Diversity (or lack thereof) in geoscience: are we hyping up the wrong things?

Posted on October 27, 2021 by Chris Rowan

Via Dr Sarah Greene, some data from a survey of student attitudes to STEM careers, including geosciences, at a college in the SW US indicates that they care more about whether their career can help people or the environment than the lure of the great outdoors.

The ‘you get to play outside!’ pitch is a popular one in geoscience recruitment, but these data support the notion that this is not broadly appealing beyond a self-selecting few.

More pertinently, digging into the data a bit more, it seems that the problem is not that students feel a career in geoscience can’t do some societal good (2nd row of the figure below), but that they’re really unsure how (1st row, and to some degree also the 4th).

From Carter et al. 2021: http://dx.doi.org/10.1038/s43247-021-00287-4

Some of this might be solved by promoting the more altruistic career options made possible by a geoscience degree. But I do wonder if there might also be some deeper structural issues with how we teach Earth Science. Are we really matching our course and degree offerings to these careers, or do we still too narrowly focus on careers in things like mapping and resource exploration, not recognizing how much doing so narrows our pool of potential recruits?

In addition, as Erik Klemetti points out in an important discussion of Earth Sciences and its confused place in the Climate Crisis:

“As a discipline, [Earth Scientists] find ourselves in the strange position of having a role in causing, finding and solving…climate change”

In order to help save the future, we must reckon with our dirty past (and arguably, present). The fact that our degree programs largely look like they did 30 years ago might be one sign that we have yet to fully do so. And perhaps this lack of reckoning further contributes to our struggles with undergraduate enrollment: we are still perceived more as part of the problem than part of the solution.

Categories: geology, science education

Why do we get earthquakes a long way from plate boundaries?

Posted on September 23, 2021 by Chris Rowan

There’s already a lot of good info out there about this week’s magnitude 5.9 earthquake near Melbourne, Australia. I wanted to dig a little more into the broader reasons you can get earthquakes like this in places you might not expect.

The Melbourne quake is a long way from any recognizable plate boundary – the closest one is more than 1000 km away. It is what we can an intraplate earthquake – one that occurs within a plate interior. This is in contrast to the interplate earthquakes that occur at plate boundaries*.

Map of Australia showing the location of an intraplate earthquake in southern Australia and the plate boundaries that are a long way from where this earthquake is. Source: USGS

To understand why we can get earthquakes inside a tectonic plate, you need to understand why we get earthquakes anywhere. To produce an earthquake, you need:

  • differential motion between two bits of the crust;
  • a fault that can accommodate that differential motion by motion along its surface (and has some friction so that the motion is not continuous)

At plate boundaries, the source of differential motion is obvious: you’ve got two chunks of crust moving in different directions. And at its most simple, a plate boundary is a planar break in the lithosphere. Add a bit of friction, and voilà: periodic interplate earthquakes accommodating that motion.

Source: IRIS

So what’s going on inside plates? The key point is that plates are moving because they are under stress. They are being pushed and pulled at their edges, and are strong enough internally that they translate rather than deform internally in response (in technical terms, they are a stress guide).

Two possible responses to a push on the edge of a block are shown. If rigid, then the whole slab moves sideways; if not rigid, then deformation near the pushed edge dissipates the stress.
Two possible responses to a push on the edge of a block are shown. If rigid, then the whole slab moves sideways; if not rigid, then deformation near the pushed edge dissipates the stress. Sketch by Chris Rowan

For example, the Australian plate is being pushed by a mid-ocean ridge to the south and pulled by a subduction zone to the north, so it is moving north.

Modified from USGS location map.

So the rocks that make up the plate are under stress due to these forces. But particularly on continents, the rocks which make up the plate are different depending on where you are, and sometimes bear billions of years’ worth of tectonic scars (pretty much everywhere has been a plate boundary at least once). In other words, there are variations in the strength of the plate.

And if the strength of the plate changes, then so does its response to stress – the strain. Weak bits will deform more than strong bits.

Response to a slab with a weaker zone in the middle; mostly translation, but some deformation in the weak zone. Sketch by Chris Rowan

There’s your differential motion. And if the point of weakness is an old fault, then voilà – an intraplate earthquake, such as the one we just saw in Australia. Or the New Madrid earthquakes of 1811–12. Or the 2011 Virginia earthquake. Or that one in North Carolina last year.

The differential motions we’re talking about here are very small compared to those that occur across plate boundaries, so it takes much, much longer to build up enough elastic strain on an intraplate fault to generate a significant earthquake. So they are very good at taking us by surprise – because there have been no large earthquakes in the relatively short length of time we’ve been paying any attention to such things, we may not have even considered the risk of an earthquake happening there. When buildings and populations are totally unprepared, even a relatively smallish intraplate earthquake can have an outsize effect.

[this post was collated from this Twitter thread]

*If you’re thinking “interplate” and “intraplate” sound very similar and easily confused – yep. I have learnt to my cost that they should be annunciated v-e-r-y s-l-o-w-l-y in lectures.

