Chapter 5 - Volcanoes

Question 1

Where do the majority of magmas originate?

Answer 1

One of three plate-tectonic settings: (1) Divergent plate boundaries (2) over subduction zones and (3) at hot spots.

Question 2

What is the composition of magma determined by?

Answer 2

By the material(s) from which the melt is derived, and the extent of melting of that source material.

Question 3

What is a mafic magma/rock?

Answer 3

One that is relatively rich in iron and magnesium.

Question 4

What are felsic rocks/magma?

Answer 4

Relatively silica-rich magmas are usually rich in aluminum as well, so they make rocks rich in feldspar and quartz.

Question 5

What are the two ways melting can occur?

Answer 5

Reduction in pressure and the addition of fluids (such as water).

Question 6

What are the two ways a magma’s composition change?

Answer 6

Through fractional crystallization - cools for a short period in magma chamber and some minerals crystallize- and interaction with crust material.

Question 7

What does the partial melting of the mantle account for?

Answer 7

Basalts (mafic volcanic rock) that form new sea floor at spreading ridges.

Question 8

How does mafic magma change comp. while traveling through a continental rift zone?

Answer 8

It can be basaltic, silica-rich rhyolite or andesite (intermediate comp.)

Question 9

What is known as a fissure eruption?

Answer 9

The eruption of magma out of a crack in the lithosphere, rather than from a single pipe or vent.

Question 10

What is known as the ring of fire?

Answer 10

The collection of volcanoes rimming the Pacific Ocean, which is actually a ring of subduction zones.

Question 11

What is a shield volcano?

Answer 11

Very flat and low in relation to its diameter, and large in areal extent, usually mafic basaltic volcanoes

Question 12

What is a cinder cone?

Answer 12

When cinders fall close to the vent from which they are thrown can pile up into a very symmetrical cone-shaped heap.

Question 13

What are pyroclastics?

Answer 13

Energetically erupted bits of volcanic material.

Question 14

What are stratovolcanoes?

Answer 14

They are volcanoes built in a layer-cake fashion and are made of more than one kind of material.

Question 15

What is a lava dome?

Answer 15

When slow-flowing rhyolithic and andesitic lavas ooze out at the surface, piling up close to the volcanic vent, which results in a compact and steep-sided structure.

Question 16

What is a pyroclastic flow (Nuée Ardentes)?

Answer 16

A special kind of pyroclastic outburst, it is a deadly, denser-than-air mixture of hot gases and fine ash. Temperatures can be over 1000 C

Question 17

What is a Lahar?

Answer 17

It is when volcanic ash and water combine into a dense mud which is hot

Question 18

What is a phreatic eruption?

Answer 18

When water enters in contact with hot lava bellow and turns into steam. This causes the volcano to blow up like an overheated steam boiler.

Question 19

When is a volcano considered active?

Answer 19

When it has erupted within recent history.

Question 20

When do we classify a volcano as dormant?

Answer 20

When it has not erupted in recent history but it does not look eroded.

Question 21

When do we call a volcano extinct?

Answer 21

When it has not recently erupted but appears eroded.

Question 22

What is the Volcanic Explosivity Index?

Answer 22

It is a way characterize the relevant sizes of explosive eruptions. It take into account the volume of pyroclastics, how high into the atmosphere they rose and the length of the eruption.

Question 23

What is a caldera?

Answer 23

It is an enlarged volcanic crater, which may be formed either by an explosion enlarging an existing crater or by collapse of a volcano after a magma chamber within has emptied.