VO L4: Where are Violent Volcanoes Found?

Question 1

At _____ margins, two plates spread apart due to upwelling of the upper mantle and the injection of magma.

Answer 1

divergent

Question 2

Where are most divergent margins located? What are they called?

Answer 2

Located in ocean basins, and are called mid-ocean ridges (MOR) because they form undersea mountain ranges.

Question 3

On-land equivalent of mid-ocean ridges (MOR)?

Answer 3

continental rift zone [e.g. East African Rift]

Question 4

Where two plates collide, one plate will subduct into the mantle beneath the other plate to form a _____ margin.

Answer 4

convergent

Question 5

Where do volcanic arcs form?

Answer 5

They form on convergent margins, in the overlying plates.

Question 6

Where two ____ plates collide, however, neither will subduct, as both are too buoyant to descend into the dense mantle. Rather, they suture together in a massive collisional zone to form a _______.

Answer 6

continental; large mountain chain

Question 7

What made the Himalayan Mountains?

Answer 7

India’s collision with Asia, a continent-continent collision.

Question 8

In what kind of collision zone is magmatism rare?

Answer 8

continent-continent collision zones

Question 9

What is a transform fault?

Answer 9

plate boundary where two plates slide laterally past one another

Question 10

What type of plate boundary rare has magmatism?

Answer 10

Transform fault

Question 11

Which part of Earth is ultramafic in composition? Why?

Answer 11

upper mantle. The rocks (called peridotites) have <45 weight percent SiO_2 (silicon dioxide), and they are high in MgO (magnesium oxide) and FeO (iron oxide)

Question 12

Which part of Earth is mafic in composition? Why?

Answer 12

Ocean crust. 45-52% SiO_2, along with less MgO and FeO and more CaO and Al_2O_3 than the upper mantle.

Question 13

What kinds of rocks are found in upper mantle? What characteristics do they have?

Answer 13

peridotites; have <45 weight percent SiO_2 (silicon dioxide), and they are high in MgO (magnesium oxide) and FeO (iron oxide). Makes upper mantle ultramafic

Question 14

What kinds of rocks are found in the oceanic crust? What characteristics do they have?

Answer 14

Igneous rocks (basalts, gabbros) with 45-52% SiO_2, along with less MgO and FeO and more CaO and Al_2O_3 than the upper mantle. Makes oceanic crust mafic.

Question 15

What kinds of rocks are found in continental crust?

Answer 15

Wide range of igneous rocks with SiO_2 contents ranging from mafic through intermediate to felsic, from 45% to >68%, with the most felsic rocks containing about 75% Si_O2.
Overall, continental crust has an average SiO2 content of 60%, which is much more SiO2-rich than oceanic crust, and which accounts for the higher buoyancy (lower density) of the continents as opposed to the ocean basins.

Question 16

Why are continental crusts lower in density compared to ocean basins?

Answer 16

continental crust has an average SiO_2 content of 60%, which is much more rich in SiO_2 than oceanic crust.

Question 17

Most felsic rocks in continental crust contains how much silicon oxide?

Answer 17

75%

Question 18

How many active sub-aerial volcanoes are associated convergent plate margins?

Answer 18

about 80%

Question 19

What does sub-aerial mean?

Answer 19

ground level

Question 20

80% of active sub-aerial volcanoes are associated with _______.

Answer 20

convergent plate margins

Question 21

Why are many active volcanoes found along the margins of the Pacific Ocean basin? Altogether, what are they called?

Answer 21

due to the convergent margin setting. called the “Pacific Ring of Fire”

Question 22

How many active sub-aerial volcanoes are on divergent plate boundaries?

Answer 22

15%

Question 23

Where is most volcanic activity on Earth taking place?

Answer 23

At the mid-ocean ridge (MOR), submarine volcanoes.

Question 24

What are hotspot volcanoes?

Answer 24

active sub-aerial volcanoes found WITHIN oceanic and continental plates, not located along a plate boundary.

Question 25

Where are hotspot volcanoes located?

Answer 25

where hot, buoyant mantle material, i.e., a mantle plume, rises into the upper mantle and initiates melting

Question 26

Where are mantle plumes thought to come from? Describe their positions

Answer 26

thought to originate at the core-mantle boundary; they appear to be fixed in the mantle; that is, their positions do not migrate with time

Question 27

A chain of hotspot volcanoes exhibiting a definite age progression along its length forms due to ….?

Answer 27

movement of a plate over a mantle plume.

Question 28

What happens when a mantle plume induces melting in the mantle near the base of an oceanic plate?

Answer 28

mafic magma is produced, and rises to the surface through mafic crust with little interaction.

Question 29

Why does mafic magma rise to the surface through the mafic crust with little interaction?

Answer 29

Because the melting temperatures of the magma and crust are similar

Question 30

What types of volcanoes form along oceanic hotspots? What kind of volcanic rocks are produced?

Answer 30

Shield volcanoes and some small cinder cones that erupt mafic magma. They produce dark-coloured, low viscosity, low silicon oxide rocks called basalts.

Question 31

What are basalts?

Answer 31

dark-coloured, low in viscosity, and low in SiO_2 rocks.

Question 32

What is a hotspot? What is NOT a hotspot?

