What are the different types of volcanoes and how are they formed?

[curious_ocean]

Hi PF,

I am teaching an introductory college level Earth Science course and have few questions about the different types of volcanoes.

I understand that the type of magma largely dictates the volcano type, where felsic (rhyolitic/granitic) magma has a higher silica and gas content than (basaltic) mafic magma. The higher silica content means that the viscosity of felsic magma is higher relative to mafic magma. Additionally, the temperatures at which felsic magma tends to erupt is lower than basaltic, further reducing felsic magma viscosity relative to mafic magma.
Q1: Why does felsic magma erupt at a lower temperature than mafic? (My guess is that this is also due to the higher viscosity?)

Hot mafic magma more readily (gently) releases gases and basaltic lava flows faster and farther before cooling, developing low slope shield volcanos.
In comparison the felsic magma tends to create some of the most dangerous explosive pyroclastic flows, due to the high viscosity and gas content of the magma.
Due to the higher viscosity (and cooler temperatures?) the flows do not travel as far before they cool, creating steeper sloped volcanoes.
Q2: Is there a name for this type of felsic magma induced volcano? Or do these not really exist?
Andesitic magma, which has a composition somewhere in between felsic and mafic magma, can create composite (/strato) volcanos, which are composed of alternating layers of cooled lava flows and pyroclastic flows.

From what I understand, mafic magma can be created in 2 ways. One way is decompression melting of the mantle, where hot rock rises so quickly that the drop in pressure causes it to melt. This can happen at divergent plate boundaries (spreading centers) or a mantle plumes (hot spots). The other way is by flux melting which happens at subduction zones where the water that is being subducted causes melting of the mantle.

I think that felsic magma is created by additionally melting continental crust. For example, at convergent plate boundaries where oceanic lithosphere runs into continental lithosphere, the mafic magma created by flux melting at the subduction zone rises until it meets the continental crust. The continental plate is less dense than this magma, so the magma pools under the continental crust until it becomes so hot that it can further melt through the crust. This is where it picks up the "extra" silica and gas content.

By this logic shield volcanoes would tend to develop at mid-ocean ridges, oceanic hot-spots, and subduction zones where 2 oceanic plate edges are colliding.
Stratovolcanoes would tend to develop at subduction zones where oceanic and continental plate edges are colliding.
This seems somewhat consistent with what I have read.
Q3: Would stratovolcanoes also tend to occur at continental hot spots, and early-stage continental rift areas, since magma would be passing through continental rocks there?

Q4: Are cinder cone volcanoes associated with a particular type of magma?

Thank you for your help!

PS - Please correct any of my mis-statements. I have a physics and oceanography background, so I have a lot of learning to do for the modules of this Earth Science class that are outside of my expertise.

 

[Astronuc]

Additionally, the temperatures at which felsic magma tends to erupt is lower than basaltic, further reducing felsic magma viscosity relative to mafic magma.
Q1: Why does felsic magma erupt at a lower temperature than mafic? (My guess is that this is also due to the higher viscosity?)

. . . igneous rocks may are classified according to their chemical composition. The most general classification is based on the relative abundance in a rock of felsic (feldspar and silica-quartz) minerals vs mafic (magnesium and ferrum or iron) minerals. Felsic minerals (quartz, K feldspar, etc) are light colored while mafic minerals (hornblende, pyroxenes) are normally dark colored. Felsic minerals have the lowest melting points (600 to 750 °C) and mafic minerals have higher melting points (1000 to 1200 °C).
Ref: http://www.columbia.edu/~vjd1/igneous.htm

https://atmos.eoas.fsu.edu/~odom/ESC1000/magmas/mag.html
https://www.tulane.edu/~sanelson/eens1110/igneous.htm

Composition will influence melting point and to some extent viscosity, and the viscosity and density is affected by temperature.

From the Tulane page - Felsic magmas usually have higher gas contents than mafic magmas.
Note also:

• Higher SiO₂ content magmas have higher viscosity than lower SiO₂ content magmas
• Lower Temperature magmas have higher viscosity than higher temperature magmas.

