides. Differences in composition and where the lavas erupt result in three types of lava flow coming from effusive eruptions. Aa lava forms a thick and brittle crust that is torn into rough and jagged pieces. Aa lava can spread over large areas as the lava continues to flow underneath the crusts surface. Pahoehoe lava forms lava tubes where fluid lava flows through the outer cooled rock crust. Pahoehoe lava is less viscous than aa lava, so its surface looks is smooth and ropy. Mafic lava that erupts underwater creates pillow lava. The lava cools very quickly, forming roughly spherical rocks. Pillow lava is common at mid-ocean ridges (Figure (a) Aa lava spread over large areas. (b) Pahoehoe lava tubes where at the Thurston Lava Tube in Hawaii Volcanoes National Park. (c) Pahoehoe lava is less viscous than aa lava so its surface looks is smooth and ropy. (d) Pillow lava.

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effusive eruptions damage

People can usually be evacuated before an effusive eruption, so they are much less deadly. Although effusive eruptions rarely kill anyone, they can be destructive. Even when people know that a lava flow is approaching, there is not much anyone can do to stop it from destroying a building or road (Figure 1.3).

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instructional diagrams

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questions

which of these are characteristics of an effusive eruption?

a) low-viscosity lava

b) mafic magma

c) lava gets slowly pushed out

-->  d) all of the above

because effusive eruptions move slowly, they are not destructive.

a) true

-->  b) false

for an effusive eruption to take place, the pressure must build as much as it does for an explosive eruption.

a) true

-->  b) false

which of these is not made by mafic magma?

-->  a) tephra

b) aa

c) pahoehoe

d) pillow lava

mafic magma create shield volcanoes.

-->  a) true

b) false

people can be evacuated from a region with an effusive eruption so there are few or no fatalities.

-->  a) true

b) false

this type of lava forms a thick and brittle crust that forms into rough, jagged pieces.

-->  a) aa

b) pahoehoe

c) pillow lava

d) none of these

high-viscosity lava flows down mountain sides.

a) true

-->  b) false

mafic lava that flows into the sea or a lake will form

a) aa

b) pahoehoe

-->  c) pillow lava

d) none of these

diagram questions

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