Hot Lava: Where does it come from?

You’ve seen it in movies, documentaries, and photographs. Mario and other video game heroes have died countless terrible deaths falling into pits of the stuff, but how much do you really know about lava?

Where does it come from?

Well, volcanoes. And the lava in volcanoes comes from deep in the earth where everything is molten, right? Wrong! It’s true that as you go deeper into the earth, things heat up, but the earth isn’t a crispy rock shell around a gooey molten center. The crust, mantle and inner core of the earth are all solid rock (or iron in the case of the core). The only large portion of the earth’s interior that is liquid is the outer core, and lava does not come from there (again, if it did, it would be molten iron).

People get confused on this point because in school we learn that the mantle flows and convects heat from the core to the surface. Well, that’s true, but it’s misleading. The mantle flows over timescales of millions of years. If you could dig down and pick up a piece of the mantle and whack it with a hammer, it would break. See, hot rocks are a lot like silly putty. Set a ball of silly putty on a table and don’t touch it and it will slowly deform and flow and flatten. Whack that same silly putty with a hammer or pull it really sharply and it will shatter or snap. So that’s how the mantle can flow, given billions of years, but still act like a hard, brittle rock over shorter time periods. The same happens with lava on the surface.

So if the whole mantle is solid, where does hot, molten, fluid lava come from? Well, in certain places, the crystals in the mantle or lower crust can begin to melt. This can happen if there is a hot upwelling from deeper in the earth, as is the case for the Hawaiian islands, or if continental crust is being dragged down into the hot mantle. Temperature, pressure and water are the three main culprits in melting rock, and I’ll explain why.

Temperature is the obvious case: heat something up and eventually it will melt. This is the main source of melting for Hawaii, since it is sitting on a mantle “hot spot” but the other two play a role as well.

When things are under higher pressure, it is much harder for them to break free from their nice, orderly crystals and become free-flowing fluids. That’s why the earth’s inner core is solid even though it is at a temperature much higher than the normal melting point of iron. Likewise, it’s why the mantle is solid, even though it is very hot. But as pressure decreases, for example when the seafloor spreads and releases the pressure on the mantle rocks below it, it becomes much easier for the crystals to melt, and some of them do.

As for water, you would think that water would cool things down and solidify the rock. But it turns out that if you dissolve water in hot rock, it lowers the melting point, much the same way that putting salt on ice lowers the melting point. The result in both cases? The solid melts. Water and other “volatiles” are especially likely to end up dissolved in rocks when an oceanic plate gets subducted, dragging with it water-rich sedimentary rocks. This is why volcanoes are common along subduction zones, such as the entire west coast of the Americas.

So that’s where lava comes from. Stay tuned for a follow-up post, where I’ll talk about the different types of lava flows and show you some pictures I took in Hawaii!

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36 Comments on “Hot Lava: Where does it come from?”

  1. Joe Says:

    So where exactly does lava come from?

  2. Ryan Says:

    It comes from the partial melting of the rocks in the upper mantle, due to either a “hot spot”, changes in pressure, the presence of water (which lowers the melting point of minerals), or some combination of the three.

  3. jane Says:

    this info is ****, it hasn’ helped me at all with my homework! where does the mantle’s heat COME FROM!!!!

  4. Ryan Says:

    Radioactive decay and gravitational contraction. But that’s not what this article was about, it was about where *lava* comes from. Lava is not heat.

  5. Dan Says:

    great article; it’s so much fun than my lousy textbook, thank you!

  6. Unknown Says:

    Lava is created by the melting of rock in the lower and upper mantle. The heat used to do this comes from the center of the earth. That heat is made by the constant motion of the inside of the earth or Friction. bah Learn Up kiddos. This is a 13 year old.

  7. Ryan Says:

    @Unknown
    Your first sentence is correct, but the heat does not come from friction, it comes mostly from the decay of radioactive elements in the mantle. So, the heat doesn’t come from the center of the earth, it is made throughout the mantle.

    • JesusLovesYou Says:

      True and not true. The mantle is not where i\the lava is. The lava is at the outer core of the Earth, which is right below the mantle. The inner core (the actual center of the earth.) is solid because of the high pressure. But like I said, the outer core, which is aprox. 3,000 miles down, is where you will hit lava, which by the way, lava is mostly made up of iron and nickel.

