Why doesn't a volcanic eruption leave empty holes in the earth's crust. Please help me reassure my mother that her concerns about the constant volcanic activity in Hawaii, where I live and work, will not result in the sinking of the Hawaiian Island chain. I have a passing knowledge of geology, plate tectonics and magma displacement, but have not figured out how to explain it satisfactorily to my mother.

Specifically, she cannot understand what takes the place of the enormous volume of lava, ash and other material spewed out by a volcanic eruption. She thinks that every eruption leaves a vast empty hole under the volcano which must inevitably collapse in on itself. I have told her that volcanoes are caused by pressure from plate tectonics which create high temperatures, melting rock into magma and forcing it up through faults in the earths crust. I explained that this relieves the pressure and that any voids are filled by magma until the pressure is reduced enough for the magma to solidify again. Is this correct, or is it even close? So far she ain't buying any of my explanations, and I could sure some help explaining this to her.

I've looked at several web sites for graphic diagrams of volcanic activity, but have had no luck so far in finding something to show her. Now if my mother happens to be right, I'd appreciate a quick reply, so I can start job hunting in Utah or somewhere else far from Kilauea. This is a serious question, and I thank you for taking the time to read it. If you ever get out to the Islands, you can count on a free margarita at the Halekulani on me.

rocky Chris Thometz

Hi Chris,

Well, I happen to live in Honolulu - do I still get the margarita?

The magma is produced by a process called "partial melting". This takes place in the upper mantle, and means that heat from the hotspot (which we don't really understand very well) causes mantle rocks to start to melt. They don't melt completely but instead, certain minerals melt more than others. That makes sense because they have different melting temperatures. Anyway, estimates of the percentage of melting that takes place rarely are higher than 20%. Additionally, this melting takes place over a wide volume so that the lava that is erupted from Kilauea is collected from a large area - no one part of the upper mantle is losing this amount of material.

This doesn't get around the fact that material is indeed transferred from the upper mantle to the surface. Is it eventually replaced? Yes - at the same time that volcanism and mid-ocean spreading is creating lava and new crust at the surface, subduction is putting material back into the mantle. This means that the size of the Earth stays the same, even with plate tectonics going on.

Is Hawai'i sinking? Most definitely. Measured at Hilo, relative sea level is going up at about 4 mm per year. 1.5 mm/year of this is due to global sea level rising, and the remaining 2.5 mm/year is due to the big island sinking. It isn't really sinking because magma is being taken away but rather because the Pacific plate is being heated by the hotspot (and therefore weakened) and because the plate is being loaded with the weight of the volcanoes. It isn't anything new though. Maui was sinking when it was over the hotspot, Moloka'i when it was over, and so on.

Hopefully this has helped to answer your question.

Sincerely, Scott Rowland 

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