Tag Archives: Mantle Plume

Mermaids offer a rare view of our oceans’ subsurface

In this case, however, MERMAIDS are seismic sensors, deployed around the world’s oceans.

Floating seismometers dubbed MERMAIDs — Mobile Earthquake Recording in Marine Areas by Independent Divers — reveal that Galapagos volcanoes are fed by a mantle plume reaching 1,900 km deep. This photo shows one rising to the surface. Image credits: Yann Hello, University of Nice.

Most of what we know about the Earth’s inside comes from seismic studies. Just like a doctor analyzes your body using ultrasound, seismic stations can pick up waves from earthquake to image the interior of the planet or deduce some of its characteristics. The problem, however, is that most of our planet is covered in water — and we don’t have too many seismic stations in water.

“Imagine a radiologist forced to work with a CAT scanner that is missing two-thirds of its necessary sensors,” said Frederik Simons, a professor of geosciences at Princeton. “Two-thirds is the fraction of the Earth that is covered by oceans and therefore lacking seismic recording stations. Such is the situation faced by seismologists attempting to sharpen their images of the inside of our planet.”

With this in mind, Simons and Guust Nolet (now Professor of Geoscience and Geological Engineering) developed a new type of seismic sensor: a hydrophone. The hydrophone’s earthy cousin, the geophone, is routinely used in surveys to detect subsurface resource. Both types of sensors are essentially a very precise microphone, capable of picking up the sounds of distant earthquakes — or to be more technical, to pick up acoustic energy from earthquakes. The resulting hydrophone was fitted with a GPS and sensors for temperature and water salinity and was mounted on a platform called a MERMAID (Mobile Earthquake Recording in Marine Areas by Independent Divers).

MERMAIDs can dive down to depths as large as 3,000 m and are easily launched from even commercial or amateur vessels. They drift passively, usually at around 1,500 m deep, and whenever they detect an earthquake, they quickly rise to the surface to accurately gather GPS data and transmit data via satellite. They are currently the first marine instruments capable of transmitting seismic data in (almost) real time.

A bathymetric map of the Galápagos hotspot
region. Yellow dots show the location of MERMAIDs when the earthquakes were recorded. Image credits: Nolet et al.

The first fleet was launched in 2017 and now, an international team of researchers has presented the first scientific results.

For starters, MERMAIDs are useful to help scientists monitor abyssal currents — but that’s not even their main function. A more striking find is that the volcanoes on the Galápagos are fed by a so-called mantle plume: a magmatic source 1,200 miles (1,900 km) deep, connected to the surface volcanoes via a narrow conduit that is bringing hot rock to the surface.

These mantle plumes were first proposed in the 1970s as an important part of plate tectonics. Their existence has been confirmed in the meantime, but they have largely resisted attempts at seismic imaging because they are found in oceans, far away from seismic stations. The existence of this mantle plume was also indicated by abnormally high water temperatures.

In addition to filling in some missing puzzle pieces, the MERMAID network could also help geophysicists settle a long-lasting mystery about the Earth, which (thankfully) refuses to cool down.

“Since the 19th century, when Lord Kelvin predicted that Earth should cool to be a dead planet within a hundred million years, geophysicists have struggled with the mystery that the Earth has kept a fairly constant temperature over more than 4.5 billion years,” Nolet explained. “It could have done so only if some of the original heat from its accretion, and that created since by radioactive minerals, could stay locked inside the lower mantle. But most models of the Earth predict that the mantle should be convecting vigorously and releasing this heat much more quickly. These results of the Galápagos experiment point to an alternative explanation: the lower mantle may well resist convection, and instead only bring heat to the surface in the form of mantle plumes such as the ones creating Galápagos and Hawaii.”

The paper, “Imaging the Galápagos mantle plume with an unconventional application of floating seismometers,” by Nolet et al., has been published in Scientific Reports. doi: 10.1038/s41598-018-36835-w.

Venus does tectonics without any plates — and it’s possible young Earth did it too

Venus might be tectonically active, a new paper reveals — but it’s not the plate tectonics we know and love from back home.

Computer-generated synthetic aperture radar mosaics of Venus from the first cycle of Magellan mapping.
Image credits NASA / JPL.

