# Determine which rock was formed from a volcanic eruption

1. Aug 24, 2016

### krbs

1. The problem statement, all variables and given/known data
Identify which rock pictured was most likely the result of a volcanic eruption. No info is provided besides the following pictures which I managed to find on the internet:

https://commons.wikimedia.org/wiki/File:Volcanic_rock_from_Unzen.jpg
https://commons.wikimedia.org/wiki/File:Museo_de_La_Plata_-_Andesita.jpg
https://commons.wikimedia.org/wiki/File:Olivine_basalt.jpg

2. Relevant equations
N/A

3. The attempt at a solution

Just based on the pictures I guessed that the third rock is from a volcanic eruption because it appears mafic and has fine grains in its matrix which are both indicative of a high cooling temperature.

2. Aug 24, 2016

### Simon Bridge

Well the first one it title'd "volcanic rock" and the third one is titled "basalt" ... which may be a clue :)
If this is a long-answer, then you should probably cite the source of your assertions. If you start by writing down the signs that a rock is from a volcanic eruption, then compare, you'll have a stronger argument.

3. Aug 24, 2016

### krbs

It may be! But I'm not sure because basalt and andesite are also volcanic rocks. I didn't go with the one titled volcanic because it appears more felsic and coarse (ie, intrusive). It's a short answer and not worth many marks, but it's bugging me that I don't know! Maybe if I explain my choices as you said it'll work out. Thank you for your feedback~

4. Aug 25, 2016

### Simon Bridge

The context where the 1st image appears suggests it is a result of phreato-magmatic explosion rather than an eruption itself. The rock fragments (and caption indicating some of the fragments are basalt) seem to support this.
The second one is used in three wikipedia pages as an example of andesite - so it is almost certainly a photo of andesite.
You can google the formation of andesite.
The third is, for similar reasons, basalt.
(If you scroll down you can see which pages the pic is used in - and, if needed, get those pages translated for context.)

5. Aug 25, 2016

### krbs

Ok, got it. Thank you!

6. Aug 29, 2016

### davenn

how can they ask you to identify them if they don't give you examples .... seems rather silly
you must have missed something or the Q is just totally screwball

they should have shown you 3 rock types, say igneous, metamorphic and sedimentary and then asked for your choice and reasoning

Yes you went and picked your own 3 eruptive volcanic rock types

Dave