Magma vs Lava?

The news articles regarding active volcanos seem to use Magma and *Lava * interchangeably. I think there is a difference and the dictionary didn’t make it clear to me. Please give me The Straight Dope on this.

If it is inside, it’s magma. If it is outside, it is lava.

I thought the magma was molten iron. And, even if magma is not…isn’t the liquid goo inside the earth liquid iron? And, is that not what volcanoes spew (besides steam)? Good question by the OP! :slight_smile:

  • Jinx

Lava is magma that has reached the surface. Both are molten rock.

The liquid iron that you are thinking of, Jinx, is the outer core of the Earth. It is predominantly made up of iron and other heavy elements that sank when the Earth was forming. The liquid outer core is located between about 1,900 miles and 3,200 miles below the surface. (Inside the outer core is a solid inner core.)

Magma that reaches the surface (lava) arises from the upper mantle, typically from not much deeper than 100 miles below the surface.

The earth’s core is believed to be iron/nickel at a high pressure and temperature – and hence with characteristics not associated with either solid or molten iron-nickel on the surface at 1 bar pressure.

But the core is over a thousand miles down. It never contributes to vulcanism except as part of the planetary heat engine.

The mantle, and melts in the crust, are what produce the lava flows and magmatic ejaculations of volcanoes, and vulcanic traps as well. Its composition varies from point to point, and you’d need an expert to get into details on this.

But among the constituents are: 1) olivine, which is fairly close to being a “consensus” component – that is, while it’s not totally composed of olivine, it behaves much as though it were, because the variants in each direction cancel each other out; 2) rhyolite, which is characteristic of Pacific Rim Pelée-form volcanoes like Mt. St. Helens, Lassen, Rainier, Krakatoa, Fuji, etc.; 3 and 4) aa and pahoehoe, the lava output from “hotspot” vulcanism like the Big Island in Hawaii; and 5) andesite, characteristic of an intermediate stage between the explosive vulcanism of Krakatoa and St. Helens and the oozy flow of Kilauea. Those with much more expertise than I can go into details on what the characteristics of these rocks are. But they’re all molten mineral forms, not molten metal.

Oh, and solidified lava that floats, or pumice, does so because because of the numerous bubble-like voids that form when the lava is rapidly quenched in water.

Finally, volcanos give off many other gases besides steam, including carbon dioxide, sulfur dioxide, hydrogen, carbon monoxide, and hydrogen sulfide.

More info:

Does every mountain have the potential to become a volcano? For all we know, couldn’t Mt. Saint Helen’s, for one, have been an ordinary mountain until an eruption blew the top off leaving the tell-tale conical crater? - Jinx

No, Not every mountan is formed as a pimple in the earth’s cust… damn that’s a good metaphor. Some are formed as wrinkles when tectonic plates push together or slide over one another.

People people people… some of you have been dozing through National Geographic instead of paying attention! :wink:

robby has the lava vs. magma definitions correct, as well as the boundaries for the Earth’s internal layers. Pumice does float because of the bubble-like voids (called vesicles), and if you look at pumice under a microscope you can see that it does have the glassy texture indicative of rapid cooling. However, that rapid cooling is happening in the air. When viscous, gassy magma reaches the surface, the sudden drop in pressure allows the gases to come out of the molten rock explosively, and the subsequent eruption is pretty violent, with gobs of lava being flung all over the place. (Think of shaking a can of soda and then opening the top; soda sprays everywhere, doesn’t it?) In the case of the gobs of lava, these too continue to “fizz out” until they solidify, which happens pretty quickly while they are sailing through the air. You’re then left with chunks of very light, spongy-textured rock that readily floats in water if it happens to land there.

Polycarp was alluding to the fact that the composition of molten rock influences the kind of eruption one ends up, but he’s mixed mineral composition, rock type and texture in one go. To clarify:

Broadly speaking, there are three classes of igneous (molten) rocks according to composition:

  1. basaltic - rich in iron- and magnesium-silicate minerals (such as olivine) and calcium-sodium feldspars, so typically very dark in color; also referred to as basic composition
  2. intermediate - in between basaltic and rhyolitic on the compositional spectrum (e.g., andesite)
  3. rhyolitic - rich in quartz (silica) and potassium feldspar minerals, so typically light in color; also referred to as acidic, alkaline or silicic composition

Each of these classes of igneous rock is typically associated with particular geologic settings and styles of volcanic eruption:

Basaltic rocks, which make up the Earth’s crust in ocean basins, typically originate in plumes of molten rock in the mantle, and erupt either though point sources in the crust (“hot spots” like the Hawaiian Islands and Iceland) or along mid-ocean ridges where tectonic plates are moving apart (e.g., the Mid-Atlantic Ridge). Basalt lavas are usually not very viscous, so when they erupt they can form smooth, ropy structures called pahoehoe, or sheet-like layers (flood basalts like the Columbia River basalts in Oregon or the Deccan Traps in India). If the basalt is a little gassy, it can produce angular chunks called aa, or scoria (the basaltic version of pumice, a.k.a the “lava rocks” you put in your gas grill).

Intermediate and rhyolitic igneous rocks are found where tectonic plates are colliding with each other, and the edge of one plate is being forced down beneath the other (a process called subduction). When ocean floor crust is colliding with continental crust, the denser ocean floor material slides underneath the lighter crust of the continent (which is similar to rhyolite in composition and density, on average) and begins to melt under pressure. The melted basaltic rock, being less dense, begins to rise up through the edge of the continent, melting and mixing with continental crust along the way to produce an intermediate composition magma. When the magma reaches the surface and erupts, a volcano on the edge of the continent is born. This volcano (or chain of volcanoes, more typically) initially yields both lava flows and ash falls of intermediate composition and viscosity, and the style of eruption can vary between relatively quiet (lava flows) and fairly explosive (lots of ash).

Under the right circumstances, as plate collision goes along the magma source may become more and more enriched in rhyolitic material, so that the lava now erupting through the volcano is distinctly rhyolitic in composition. Rhyolites are pretty viscous, so when an eruption occurs, it’s often violent, with lots of ash and chunks of rock spewed into the air.

As Padeye said, some mountains are formed by “wrinkling” or folding of the Earth’s crust during a plate collision. This typically happens when continent collides with continent, and neither plate is subducted. The Appalachians, European Alps and Himalayas are all examples of non-volcanic mountains formed by folding.

Lava is magma that has came to the surface and spewing out; and vice versa.

Magma is in a chamber, under pressure. As it reaches the surface, the lesser pressure lets gasses bubble out. :eek: :smiley: