Magma Can Survive in Earth's Upper Crust for Hundreds of Thousands of Years

When volcanoes erupt, silica-rich magma can burst through the Earth's crust, burning the surrounding area in a massive explosion. Now, it turns out that this magma can lurk in Earth's upper crust for hundreds of thousands of years without triggering an eruption, which means that current magma reservoirs may not necessarily indicate that there will be an eruptive event in the near future.

Previous models indicated that reservoirs of silica-rich magma, or molten rock, form on and survive for geologically shorts time scales in the Earth's upper crust before they solidify. The models also showed that this magma had to be injected into the curst at a high rate in order to reach a large enough volume and pressure to cause an eruption. The new model, though, seems to show otherwise.

In this latest study, the scientists took the models further and incorporated changes in the crystallization behavior of silica-rich magma in the upper crust and temperature-dependent heat conductivity. In the end, they found that magma could accumulate more slowly and remain molten for a much longer period that the models previously suggested.

There are generally two different types of magma. Plutonic magma freezes in Earth's crust and never erupts. Instead, it becomes a craggy granite formation like those commonly seen in Yellowstone National Park. The other type is volcanic magma, which is associated with eruptions.

It's possible that magma chambers in the Earth's crust could consist of a core of partially molten material feeding volcanoes surrounded by more crystalline regions that ultimately turn into plutonic rock. Yet more research needs to be conducted before this theory is confirmed.

However, there is a way to learn a bit more about the magma itself. Silica content is a good way of judging how magma has been affected by being in the crust. As the magma is forced up a column from lower in the Earth to the crust, it begins to crystallize. These crystals start to drop out as the magma moves higher, leaving a higher silica content in the remaining molten rock. Yet even if the molten magma base begins to solidify before it erupts, it's a long process. As the magma cools, more crystals form and give the rock a "mushy" consistency. This means it's still capable of erupting.

The fact that this magma could remain beneath the crust for far longer than expected is significant for volcanic "arcs." These arcs are found near subduction zones where one of Earth's tectonic plates is driving beneath another. While scientists have developed techniques to detect magma pools beneath these arcs, they haven't been able to determine how long the pools have been there.

"If you see melt in an area, it's important to know how long that melt has been around to determine whether there is eruptive potential or not," said Sarah Gelman, the lead author of the paper, in a news release. "If you image it today, does that mean it could not have been there 300,000 years ago? Previous models said it couldn't have been. Our model says it could. That doesn't mean it was there, but it could have been there."

The findings are published in the journal Geology.