Has Earth’s mantle always worked like it does today?

Posted on July 28, 2023 by Chris Rowan

This seems to be the latest round in the eternal battle between the geophysics data which strongly suggests whole mantle convection (and for quite some time, if ideas about the origin of all the weird junk at the core-mantle boundary are correct) and the geochemistry data which suggests long-lived and distinct mantle reservoirs. The authors of this study in Nature are in latter camp.

I’m not against idea that this conundrum might be solved by changes to mantle convection regime over time. Over the 4.5 billion years of Earth history, the mantle has cooled substantially; and mantle convection is driven mostly by the stuff sinking into the mantle by above, which has changed – in nature and possibly also mechanism – too.

But in proposing a ‘recent’ transition between an impermeable 660 km mantle transition (isolating the upper and lower mantle from each other) and an impermeable one (as seems to be the case for the Earth of the present), the authors of this paper are frustratingly coy about exactly how ‘recent’ they are thinking. If recent is sometime in the late Neoproterozoic (700–800 million years ago)1, this seems more plausible than say, in the last 100 million years.

Nonetheless, I’ve always found this particular debate a fascinating example of how scientists approach different datasets. Both geophysics and geochemistry data are complicated – you are trying to get to what you really want to see (structures and processes) indirectly, and you have to make assumptions and approximations about how what you can measure relates to those things.

When, like here, two datasets seem to be telling you different things, scientists tend to trust the interpretation of the one they are more familiar with and assume the problem is with the other one.

Everyone agrees we’re missing something – the disagreement is over where. And you trust what you know.

  1. All the weird stuff was happening then, so why not one more? 

Categories: deep time, geochemistry, geology, geophysics, past worlds

How the UK’s tectonic past is key to its seismic present

Posted on July 8, 2023 by Chris Rowan

Today I learnt something very interesting that I didn’t know before – that intraplate earthquakes in the UK mostly occur in western England and Scotland, not Ireland, eastern Scotland or southeast England (where I grew up).

The cause of this is that the lithosphere in that region is thinner – 80 km rather than around 100 km – and presumably weaker; I have written before about how intraplate earthquakes tend to be focussed in regions of relative weakness within the plate, where the strain rate is higher.

Two maps of the area encompassed by the United Kingdom. On the left, earthquakes are plotted as circles, with larger red and purple circles representing larger earthquakes and smaller yellow and orange circles representing smaller ones. The earthquakes are concentrated in southwest England, Wales and Western Scotland, with relatively few outside this area. On the right, estimated thickness of the lithosphere in the same region is plotted, with red regions being relatively thin and blue regions relatively thick. The red/thin region is in the same area as the area where more earthquakes are occurring.
The record of seismic activity in the UK (left) compared to the thickness of the underlying lithosphere (right). Source: Sergei Lebedev

As for what is causing this variation in lithospheric thickness, there is a fairly good correspondence between the thinned lithosphere and extent of the British Tertiary Igneous Province – the UK part of the North Atlantic Igneous Province, a region of widespread (probably mantle plume related) volcanic activity that accompanied the rifting of the North Atlantic Ocean around 55 million years ago. Since lithospheric mantle is just sufficiently cooled asthenosphere, heating = thinning.

Map of the United Kingdom showing outcrops of igneous rocks in the British Tertiary Igneous Province. These are mostly in Western Scotland - Skye, Mull, Arran - and Northern Ireland, but the Province also includes areas the Bristol Channel (Lundy)
Map of the United Kingdom showing outcrops of igneous rocks in the British Tertiary Igneous Province. Source: British Geological Society

It’s pretty cool how earthquake patterns can so tangibly reflect a deep tectonic history that played out tens of millions of years before the combination of plate motions and stresses that proximally cause them.

Of course, there are potentially damaging intraplate earthquakes outside of this region – the most destructive earthquake in the last 400 years – estimated magnitude 4.6 – had an epicentre very close to where I grew up. And whilst all of these earthquakes might seem very small in the grand scheme of things, but remember the UK is not a country that builds anything with earthquake shaking in mind, so damage from even a minor shake is potentially significant.

Categories: deep time, earthquakes, geohazards, tectonics

A new recipe for Large Igneous Provinces: just add BIF, then wait a couple of hundred million years

Posted on May 26, 2023 by Chris Rowan

Here’s a new paper that proposes the biggest of big ideas: a 240 million year causal chain that runs from the Earth’s surface, to the core mantle boundary, and back again! Here’s how it supposedly goes:

1. Banded iron deposits form on the ocean floor.
2. After subduction, they slowly sink to the bottom of the mantle.
3. The presence of an iron-rich and conductive blob of subducted stuff on the core-mantle boundary boosts local heat flow, and triggers the formation of a mantle plume.
4. Once the plume rises to the base of lithosphere, vast amounts of magma forms by decompression melting, and a new Large Igneous Province is born.

A series of cross sections through the Earth showing how iron formations formed at the surface (left) are subducted and sink to the bottom of the mantle, where they promote heat transfer across the core mantle boundary (centre), which creates a mantle plume that rises to the surface and causes the formation of a large igneous province (right).
The proposed mechanism by which subducted banded iron formations (BIFs) trigger mantle plumes by messing around with heat flow when the reach the core-mantle boundary, and eventually lead to the formation of a Large Igneous Province back on the Earth’s surface. Sketch by Chris Rowan

A few quick thoughts on this:

  • Most BIF formation occurs in the Proterozoic – most famously they are associated with the Great Oxygenation Event around 2.4 billion years ago, but even after that the ocean remained poorly oxygenated. Does this imply that Large Igneous Provinces were much more frequent in the Proterozoic (which would make the ‘Boring Billion‘ a mite less boring, IMHO)?
  • There were episodes of BIF formation associated with the ‘Snowball Earth’/Cryogenian period at the end of the Proterozoic, which could be linked to Large Igneous Province formation in the Paleozoic (although the Siberian Traps is pushing the proposed time delay a bit).
  • It assumes/requires that the Proterozoic was a time of active ‘modern’ subduction, which is somewhat disputed ground in the ‘When did plate tectonics start?’ debate…
  • Although this is a cool idea, correlating events with such a large, and inherently noisy due to the details of subduction, lag time is something to do very carefully.

