VO L4: Where are Violent Volcanoes Found? Flashcards
At _____ margins, two plates spread apart due to upwelling of the upper mantle and the injection of magma.
divergent
Where are most divergent margins located? What are they called?
Located in ocean basins, and are called mid-ocean ridges (MOR) because they form undersea mountain ranges.
On-land equivalent of mid-ocean ridges (MOR)?
continental rift zone [e.g. East African Rift]
Where two plates collide, one plate will subduct into the mantle beneath the other plate to form a _____ margin.
convergent
Where do volcanic arcs form?
They form on convergent margins, in the overlying plates.
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 _______.
continental; large mountain chain
What made the Himalayan Mountains?
India’s collision with Asia, a continent-continent collision.
In what kind of collision zone is magmatism rare?
continent-continent collision zones
What is a transform fault?
plate boundary where two plates slide laterally past one another
What type of plate boundary rare has magmatism?
Transform fault
Which part of Earth is ultramafic in composition? Why?
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)
Which part of Earth is mafic in composition? Why?
Ocean crust. 45-52% SiO_2, along with less MgO and FeO and more CaO and Al_2O_3 than the upper mantle.
What kinds of rocks are found in upper mantle? What characteristics do they have?
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
What kinds of rocks are found in the oceanic crust? What characteristics do they have?
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.
What kinds of rocks are found in continental crust?
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.
Why are continental crusts lower in density compared to ocean basins?
continental crust has an average SiO_2 content of 60%, which is much more rich in SiO_2 than oceanic crust.
Most felsic rocks in continental crust contains how much silicon oxide?
75%
How many active sub-aerial volcanoes are associated convergent plate margins?
about 80%
What does sub-aerial mean?
ground level
80% of active sub-aerial volcanoes are associated with _______.
convergent plate margins
Why are many active volcanoes found along the margins of the Pacific Ocean basin? Altogether, what are they called?
due to the convergent margin setting. called the “Pacific Ring of Fire”
How many active sub-aerial volcanoes are on divergent plate boundaries?
15%
Where is most volcanic activity on Earth taking place?
At the mid-ocean ridge (MOR), submarine volcanoes.
What are hotspot volcanoes?
active sub-aerial volcanoes found WITHIN oceanic and continental plates, not located along a plate boundary.
Where are hotspot volcanoes located?
where hot, buoyant mantle material, i.e., a mantle plume, rises into the upper mantle and initiates melting
Where are mantle plumes thought to come from? Describe their positions
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
A chain of hotspot volcanoes exhibiting a definite age progression along its length forms due to ….?
movement of a plate over a mantle plume.
What happens when a mantle plume induces melting in the mantle near the base of an oceanic plate?
mafic magma is produced, and rises to the surface through mafic crust with little interaction.
Why does mafic magma rise to the surface through the mafic crust with little interaction?
Because the melting temperatures of the magma and crust are similar
What types of volcanoes form along oceanic hotspots? What kind of volcanic rocks are produced?
Shield volcanoes and some small cinder cones that erupt mafic magma. They produce dark-coloured, low viscosity, low silicon oxide rocks called basalts.
What are basalts?
dark-coloured, low in viscosity, and low in SiO_2 rocks.
What is a hotspot? What is NOT a hotspot?
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.
How do hotspots begin?
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.)
How deep can magma be generated?
1500 km deep
What does erupted lava build?
Mountains that emerge as islands
What happens to volcanoes that spend time on the thermal plume?
Slowly get rafted away on the moving plate and new volcanoes are built in their place.
Why can volcanoes erupt again even after moving away from the hotspot?
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.
What differs mantle plumes located beneath continental plates and mantle plumes found beneath oceanic plates?
No difference
Why are continental hotspot volcanoes very different from the shield volcanoes in oceanic hotspot settings?
Because of the higher silica content of the continental crust and the resulting interactions of mantle-derived magmas with that of continental crust
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?
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.
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 _____.
giant calderas; felsic; ash flow tuff
_____ are the most explosive type of volcano, but luckily for us, they have the longest recurrence interval between eruptions.
Calderas
What are rhyolites?
The light-coloured, high silica magmas produced by eruption of SiO_2-rich magmas
What chain of volcanoes is Yellowstone at?
the Yellowstone hotspot chain
What do ocean-ocean convergent margins produce?
Oceanic arc volcanism
At ocean-ocean convergent plate boundaries, the ____, ____, thus more dense oceanic lithosphere is _____.
colder; older; subducted
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.
lithosphere; asthenosphere; melting of the mantle; mafic; mafic
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.
oceanic; mafic; SiO_2; mafic
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:___.
composite; andesites; The western Aleutian arc
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.
continental arc volcanism; oceanic lithosphere; intermediate and felsic magmas; felsic; intermediate
Another name for the Garibaldi-Cascade arc?
the Cascade Range volcanoes
How are the Cascade Range volcanoes formed?
formed from subduction of the Juan de Fuca plate beneath the North American Plate
Although any volcano is potentially hazardous, the most hazardous volcanoes are ______ and ______ because of their tendency to erupt gas-rich viscous magmas.
composite volcanoes and calderas
The _____ of these explosive eruptions may be high, especially for eruptions associated with a caldera. On a positive note, calderas erupt infrequently
volumes