Look at the types of magma and their melting points.

Q4: Are cinder cone volcanoes associated with a particular type of magma?

Some trivia - "Some of the most conspicuous and beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount St. Helens and Mount Rainier in Washington."
Ref: https://pubs.usgs.gov/gip/volc/types.html

Ref: https://www.tulane.edu/~sanelson/eens1110/volcanoes.htm
Cinder cones are small volume cones consisting predominantly of ash and scoria that result from mildly explosive eruptions. They usually consist of basaltic to andesitic material.

Stratovolcanoes show inter-layering of lava flows and pyroclastic material, which is why they are sometimes called composite volcanoes. Pyroclastic material can make up over 50% of the volume of a stratovolcano. Lavas and pyroclastics are usually andesitic to rhyolitic in composition.

http://www.soest.hawaii.edu/coasts/lecture/gg101/powerpoints/Volcanoes.pdf

I have some volcanic rock that I found about 200 miles from the likely volcano. There were a variety of rocks that did not seem to be related to the surrounding area.

See also this discussion - https://www.physicsforums.com/threa...k-fagradalsfjall-volcano.1000553/post-6565542

One could compare the lava (or ash) from Fagradalsfjall with that of the larger Eyjafjallajökull, Grímsvötn or Katla.

https://en.wikipedia.org/wiki/Fagradalsfjall
https://www.volcanodiscovery.com/fagradalsfjall.html

https://en.wikipedia.org/wiki/Eyjafjallajökull
https://volcano.si.edu/volcano.cfm?vn=372020

https://en.wikipedia.org/wiki/Grímsvötn
https://volcano.si.edu/volcano.cfm?vn=373010

https://en.wikipedia.org/wiki/Katla_(volcano)
https://volcano.si.edu/volcano.cfm?vn=372030
Claim: "Katla is one of the largest volcanic sources of carbon dioxide (CO2) on Earth, accounting for up to 4% of total global volcanic carbon dioxide emissions." See note.

 

[curious_ocean]

@Astronuc Thank you!

Regarding Q2:
It looks like there are volcanoes associated with very high silica content magma. From the last page of the slides at this link that you sent http://www.soest.hawaii.edu/coasts/lecture/gg101/powerpoints/Volcanoes.pdf

It looks like rhyolite caldera complexes fit this description as having very high silica content.
https://volcano.oregonstate.edu/rhyolite-caldera-complexes

Regarding Q2 and Q3:
Looks like the Yellowstone rhyolite caldera is thought to be from hotspot activity. Does it have such felsic characteristics because the magma is rising through the continent?
Yellowstone is also associated with flood basalts... I assume that happens after the continent bits are exploded off so that the rest of the magma plume that flows out is more mafic?
In the case of flood basalt plateaus that were created in the ocean - did these have rhyolite calderas or did it look like something else since there was no continental crust there?
Looks like other rhyolite calderas may have other driving mechanisms...do most of these locations/mechanisms involve continental crust to explain the association with felsic magma?

Lastly, what kinds of volcanoes tend to form at continental rifts? In one sense I might expect something similar to what happens at mid-ocean ridges since these are both divergent boundaries, but I could also imagine it being different because of the continental crust that is still there in the earlier stages.

Thanks again!

 

[curious_ocean]

This is a table from our textbook that I'm still working through

 

[Astronuc]

Looks like the Yellowstone rhyolite caldera is thought to be from hotspot activity. Does it have such felsic characteristics because the magma is rising through the continent?
Yellowstone is also associated with flood basalts... I assume that happens after the continent bits are exploded off so that the rest of the magma plume that flows out is more mafic?