Categories: deep time, geology, past worlds, volcanoes

A volcano erupted on Venus in the 1990s!

Posted on March 29, 2023 by Chris Rowan

Exciting news that there was a volcanic eruption on Venus in the early 1990s, shown by changes in the size of crater between two passes of the Magellan probe‘s radar over the same area.

Two pictures of the same area on the surface of Venus, acquired with radar, taken about 8 months apart. Of the several craters or circular depressions seem in the earlier image on top, the one in lower centre appears to have grown bigger by the time the later image on bottom was taken.
Two radar images of the same area on the surface of Venus, taken about 8 months apart, showing the changes interpreted as being due to a volcanic eruption. Source: Herrick & Hensley (2023).

There have been hints of volcanic activity observed before, but this is the first direct observation – even if it took 30 years to take note of it. According to the paper, about 42% of Venus was imaged two or more times during the Magellan mission, but the radar energy was bounced off the surface in different directions each time. This means that you can’t just automatically compare images – the different angles of reflection mean that things might “look” different even if nothing has changed.

Instead, this apparent eruption was found by manually searching data in an area considered likely to be volcanically active. The change in vent size and shape at the bottom right is pretty definitive, but the different viewing angles between the two images are the reason that the ‘new flows’ are labelled with a question mark – their appearance is suggestive but you can’t rule out the change just being due to the radar energy being scattered differently at a different incident angle.

It makes the case for a follow up mission to Magellan, that can be designed to make more easily comparable repeat observations, even more compelling. Unfortunately, the recently announced delay to NASA’s VERITAS Venus radar mapper mission is more of a ‘shut it down, and hope we can spin it up again in a few years without any further delays or cost overruns.’ I have some sympathy – the non human spaceflight part of NASA consistently gets the short end of the funding stick, and produces a lot of awesome science nonetheless – but this appears to be a pretty big screw-up that may mean the mission never happens at all. Which would suck – we need another radar mapper not just to hunt down more Venusian volcanoes, but because ‘what does Venusian plate tectonics (or not plate tectonics) look like?’ is a pretty fundamental question I’d very much like to know the answer to.

To put it another way, one of the biggest questions in planetary science is: why is Venus so different than Earth, despite their size, composition and internal structure being so similar? Answering this question might give us a hint about whether any “Earth-like” rocky exoplanets we discover are actually more likely to be like Earth, or whether they are more likely to be friends of our evil planetary twin.

Categories: planets, volcanoes

Earth’s inner core has an inner core?

Posted on February 25, 2023 by Chris Rowan

We all know that the Earth’s mostly iron core is divided into a molten outer core and solid inner core. But that may not be the whole story: some just-published seismic data suggests that the Earth’s inner core is divided into distinct inner and outer portions.

In most materials, the speed at which seismic waves move through varies by a few percent depending on which direction they are travelling. The new data measures the seismic anisotropy within the core in much more detail than has been achieved before, and indicates that the orientation of the directions of fastest and slowest travel, and the actual difference in speeds between these two directions, is different in the central ~650 km of the inner core compared to the outermost ~650 km.

Cross-section through Earth's core, with the Earth's rotation access orientated in the vertical direction, showing proposed division into an outermost and innermost inner core (abbreviation IMIC). This division is on the basis of changes in the directions of the fastest and slowest speeds of seismic waves travelling through the core, as shown by the crossbar symbols where the red bars show the 'slow' direction and the black bars the 'fast directions'. The red bars are parallel to the equator in the outermost inner core, and at an angle of about 45 degrees to the equator in the innermost inner core. From Figure 4 of the paper linked in the post.
Cross-section through the Earth’s core (ERA=Earth’s rotation axis), showing measured variation in seismic anisotropy between the outermost inner core and innermost inner core (IMIC). Sketch by Chris Rowan.

The different seismic properties of the ‘innermost inner core’ has been observed previously, but this new study is strongly arguing that it is due to distinct layering – there is a sharp change at a particular depth within the inner core, rather than a more gradual change with depth.

How they got this information is pretty cool – they found seismic stations close to the antipode – the exact other side of the world – from a big earthquake, which is what you need to pick up seismic waves that travel through the core before returning to the surface. But those waves also get reflected back down back through the centre of the Earth close to where they originated, and then reflected back through the earth *again*. Then the same thing happens again… and again… and again. Each time the signal is weaker, but they managed to capture up to 5 of these reverberations from the same earthquake! And because the stations are not exactly antipodal, each time the wave samples a slightly different part of the inner core, which allows a high-resolution mapping of its properties.

If there is indeed an ‘innermost inner core’, it might give us a clue about the deep history of our planet, because it would imply potentially two distinct phases of inner core formation, rather than it simply being a product of gradual cooling over time. That would also have an impact on the evolution of the Earth’s magnetic field (which is generated in the liquid outer core, but the inner core appears to play a role in stabilising it).

I’m not sure I’m a fan of ‘innermost inner core’ though – it’s a little clunky.

Categories: earthquakes, geophysics