What's left after lava leaves?
Kilauea has erupted about 0.13 cubic kilometers (170 million cubic yards) of lava yearly for more than 20 years. That’s enough to fill almost 2 1/2 craters the size of Halemaumau each year. The entire eruption has produced enough lava to fill 50 such craters. Does this mean that there’s a hole beneath Kilauea with a volume equal to 50 Halemaumaus?
That’s one of the most frequently asked questions about Kilauea. The question is a reasonable one. In our everyday experience, we would expect such a hole.
But, in this case, everyday experience doesn’t work. We aren’t used to thinking about how rocks behave under very high pressure, and we aren’t used to thinking on a global scale. When we force ourselves to take this unusual view, we can understand that there will be no such hole.
The magma erupted at Kilauea comes from a hot spot in the earth’s mantle at a depth of 60-80 km (35-50 miles), give or take a few kilometers (miles). The exact depth is unimportant for our discussion. The point is that the depth is so great that the pressure on the rocks is tremendous, enough to cause them to actually flow very slowly. Any empty space created by the departure of liquid would be filled as the liquid leaves, perhaps as the island continues to subside or as rocks creep into the emptied space from all sides.
An analogy might be a saturated sponge. If you squeeze the sponge, water comes out but no void is left. The pressure on the sponge both forces out the liquid and closes the volume occupied by the liquid. In the case of the earth, the squeezing is caused by the great pressure of the overlying tens of kilometers (miles) of rock. In something akin to this process, no hole is left when magma rises beneath Kilauea.
But, continuing our analogy, the sponge gets smaller when you squeeze it. Maybe no voids are left, but the sponge occupies less volume than it did before. If the earth acted like the sponge, the source area for the magma would be shrinking, even if it lacked holes. So, something else is going on.
That something else is related to plate tectonics. The relatively thin plates that form the ground we live on are moving steadily in different directions, some about as fast as our fingernails grow and some a little faster or slower. With all this movement in different directions, collisions of plates are inevitable. At sites of collision, something has to give, and generally one plate sinks beneath the other in a process called subduction.
Carried down as the plate sinks are not only the original plate itself but all material that has been added to it, such as sediments deposited on the ocean floor or volcanoes built on the floor. This is the ultimate recycling. Whatever appears on the earth’s surface eventually is recycled into the earth’s mantle.
What leaves the mantle as magma ultimately returns as rock that, as the plate sinks and pressure increases, merges into the mantle and becomes able to flow slowly again.
The process just described is generalized from exceedingly complex geophysical processes. Other factors, such as changes in density as rock melts, come into play in the real world. The bottom line, though, is essentially a zero-sum game in terms of the balance between outflow and income. What the mantle spends in giving up magma to the volcanoes is approximately balanced by what it earns from the incoming rock subducted along plate margins.
As a consequence of all this, there is no hole beneath Kilauea equal to 50 Halemaumaus–or even one–despite all the lava erupted in the past 20 years. Subsidence and lateral flow in the mantle keep voids from forming, and the mass loss by eruption is roughly offset by the mass added back into the mantle by subduction.
Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Surface activity is mainly visible in the westernmost section of the pali flow field. The Kohola arm of the Mother’s Day flow has some breakouts on the coastal flat. The breakouts, small and sluggish, are scattered through the flow. The east-side lobe of the main Mother’s Day flow also remains visible as a series of incandescent patches from the top of Pulama pali out onto the gentle slope below. No lava is entering the ocean.
One earthquake was felt on the island, a magnitude-3 shock reported by a resident of Kailua-Kona at 7:31 a.m. on September 2. The earthquake was located offshore, about 15 km (9 miles) northwest of Kailua, and came from a depth of about 11 km (7 miles).
Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low, with only one earthquake located in the summit area during the last seven days.
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