September 26, 2022 | David F. Coppedge

Major Rethink of Magma Dogma

How fast does it take to fill a big magma chamber?
Millions of years? No. Just a few months, actually.

 

A new paper revises estimates of how quickly a big magma chamber formed. It was “unexpectedly fast.”

The magma chamber under consideration, the Skaergaard intrusion in East Greenland, “has been important to the development of key concepts in igneous petrology, including magma differentiation and fractional crystallisation and the development of layering,” says Wikipedia. According to the new study, its formation was “catastrophically rapid”—orders of magnitude faster than previously believed. By implication, other magma chambers also were filled with unexpected speed: not requiring millions of years, but just centuries, decades, or even months in some cases.

Volcano eruption in Tonga Jan 15, 2022. (Earth Observatory satellite, NASA)

Spectacular volcanic eruptions have been observed throughout human history. They are the stuff of legends: Thera, Vesuvius, Krakatoa. In recent years, Lassen, Pinatubo, and Mt. St. Helens awed viewers with their power. Just this year, the Hunga Tonga eruption in the South Pacific surprised geologists with its explosive force, generating sonic waves and tsunamis that were felt around the world. This one blast sent 50 million tons of water vapor into the atmosphere, Live Science reported on Sept 23, raising atmospheric water vapor by 5%. This one volcano could warm the Earth for years, the headline states, since water vapor is a potent greenhouse gas.

Geologists can also infer spectacular eruptions of supervolcanoes from their remains around Yellowstone, in California, and elsewhere. Large igneous provinces (LIPs) are not rare on our planet. Some of the most notable are the Columbia River basalts in Washington and the Deccan Traps in India, but many more are known. Volcanic eruptions are typically short-lived, lasting a year or less; the longest above-ground eruptions may continue for a century or so. But what happens underground? How quickly does magma move into the chambers that sometimes produce volcanic eruptions? Does it take millions of years?

Speeding Up a Model

The Skaergaard magma chamber was first described in 1931. In size, it is 8 km by 11 km. Having a box-like shape, it lies 2 km below the surface and is estimated to be 4 km thick. This makes its volume 300 cubic kilometers of solidified basaltic magma. Geologists believe it formed 55 million years ago, but for present purposes, the speed of its formation is the issue. Previous estimates for magma flow using radiometric dating ranged from 10,000 years to 10 years for the magma to move one kilometer into the chamber, resulting in 40,000 years to 400 years to fill entirely. The authors of a new paper move that speed to almost instantaneous in geological terms – as if there was a super-eruption underground.

Source: Annen, Latypov and Nielsen, Catastrophic growth of totally molten magma chambers in months to years, AAAS Science Advances, 23 Sep 2022, Vol 8, Issue 38 (open access), DOI: 10.1126/sciadv.abq0394.

The vertical growth rate of basaltic magma chambers remains largely unknown with available estimates being highly uncertain. Here, we propose a novel approach to address this issue using the classical Skaergaard intrusion that started crystallizing from all margins inward only after it had been completely filled with magma. Our numerical simulations indicate that to keep the growing Skaergaard magma chamber completely molten, the vertical growth rate must have been on the order of several hundreds to a few thousands of meters per year, corresponding to volumetric flow rates of tens to hundreds of cubic kilometers per year. These rates are several orders of magnitude higher than current estimates and were likely achieved by rapid subsidence of the floor rocks along faults. We propose that the Skaergaard is a plutonic equivalent of supereruptions or intrusions that grow via catastrophically rapid magma emplacement into the crust, producing totally molten magma chambers in a matter of a few months to dozens of years.

Aerial photo of Greenland (DFC)

Thermodynamics Instead of Proxies and Assumptions

The authors ditched radiometric dating with its “highly uncertain” estimates for a thermal model. Their reasoning hinged on the fact that crystallization is found from the sides toward the middle, not from bottom and top as the Wikipedia article says. Also, the lack of large crystals indicates that the magma must have been fluid the entire time the chamber was filling before crystallization began.

Geologists have long assumed that magma moves very slowly underground, creeping upward at similar speeds to plate movements that are hardly perceptible. The authors indicate that this assumption has led to false conclusions that do not fit the observational facts.

In felsic plutons, large spreads of zircon ages from individual samples have led to the conclusion that the ages of many zircon grains do not date the timing of magma emplacement. Geochronology must therefore be used with great caution for rigorous estimation of the growth rate of plutonic bodies. There is also a tacit assumption that magma chamber growth rates must be similar to rates of tectonic processes, which are inherently controlled by horizontal plate motion velocities (1 to 5 cm/year). However, at such slow rates, small batches of melts arriving into a chamber will totally or partially solidify between injections, leaving little chance for the formation of large crystal-poor magma chambers.

And so this paper used a thermal model that could explain the observations at Skaergaard. The implications of their results would have far-reaching implications for magma flows around the world.

Here, we explore a simple and straightforward thermal approach to obtain the first rigorous estimates of the growth rate of the Skaergaard layered intrusion, Eastern Greenland—a classic example of a solidified basaltic magma chamber, whose study has, historically, constrained many of the fundamental principles of igneous petrology.

Radiometric dating misled geologists. These new estimates speed up the accepted rate of magma flow by 3 to 5 orders of magnitude: i.e., by factors of a thousand to 100,000.

The thermal modeling thus indicates that the volumetric flow rate of magma required for the Skaergaard magma chamber to remain crystal-free is three to five orders of magnitude larger than those typically inferred from the geochronology of felsic and mafic intrusions. This corresponds to an emplacement time for the entire mafic intrusion of a few years to a few decades.

Moreover, the new results “are similar for both cold and hot geotherms” and for both crystal-free and crystal-bearing magmas.

Catastrophic Magma vs Slow Dogma

  • At a minimum, the magma must have moved 200 m to 2000 m (two km) per year to form the crystal-free intrusion.
  • This estimate yields 17.6 to 176 cubic kilometers of magma movement per year at Skaergaard.
  • A single dike from below, providing on open conduit for fluid magma, could have filled the entire chamber quickly.
  • Depending on the dike width, the 300 km3 chamber could have filled anywhere between 2 to 20 years.
  • The process could have been further accelerated by lubrication of the slip faults that dropped the chamber floor.
  • “Since our numerical simulations constrain the minimum rate of magma emplacement, the actual filling time for such a body may be even faster, perhaps several months or even a few weeks.

In conclusion, the authors have overturned magma dogma for large igneous provinces all over the Earth. They accept long ages for the planet; they are not trying to overturn the geologic column. They’re just sayin’—

We propose that the Skaergaard and possibly some other layered intrusions can be viewed as products of catastrophic events associated with a rapid and voluminous emplacement of basaltic magmas into the Earth’s crust. In other words, some layered mafic intrusions may represent the plutonic analogs of LIP-related volcanoes that are responsible for eruptions of enormous volumes of flood basalts on the Earth’s surface

Even the world’s largest magma chambers could have formed in a matter of weeks or months, they calculate, or perhaps a few years at most. See the numbers in the open-access paper.

Geologists need to think outside the box of the traditional millions-of-years mindset. Things can happen catastrophically fast both above ground and below ground.

Vulcan’s Throne, part of the Uinkaret Volcanic Field (DFC). This cinder cone formed above all the Grand Canyon Layers; its lava is pouring into the canyon. Yet radiometric dates put it older than lava underneath all the Grand Canyon strata! Click photo for more information on discordant radiometric dates.

 

 

 

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