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Geology People- Why is the most common Volcanic Rock basalt?

N

Nutdotnet

Diamond Member
This is a question that I know is going to be on my exam tommorow. I've been searching the net with limited success. Just curious if anyone here may know.

Thanks!
 
I think I understand what you are saying. So the volcanic rock cools as basalt since the most common cooling "device" is the air around it?
 
Originally posted by: Kiyup
Just found this
Click to expand...

I found that too (thanks for looking!) but it really didn't answer my question. It told me what extrusive igneous rocks are (LIKE basalt), but it didn't say why basalt is the most common. I know that some volcanoes erupt Andesite and/or Rhyolite. Does it have something more to do with the Silica content?
 
i think i remember something about silicic lava in pyroclastic eruptions only..something that doesnt happen as often...

im pretty sure i m wrong tho 😛
 
I think it's because volcanic rocks are extrusive and as a result cool fast preventing the formation of large crystals which defines the rock as basalt.
 
Originally posted by: Moonbeam
I think it's because volcanic rocks are extrusive and as a result cool fast preventing the formation of large crystals which defines the rock as basalt.
Click to expand...

Wtf? How do you know? Did Voltaire write about rocks?
 
This is all speculation from facts I know. Beware.

Continental crust is made of granite with a density of around 2.7 g/cm³. Oceanic crust is made of basalt with a density of around 3.0 g/cm³. In areas of subduction, which result in volcanoes, the oceanic crust would subduct below the continental crust. This would mean that the oceanic crust is the crust that is being liquified and turned into magma. The continental crust is not deep enough to be heated, or not heated as much because it is not as deep. There is volcanic activity away from subduction and it occurs at hot spots. However, hot spots are concentrated in the Superswell near the French Polynesian islands. Therefore, mostly oceanic crust is liquified here.
 
Originally posted by: jarsoffart
This is all speculation from facts I know. Beware.

Continental crust is made of granite with a density of around 2.7 g/cm³. Oceanic crust is made of basalt with a density of around 3.0 g/cm³. In areas of subduction, which result in volcanoes, the oceanic crust would subduct below the continental crust. This would mean that the oceanic crust is the crust that is being liquified and turned into magma. The continental crust is not deep enough to be heated, or not heated as much because it is not as deep. There is volcanic activity away from subduction and it occurs at hot spots. However, hot spots are concentrated in the Superswell near the French Polynesian islands. Therefore, mostly oceanic crust is liquified here.
Click to expand...

That's a very convincing speculation! Oceanic crust is mafic, with the first layer being basalt. It is denser than continental crust which is felsic (salicic) or intermediate which is less dense than mafic rocks. Since the oceanic crust is being subducted, and the first layer is basalt. It is a good assumption that the basalt is the first rock that gets melted and is the first to erupt from the volcanoes that are formed at subduction zones.

Does that make sense?

 
Originally posted by: Moonbeam
I think it's because volcanic rocks are extrusive and as a result cool fast preventing the formation of large crystals which defines the rock as basalt.
Click to expand...

Actually large crystals define the rock as phaneritc. Which are rocks like Gabbro, Granite, and Diorite. These are non-volcanic rocks, the opposite of Basalt.
 
Originally posted by: Nutdotnet
Originally posted by: jarsoffart
This is all speculation from facts I know. Beware.

Continental crust is made of granite with a density of around 2.7 g/cm³. Oceanic crust is made of basalt with a density of around 3.0 g/cm³. In areas of subduction, which result in volcanoes, the oceanic crust would subduct below the continental crust. This would mean that the oceanic crust is the crust that is being liquified and turned into magma. The continental crust is not deep enough to be heated, or not heated as much because it is not as deep. There is volcanic activity away from subduction and it occurs at hot spots. However, hot spots are concentrated in the Superswell near the French Polynesian islands. Therefore, mostly oceanic crust is liquified here.
Click to expand...

That's a very convincing speculation! Oceanic crust is mafic, with the first layer being basalt. It is denser than continental crust which is felsic (salicic) or intermediate which is less dense than mafic rocks. Since the oceanic crust is being subducted, and the first layer is basalt. It is a good assumption that the basalt is the first rock that gets melted and is the first to erupt from the volcanoes that are formed at subduction zones.

Does that make sense?
Click to expand...

Thanks. I've been studying for National Ocean Science Bowl, but the textbook I'm reading doesn't get nearly as specific as mafic/felsic/salicic and all those things, it just classifies oceanic crust as basaltic and continental crust as granitic. Your explanation makes sense to me from what I know though.

 
preventing the formation of large crystals means the crystals are small, therefore difinitive of basalt. we're talking about the absence of large crystals, small crystals, that make it basalt.