Short answer:

Yellowstone is underlain by two magma bodies. The shallower one is composed of rhyolite (a high-silica rock type) and stretches from 5 km to about 17 km (3 to 10 mi) beneath the surface and is about 90 km (55 mi) long and about 40 km (25 mi) wide. The chamber is mostly solid, with only about 5-15% melt. The deeper reservoir is composed of basalt (a low-silica rock type) and extends from 20 to 50 km (12 to 30 mi) beneath the surface. Even though the deeper chamber is about 4.5 times larger than the shallow chamber, it contains only about 2% melt.

https://www.usgs.gov/faqs/how-big-m...s_science_products=0#qt-news_science_products

https://www.usgs.gov/volcanoes/yellowstone/lava-flows-and-associated-hazards-yellowstone

I've visited Yellowstone a few times. When I was there the last time, which was about 30 years from the first time, some thermal pools (hot spots) had become inactive, since the hot magma underneath had moved, eastward I believe.

https://www.nps.gov/yell/learn/nature/volcano.htm
See the image/figure Yellowstone Geologic History with respect to the location of volcanic activity from northern Nevada to Yellowstone. Note also the direction of the North American plate.

Lastly, what kinds of volcanoes tend to form at continental rifts? In one sense I might expect something similar to what happens at mid-ocean ridges since these are both divergent boundaries, but I could also imagine it being different because of the continental crust that is still there in the earlier stages.

See - https://www.nps.gov/subjects/geology/plate-tectonics-continental-rift.htm

I recall a visit to Grand Canyon National park in which a ranger discussed fish fossils up toward Colorado. At one time, the area northeast of Grand Canyon was undersea, as evidenced by fish and shell fish fossils. I also recall the shellfish fossils are found on Mt. Everest, which is consistent with the Indian plate moving into the Asia plate and pushing the boundary vertically.

I don't know if this answers any of the questions, but see Figure 1 in
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2002GB001891

Rhyolites in continental mafic Large Igneous Provinces: Petrology, geochemistry and petrogenesis
https://www.sciencedirect.com/science/article/pii/S1674987120301559

Along the Pacific Ring of Fire, one finds subduction zones that give rise to a variety of volcanoes.

With respect to mid-ocean ridges, see - https://volcano.oregonstate.edu/mid-ocean-ridges
Note the East Pacific Rise and proximity the N. American Plate the subduction zones along the west coast of N. America.

It would be worthwhile to sample mid-ocean volcanoes and compare compositions of magma and ash. For example,

The commonest lava of all the Hawaiian volcanoes is the type known as olivine basalt. Its most abundant constituent is the light-colored mineral labradorite, which is a variety of plagioclase feldspar (see vocabulary page 60) containing more lime than soda. It generally comprises nearly half of the crystallized rock. Next in abundance is pyroxene (see vocabulary). A little iron oxide (magnetite) and titanium-iron oxide (ilmenite) also are present.
. . .

and

The olivine basalts grade into rocks known as andesites, richer in silica and alkalies than the basalts (Table 1), and generally lighter in color. Andesite is unknown at Kilauea and Mauna Loa but is abundant in Mauna Kea and Haleakala. The commonest type of andesite of the Hawaiian Islands goes by the special name of "hawaiite."

Hawaiian volcanoes seem to represent a unique condition.
https://www.soest.hawaii.edu/GG/HCV/haw_formation.html

I was trying to find examples of similar volcanoes elsewhere, and not along a mid-ocean ridge or too close to one, or close to a subduction zone.

This is interesting, but not what for which I was looking.
https://manoa.hawaii.edu/exploringourfluidearth/node/1351

Edit/update: After looking around further, I found: https://uwaterloo.ca/wat-on-earth/news/volcanoes-and-oceanic-crust

and, https://en.wikipedia.org/wiki/Mantle_plume

Because the plume head partially melts on reaching shallow depths (lower pressure), a plume is often invoked as the cause of volcanic hotspots, such as Hawaii or Iceland, and large igneous provinces such as the Deccan and Siberian Traps. Some such volcanic regions lie far from tectonic plate boundaries, while others represent unusually large-volume volcanism near plate boundaries.

Perhaps Condie's text https://www.physicsforums.com/threads/future-of-tectonic-plates.1009325/post-6566653 would provide some insight.