Answer 32

A localized source of high heat energy that sustains volcanism. A hotspot is not simply a shallow reservoir nor a pipe from the liquid outer core.

Question 33

How do hotspots begin?

Answer 33

Theory: begins as a blowtorch-like thermal perturbation in a zone between the liquid outer core and overlying mantle about 2900 km heat. The thermal plume allows solid, yet mobile mantle to rise slowly and convect outward. (convection is process by which hotter material rises and cooler material sinks.)

Question 34

How deep can magma be generated?

Answer 34

1500 km deep

Question 35

What does erupted lava build?

Answer 35

Mountains that emerge as islands

Question 36

What happens to volcanoes that spend time on the thermal plume?

Answer 36

Slowly get rafted away on the moving plate and new volcanoes are built in their place.

Question 37

Why can volcanoes erupt again even after moving away from the hotspot?

Answer 37

Plate carries the volcanoes away from the hotspot and drags the plume with it. Eventually volcano will subside due to erosion, which outpaces volcanism as eruptions wane and the buoyant effect of the plume diminishes.

Question 38

What differs mantle plumes located beneath continental plates and mantle plumes found beneath oceanic plates?

Answer 38

No difference

Question 39

Why are continental hotspot volcanoes very different from the shield volcanoes in oceanic hotspot settings?

Answer 39

Because of the higher silica content of the continental crust and the resulting interactions of mantle-derived magmas with that of continental crust

Question 40

A rising mantle plume will induce melting in the _____ or the ______ near the base of a continental plate. Resulting melts are ____. As the magma rises, crust melts. Why?

Answer 40

upper mantle; lithosphere; mafic.

Because continental crust has, on average, a higher silica content and hence a lower melting temperature than mafic crust. Thus in this manner, mafic magmas can produce large volumes of felsic melt.

Question 41

The production of large volumes of high silica, high viscosity magma in continental hotspot settings leads to the formation of _____. Because of their high viscosity and high gas content, these [mafic]/[felsic] magmas typically erupt as large volumes of _____.

Answer 41

giant calderas; felsic; ash flow tuff

Question 42

_____ are the most explosive type of volcano, but luckily for us, they have the longest recurrence interval between eruptions.

Answer 42

Calderas

Question 43

What are rhyolites?

Answer 43

The light-coloured, high silica magmas produced by eruption of SiO_2-rich magmas

Question 44

What chain of volcanoes is Yellowstone at?

Answer 44

the Yellowstone hotspot chain

Question 45

What do ocean-ocean convergent margins produce?

Answer 45

Oceanic arc volcanism

Question 46

At ocean-ocean convergent plate boundaries, the ____, ____, thus more dense oceanic lithosphere is _____.

Answer 46

colder; older; subducted

Question 47

On Convergent Plate margins: At depth, the ____ dehydrates and water is introduced into the overlying _____ (upper mantle). This water induces ________ (by lowering the melting temperature), which leads to the formation of ____ magma. Being less dense than the surrounding mantle, ___ magma rises into the crust and towards the surface.

Answer 47

lithosphere; asthenosphere; melting of the mantle; mafic; mafic

Question 48

On Convergent Plate Margins: Because the plate overlying the rising magma is ____, the ____ magma will have limited interaction with it. Intermediate magmas may form from differentiation processes that change the composition of the magma and slightly raise the ____ content. Thus, magmas produced in oceanic arcs are mostly ___, with some intermediate magmas.

Answer 48

oceanic; mafic; SiO_2; mafic

Question 49

By-and-large, volcanoes in oceanic arcs are _____ volcanoes, and the rocks that form from them range from basalts to ____. Example of an arc from from ocean-ocean plate convergence:___.

Answer 49

composite; andesites; The western Aleutian arc

Question 50

Ocean-continent convergent margins produce ___________. At ocean-continent convergence zones, the denser ________ subducts beneath the continental plate. Mafic magma forms in the mantle through the same mechanism described in the previous section; that is, water introduced by the subducted slab lowers the melting temperature of the mantle. However, as the resulting mafic magmas rise through continental crust, they interact with it to produce _______ and ________. These interactions include melting of the continental crust (to produce ____ magma) and/or changing (or differentiating) the composition of mafic magma to a more SiO2-rich composition (to form _____ and/or felsic magmas).

Volcanoes that result from eruption of magmas with a wide range of silica contents are called composite volcanoes, and they consist of rocks that range from basalt, to andesite and rhyolite. The higher silica magmas produce the most explosive and therefore most hazardous eruptions at these volcanoes.

Answer 50

continental arc volcanism; oceanic lithosphere; intermediate and felsic magmas; felsic; intermediate

Question 51

Another name for the Garibaldi-Cascade arc?

Answer 51

the Cascade Range volcanoes

Question 52

How are the Cascade Range volcanoes formed?

Answer 52

formed from subduction of the Juan de Fuca plate beneath the North American Plate

Question 53

Although any volcano is potentially hazardous, the most hazardous volcanoes are ______ and ______ because of their tendency to erupt gas-rich viscous magmas.

Answer 53

composite volcanoes and calderas

Question 54

The _____ of these explosive eruptions may be high, especially for eruptions associated with a caldera. On a positive note, calderas erupt infrequently

Answer 54

volumes