What is lava, exactly?

I did read some Wiki and Google articles about lava, but it puzzles me. Many articles describe it as liquid rock, but isn’t lava from so deep within the earth, it would be high in iron content*? And if so, why would it cool into very light rock (typically pumice)?

*Rock implies it is not ore. Yet, I thought it was closer to ore than just rock.

I think rock now a generic is a term that refers to naturally occurring mixing of hard stuff. :wink:

Ore is a type of rock.

Lava is the ‘flow’ of magma after it exits the chamber and the same can be used to describe the hardened state. There are different types of lava with different mineral properties.

i hated geology, but I always thought lava was (terrifyingly) cool.

different types of lava come out. it’s basically magma that liquifies somewhere at the upper-mantle to lower crust. on a steady state, the heat and pressure makes hard rock behave in a plastic manner --neither hard substance nor liquid. if for some reason the pressure somewhere down there is reduced, the plastic will liquify and try to punch its way up through the solid/plastic country rock. all magma is intrusive rock. intrusive rocks that find their way to the surface as liquid/molten rock are called lava or extrusive rocks.

two main types of lava --basaltic which is iron and manganese-rich (rocks containing dark, iron-rich minerals) and andesitic (rocks with somewhat whiter and lighter minerals, usually aluminum and magnesium-rich.) basaltic lava you see in iceland and hawaii. these are generally very liquid lava that flow out easy, usually with no volatile components like CO2 or H2O so they don’t come out in an explosive manner but usually as “quiet” eruptions and lava flows. sometimes, the basalt has volatile contents and cooled basalt lava may have a pumice-like features (called scoria) and also voids inside.

andesitic lava is found in islands typical of the pacific rim of fire and also in continents that have some sort of subduction activity and gives rise to similar volcanism as island arcs. andesite is much more viscous. you could actually stand in front of some lava flows as they travel less than a foot a day (a friend even lights the cigarette in his mouth by pressing the end against the face of the lava.) and they tend to contain a lot of volatiles like water and CO2. these tend to give andesitic eruptions an explosive nature. the volatile gasses expand as they come out and so you have peculiar textures in the cooled lava. pumice and amygdules (bubbles in the cooled rock).

ore is a concentration of economic minerals sufficient to be mined profitably (or maybe sufficient for some strategic purpose.)

I’m thinking Jinx: may be confusing what “deep within the Earth”. To simplify what mac_bolan00: said, as I understand it, lava coming from volcanoes is from pockets of melted liquified crust material, caused by localized hot spots. It sometimes doesn’t even contain mantle material (that surprised me), and it certainly doesn’t contain material from the Earth’s liquid nickel-iron core. Is that why you think lava should be richer in iron?


nice one. all lava comes from the near surface of the earth (crust and upper mantle) so it’s mostly silicates, whether Fe-Mn -rich or Al-Mg -rich.

there is hardly any movement of mass from the Ni-Fe core to the liquid outer manlte, to the lower mantle upwards. just heat. it’s this heat plume that disturbs the upper mantle and crust, giving rise to things like earthquakes and volcanism.

I think most of the above answers miss the key point:

Pumice is very light because it is highly porous and is substantially gas or air. It is essentially cooled and solidified liquid rock froth. If you took all the gas out of it before it cooled, it would be heavy.

not enough to point out how pumice is influenced by the lava’s volatile content since the OP might be implying pure molten iron-nickel and not just iron-rich silicate lava.

i didn’t miss it. some things just take long explanations. :wink:

Weird trivia: if you melt some obsidian, it puffs up like popcorn. End result is pumice. Ah, now I know where ash clouds come from. Similar to mentos+cola eruption.

So, melt and squeeze pumice to re-dissolve the bubbles, and obtain obsidian?

Fast cooling => volcanic glass? Slow colling => basalt w/large crystals?

regarding the pumice to obsidian and the reverse, i’m not sure. that’s for petrographers and lab-based geologists to say. pop-corn, eh? i would rather think obsidian doesn’t have that much gas content and it’s primarily a silica-rich portion of the lava, meaning glass, so it will cool as such.

basalt is an extrusive rock (cooled lava) and very rarely will develop crystals (in fact i’ve never seen them.) the intrusive form of basalt (diabase, dolerite, etc.) is greenish with some dark crystal minerals visible but still somewhat fine-grained. on the other hand, the dark minerals contained in basalt (pyroxenes mainly) do grow into very large and defined crystals. but these are no where near volcanic eruptions. they happen deep undergound.

Pretty much. The slower the cooling, the bigger the crystals, in general. But there are lots of other things affecting how it turns out, like the presence of water vapor or minerals that act as flux by lowering the solidification temperature. Whether basalt or not depends on the crustal composition (basalt is typical of oceanic and not continental crust). Also, basalt is lava at the surface, so it cools way quicker. You may be thinking of rock that is chemically similar to basalt but that’s plutonic (i.e. way down deep), making coarse crystals; it’s called gabbro.

Pegmatite is the big-crystal champ. But its composition is chemically similar to granite, quite different from basalt. Pegmatite’s big crystals are probably more the outcome of accelerated crystallization than the slowness of cooling. Coarse-grained granite is a slow cooler. Pegmatite is formed by certain other influences acting on granitic magma. Also, different minerals crystallize in different conditions; as crystallization takes them out of the molten mixture, the remaining magma changes chemically, which in turn affects how it crystallizes. Both pegmatite and granite are formed at depth and so they don’t come from lava. Granitic lava produces a chemically similar but structurally different rock called rhyolite.

LOL! gabbro was the name i was looking for in my mind! :slight_smile:

the big crystal champion to me would be those formed hydrothermally (fluids and gasses depositing/condensing.)

look up cave of the giants.

It’s what the floor is.

Like they said recently on PBS (it was an episode of Secrets of the Dead, about the crash of the Minoan civilization,) pumice only floats on water for a little while. Soon, the water fills the gas spaces, and the rock sinks. So yeah, what you described happens all the time.