I’m surprised that Stephen was capable of that level of subtlety.  I had to think about it for a minute.

Can we get a geologist in here to clear it up????

“Lava” has to originally be molten rock, but even after it cools and solidifies it can still be loosely classified as being lava.  It’s just no longer molten lava.  Non-molten lava then gets further classified by how it cooled and its composition into narrower catagories, such as rhyolite or basalt or hard pointy blackish stuff.

There are two basic types of molten lava:  viscous and non-viscous.  When the non-viscous type cools, it will often form long, (relatively) smooth sheets of cooled lava.  This is probably the type that people would go sliding on.  If it was on the side of a volcano, then it could very well be like a giant playground slide, only more likely to get your limbs ripped off from your body.

I thought Lava, was theoretically magma? Isn’t lava just a general word for molten rock? Or is it the other way round?

Magma is a molten silicate substance, and can be found above or below the surface of the Earth.  It’s not just molten rock, but also super-heated water and some other gases.  It’s a very complicated substance, due to the way all the high pressures affect it.  It’s not a uniform substance.  Lava (extruded magma) is just one part of it, a less dense portion of the magma that’s forced to the surface by the denser parts beneath.

So in general terms, when people refer to “magma” they’re talking about magma still underground.  “Lava” is also magma, but it’s the term for magma that’s on the Earth’s surface.

Thanks for clearing that up mate. Much Obliged.

I’ll send you the bill for the consulting fee later, Mort.

Although I am not a geologist, I have been geogicaly trained, and I fully agree with the class Acci has been giving here.

It also depends on whether you take a petrologist view, or a geomorphologist or general geologists view. A petrologist will likely use names like Tephrite, Basalt, Rhyolite, Andesite etc. to refer to solidified “lava”. I.e., he wil go down to the petrologic level in his description. A geomorphologist will talk about “lava flows”. Vulcanologists have their own set of esoteric names.

Referring to solidified lava: there is a typology based on the structural aspects of the solidified rock, the main categories of which are:

* Pahuhu or ‘ropy’ lava: a wrinkled, ropy surface. Name is Hawaiian
* Aa: a very ragged, sintered surface. Name is Hawaiian.
* block-lava: how surprising, consisting of a blocked surface.
* pillow lava: a mass of pillow like blocks, typical for under water extrusion

One of my own photo’s of Yaks (made in Qinghai, Chiina, 1998):

Cool, and thanks to you too Lama. Wow, two geniuii in one day…

I would LOVE to go to China! Nice pic, LaMa!