      Hope this helps some one!!

      • Ryan Says:

        I think you’re confused. It’s true that the outer core is made of liquid Fe/Ni, but lava is formed by partial melting in the upper mantle and lower crust, and is mostly silicate minerals.

  8. A Says:

    Thank you so much. This has really helped me! I’ve used some stuff for my homework, hope that’s OK. I used the silly putty bit;) So the magma has characteristics of both liquid and solid…? And the convection current occurs because of the heat generated from radioactive decay and gravitational contraction…What exactly is gravitational contraction? Thanks so much xo

    • Ryan Says:

      You’re welcome! Make sure you cite your sources (a.k.a. this site) and that blogs are an acceptable resource for your homework. If not, then you can use the info here to track down more “trustworthy” information elsewhere, such as a geology textbook.

      Gravitational contraction is when something gets smaller due to gravity pulling things toward the center of mass. The planets form from gas and dust and as that stuff coalesces, it gives off heat. And even once the planet is formed, heavy things like iron will tend to sink to the core while light things like crustal rock will tend to “float”.

      Whenever something falls from high to low, it is giving off potential energy. So when you hold a book up over your head it has lots of potential energy. You let go, and it begins to fall and convert that potential energy into kinetic energy (energy of motion). But then when the book hits the ground what happens to that energy? It doesn’t just disappear, it is transformed again: this time into sound waves and thermal energy.

      The same thing happens when gas and dust and rocks come together to form a planet, they transform potential energy, into kinetic energy, into thermal energy. And the same thing happens when iron in the planet sinks to the core: it has to give up some of its potential energy in the form of heat.

  9. fat twankie Says:

    thanks u helped me alot.

  10. sbb Says:

    the core is the hottest part of the sun isnt it i just dont get it,so the volcano eruption is led from the core of the earth

    • iantouch Says:

      No, the core of the sun is not the hottest part. The hottest part of the sun is the area surrounding it. The surface of the sun is much cooler, and it appears to get cooler the further in you go. Sunspots are the coolest parts on the sun and they are apparently “holes” (or something) into the interior.

      • Ryan Says:

        The core of the sun is the hottest part. Sunspots are cool because magnetic fields trap plasma at the surface, where it can radiate its heat to space.

        But the sun’s structure has little to do with the earth’s structure. The earth is hottest in the core too, but lava and eruptions are derived from the lower crust and upper mantle.

    • iantouch Says:

      And a volcanons eruption is not led from the core of the earth. The core of the earth has nothing to do with a volcano erupting. All of that business happens in the crust.

  11. ashley Says:

    i think rocks are coooool

  12. emerson Says:

    very confusing.How can it be there so much heat so deep inside the earth.How about earthguake!are vulcanoes and earthguake related?

  13. david Says:

    thanks this has helped me alot!

  14. shweta Says:

    I think lava comes from water,tempreture and pressure. well actually it kind of helped me!

  15. shaina Says:

    i think lava comes from alot of sources

  16. unknown Says:

    hi! this was really useful for my project thanks

  17. unknown Says:

    ummmmmmmmmmmm. i dont understand how people are questioning this brilliant article, it must be true if its so magnificent

  18. unknown Says:

    my best friend would like to know where and how the lava was formed. her project would like to be more explanitory that mine. If anyone answer this question would be great!

  19. murtaza Says:

    how did the gass come inside the earth

    • marc Says:

      Link is to BBC doco “The Planets” and explains planet formation and why there are craters on the moon & mars and even arizona.

  20. angel dzerikah Says:

    where did volcano came from???

    • Miguel Says:

      Volcanoes are mountains from when the continents move by tectonic plates and crash into eachother. At that point the two lands start to fold the crust and create mountains. Lastly, the magma from the mantle find it’s way up to the crust then up to the mountain and shoots out of it. So therefore it is called a volcanoe.

  21. JesusLovesYou Says:

    I think I agree with almost everything on this page except for the part about the Earth being billions of years old…

  22. Wendy Says:

    I am interested in knowing if lava can from anywhere except a volcano? I mean water randomly comes up from gysers just wondering if it possible that lava can just come up from the ground?


  23. i love justin bieber


  24. indiegh eats pooop.


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