Density Demo: Pumice

Pumice rock, also known as volcanic bomb, is the only rock that can float on water, because it has a low enough density. When the magma that forms pumice rock erupts it does so violently which causes it to fly up high, cool quickly, and have a lot of gas dissolve in it and escape again. This results in a rock that is very porous and so it has a lot of empty space in its volume which causes it to have a low enough density to float on water.

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[Gertywow]

Hole, is actually called volcanolic gases trapped to make gas bubbles, and it doesnt weight 40 pounds, either.

[mrericsully]

@Gertywow The holes/pores are caused by volcanic gases, but they themselves are not volcanic gases. The texture is called porous.

I don't know where you got the 40 pounds from because I don't say that in the video or the description- I'd estimate this chunk is between 5 & 10 pounds.

[fpgt91]

Just a correction to one of his comments - the pumice does not get "filled" with gases, it already IS filled with gases when it is ejected from the volcano. As it rises, the decrease in pressure allows the gases (like H2O and CO2) that are dissolved in the magma to rapidly exsolve and expand. At the same time, the rapid cooling freezes the bubbles in place.

[mrericsully]

@fpgt91 Thanks for the information. I know that there are gases in the lava and that the pressure release would cause them to expand (although ironically the temperature decrease causes them to shrink some), but I had been told that more gases dissolved in the lava as it flew through the air (I've never verified this or looked it up on my own).

[raulbcd554433]

That's pretty cool, Is pumice the only rock that floats in water?

[mrericsully]

As far as I know.

Have said that some stores will mislabel pumice and scoria and some will just call it "volcanic rock" so you might end up purchasing pumice under a different name.

Pumice is grey in color and scoria is rust in color [actually the color comes from rusted iron].

[G00g284]

Cool video, randomness got me here though

all the way from England

[mrericsully]

I don't see why not. The biggest problems would be balance [related to the shape of the rock], finding a large enough piece [or holding enough together without affecting density], and its inability to have support large loads [multiple people].

[ukulelemike]

So, can a large enough piece hold a person's weight? Like if a floating bridge was built from it, could you walk on it?

[nithun4]

yes, i think that you will be an amazing teacher for me. im in year 8 and we are studying this. but i thought that the pumice couldn't do damage to you much? my teacher said that even a really big pumice part can be easily picked up by anyone? im confused help me out.