It’s so similar to Earth that it’s often called its twin, and yet, Venus seems to lack one of the defining features of Earth: tectonic plates. This has been bugging planetary scientists for a long time now because our pearly neighbor should be an ideal host for such processes — but they insist on popping up on Mercury or Europa — a moon out of all places — and pointedly absenteeing from Venus.

So why is Venus such a good candidate for plate tectonics? Well, it’s really similar to the Earth as far as size and chemical composition are concerned. Its surface is also littered with volcanoes. We don’t know for sure which are active or not — the surface is hidden behind thick clouds that make repeated observations nigh impossible — but at least they’re proof that there once was a lively geology in the mantle. The proverbial coffin nail is that Venus is pockmarked by craters and material accumulated over hundreds of millions of years or successive eruptions. So tectonic recycling clearly doesn’t take place here.

Men come from Mars, evidence of tectonics comes from Venus

But it may be the case that Venus just has its own flavor of tectonics. For instance, there are some features on Venus, such as trenches and rifts, that point to some kind of tectonic activity. Under normal circumstances, researchers would try to digitally model internal processes and progressively tweak them until the model matched what we see on the surface — but, according to the team of French and US researchers behind the paper, the models currently at our disposal simply aren’t powerful enough to handle a 3D model of Venus’ mantle and crustal activity in enough detail.

Instead, they used an old-fashioned physical model to figure out what was happening. The team used a solution of silica nanoparticles (i.e. finely ground sand) in suspension in water to match the physical properties of semi-solid rock. Put over a heating plate, this medium re-created the convection cells generated in Venus’ mantle as hot (and thus less dense) material pushes up from the planet core towards the crust.

The team used cameras to monitor the evolution of their model. Over time, as water evaporated, a thin crust started to form on top of the container. On this crust, something similar to distinctive features of Venus’ crust (formations called coronae) began forming on the simulated crust. These “volcano-tectonic features unique to Venus” are circular structures which can grow to a few thousand kilometers across, with a mound-like rise in the center. The rise is predominantly made up of igneous rocks, while the edge is bumpy and ends with a deep trench. This trench is very similar to what happens in the areas where tectonic plates get subducted back on Earth, although the rest of the corona isn’t.

Experiment picture.

Side views of the model. Plume shown in red, effect on surface in yellow, subduction zones in white. Blue arrows show the movement of rocks.
Image credits A. Davaille et al., (2017), Nature.

The team believes that a phenomenon underlying plate tectonics on Earth also creates Venus’ coronae: mantle plumes. Think of them like really big convection cells, with the upwelling material burning through the crust like a blowtorch. On Earth, mantle plumes are responsible for hotspots of volcanic activity especially for volcanoes that are smack-dab in the middle of plates, like Yellowstone or Iceland, and those stringy volcanic island arcs like Hawaii. Sometimes, they can be powerful enough to burn through whole tectonic plates and break them apart.

But that happens because Earth’s crust is pretty thick and solid. Venus is a much hotter place, with average surface temperatures revolving around 450 degrees C (840 F), making its crust thinner and more flexible — so mantle plumes have a different effect. As the team’s model showed, when a plume hits the crust, molten rock is pushed up fractures and faults to the surface. This added weight causes the crust to sag, widening the fractures, making more material pile on, and so forth. Eventually, the whole section of the crust will rupture, sink, and melt into the mantle.

Artemis corona.

The Artemis corona is actually made up of 5 different coronae. a) Combined radar (greyscale) and topography (color scale) image; corona-like features labeled ‘c’. Rift segments radiating out from the trench labeled ‘r’. b) Terrain shape along the B-B′ line. c) Radar image showing a 200-km-wide area with graben-like lineation, on average 300 km in length and 1 km in width, spaced at 6-12 km.
Image credits A. Davaille et al., (2017), Nature.

This unique take on tectonics is what we see as a corona. The central igneous bit is deposited by material pushed up by the plume, surrounded by a circular equivalent of a subduction trench. The authors compare their model’s result to two important coronae, Artemis and Quetzalpetlatl, and find that the features observed in the lab line up pretty well to those seen on Venus.

Venus’ coronae might hold an unexpected glimpse into the Earth’s past, too. The team notes that temperature conditions on Venus today are very similar to what Earth had for much of its early history. So it’s possible that there was a more Venus-like tectonic system in place down here before the Earth cooled down enough for plate tectonics to take a hold.

The full paper “Experimental and observational evidence for plume-induced subduction on Venus” has been published in the journal Nature Geosciences.