I think the oceanic continental thingi is irrlevant because we are talking ignious silicate rocks in either case. If you melt granite and freeze it quick you get basalt, no? Granite is a large crystal stone. Could be wrong cause there usually's a lot of mass to basalt which implies a lot of iron to me. The iron's in the hornblend in granite, I think.

PS the lack of iron is probably why the continents float.
 
Originally posted by: jarsoffart
Originally posted by: Nutdotnet
Originally posted by: jarsoffart
This is all speculation from facts I know. Beware.

Continental crust is made of granite with a density of around 2.7 g/cm³. Oceanic crust is made of basalt with a density of around 3.0 g/cm³. In areas of subduction, which result in volcanoes, the oceanic crust would subduct below the continental crust. This would mean that the oceanic crust is the crust that is being liquified and turned into magma. The continental crust is not deep enough to be heated, or not heated as much because it is not as deep. There is volcanic activity away from subduction and it occurs at hot spots. However, hot spots are concentrated in the Superswell near the French Polynesian islands. Therefore, mostly oceanic crust is liquified here.
Click to expand...

That's a very convincing speculation! Oceanic crust is mafic, with the first layer being basalt. It is denser than continental crust which is felsic (salicic) or intermediate which is less dense than mafic rocks. Since the oceanic crust is being subducted, and the first layer is basalt. It is a good assumption that the basalt is the first rock that gets melted and is the first to erupt from the volcanoes that are formed at subduction zones.

Does that make sense?
Click to expand...

Thanks. I've been studying for National Ocean Science Bowl, but the textbook I'm reading doesn't get nearly as specific as mafic/felsic/salicic and all those things, it just classifies oceanic crust as basaltic and continental crust as granitic. Your explanation makes sense to me from what I know though.
Click to expand...

Yeah, my book says that the first layer of the Ocean floor is sediment, the second is Basalt, and the third being gabbro.

 
Originally posted by: Moonbeam
preventing the formation of large crystals means the crystals are small, therefore difinitive of basalt. we're talking about the absence of large crystals, small crystals, that make it basalt.

I think the oceanic continental thingi is irrlevant because we are talking ignious silicate rocks in either case. If you melt granite and freeze it quick you get basalt, no? Granite is a large crystal stone. Could be wrong cause there usually's a lot of mass to basalt which implies a lot of iron to me. The iron's in the hornblend in granite, I think.

PS the lack of iron is probably why the continents float.
Click to expand...

Lack of magnesium is why the continents flat. Continents are SiAl, oceanic plates are SiMa. Granite is intrusive and has more time to solidify underground, basalt solidifies quickly so it has smaller crystals.
 
Originally posted by: Jellomancer
What's funny is we are doing this stuff in Geography... and it's a liberal arts class!
Click to expand...

dont take geo401 or geo404c

i did, see how i dont know jacksh1t 😀
 
Originally posted by: Moonbeam
preventing the formation of large crystals means the crystals are small, therefore difinitive of basalt. we're talking about the absence of large crystals, small crystals, that make it basalt.

I think the oceanic continental thingi is irrlevant because we are talking ignious silicate rocks in either case. If you melt granite and freeze it quick you get basalt, no? Granite is a large crystal stone. Could be wrong cause there usually's a lot of mass to basalt which implies a lot of iron to me. The iron's in the hornblend in granite, I think.

PS the lack of iron is probably why the continents float.
Click to expand...

Yeah I re-read your first post, I understand what you are saying now. If you melted gabbro and cooled it quickly then it makes sense that it would become basalt. I don't think granite would since granite has a much higher silica content.

Saying that, I am a little skeptical about the oceanic crust theory as well. Since melted phanertic rock cooled quickly would probably become aphanetic.
 
From the web:

Granite, along with other crystalline rocks, constitutes the foundation of the continental masses, and it is the most common intrusive rock exposed at the earth's surface.

Basalt is the most common volcanic rock in the earth's crust. The ocean floor is covered with basaltic lava that flowed from the mid-ocean ridges.
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Basalt is extrusive and it's intrusive cognate is gabbro.

For granite it's ryolite.

The ocean covers 70% of the earth so if its basalt, I guess that makes it the most common in total.
 
What about this "theory"?

Since Basalt is the least viscous from all the Volcanic rocks (since it has the lowest amount of silica) it is able to travel greater distances once erupted from the volcano? Then again, that may have nothing to do with it...just a thought.
 
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