Categories: earthquakes, geohazards, geology, tectonics

The unthanked shoulders we stand on

Posted on August 3, 2021 by Chris Rowan

Via Liz Hide on Twitter, a thought-provoking acknowledgement of the important role the in discovering and excavating the paleontological treasures in many museums’ collections.

On a similar theme, I think of the story of Alfred Wegener and continental drift. The data Wegener used to such great effect to hypothesise the existence and break-up of Pangaea was not collected by him, but collated from many other sources.

Those sources – maps and descriptions of rock units, fossils and geological structures from every continent – surely relied on indigenous knowledge of where to find the good outcrops, and detailed exploration surely occurred with the help of locals.

Wegener’s achievement is not tarnished by acknowledging he was standing on the shoulders of the many people, all over the world, who built the foundations of the geological datasets he used – and we still use today.

[collated from this Twitter thread]

Categories: fieldwork, geology, history of science, society

The Cuyahoga River burned today for the first time in 51 years. Here’s what we can learn from it.

Posted on August 25, 2020 by Anne Jefferson
Cuyahoga Falls Fire Department photo of the August 25, 2020 fire on the Cuyahoga River. This photo was published by the Akron Beacon Journal.

How many of you had “Cuyahoga River catches fire” on your 2020 bingo card?

Yet that’s what happened today. 

A tanker-car collision/fire near the Cuyahoga River in Akron this morning spilled burning fuel into a storm sewer and then the river, so the river caught fire. This is the first time in 51 years and 2 months that the Cuyahoga River has burned. 

When it comes to fires on the Cuyahoga River, the burning river jokes are inevitable. But there are some real, substantive differences between this small fire and the fires of 50+ years ago. 

Today we have a clear point source of the fuel and fire making it into the otherwise nonflammable river in Akron. 

50+ years ago, there were many, many point sources & non-point sources of pollution that made the river itself flammable (in Cleveland, near the mouth), and all it took was a sufficient spark. The Cuyahoga burned more than once (13 times before today), and so did rivers in other industrial cities in the US.

The Cuyahoga’s historical fires made it the burning river that “sparked” the environmental movement in the late 1960s/early 1970s. Both local grassroots and national efforts have led to dramatic improvements in water quality since then. The Cuyahoga River still has some issues, but flammability isn’t among them. NE Ohio is justifiably proud of the rebirth of its rivers and its history of environmental work. And we should be. 

Today’s event reminds us that we need to keep our environmental protections strong and not backslide on regulation. We need to fill the gaps in those regulations, not widen them.

River fires need to remain extraordinarily rare, small, easily-contained events that are sobering reminders of our history, not re-enactments. 

And today’s events should spur us to better manage stormwater, so that the river to roadway connection is indirect, not a pipeline for a blaze. The same connection that allowed burning fuels to reach the river today, allows road salt, metals, and many other pollutants to enter the river every time it rains or the snow melts. These direct connections between roads and rivers are everywhere and they are a major remaining source of pollution for rivers like the Cuyahoga. If you care about the health of rivers and streams (and apparently, if you don’t want them to risk catching fire), you should be pushing for stronger stormwater management, including retrofitting stormwater controls into existing roadways and developed areas.

If you want to learn more about the environmental history of the Cuyahoga River, I highly recommend this documentary. Today’s accident and fire occurred a short distance upstream of the Akron Gorge Dam, which features in the film as one of the last major impediments to water quality on the middle river. The dam is currently slated to be removed sometime in the next 3 years or so.

[Photo by Cuyahoga Falls Fire Department, via Akron Beacon Journal. Story here: https://www.beaconjournal.com/…/one-dead-in-fiery-route-8-n…. My heart goes out to the families of the people killed and injured in the accident, and my sympathy goes to those who faced the disruption of evacuation from nearby homes and workplaces or a significant time spent stuck on the road.]

Categories: by Anne, environment, geohazards, geology, hydrology, public science, society

Why did North Carolina experience a magnitude 5.1 earthquake yesterday?

Posted on August 10, 2020 by Chris Rowan

The location of this earthquake seems a little odd because North Carolina is about as far as it’s possible to get from an active plate boundary – thousands of km from the mid-Atlantic spreading ridge to the east and the San Andreas/Cascadia strike-slip subduction combo to the west.

Map showing extent and boundaries of the North American plate, which is shaded read.
The extent and boundaries of the North American plate. Modified from original on Wikipedia.

The whole point – and power – of plate tectonics is that most geological activity, including earthquakes, occur at the boundaries of the Earth’s tectonic plates. Those plates are constantly moving and rearranging, but they are rigid enough to do without changing shape.

However, it is more accurate to say that they do so without changing shape much. “Intraplate” earthquakes like this one, or the M5.8 that shook Virginia in 2011, show that in reality, the interiors of plates can buckle and stretch. It just happens at much, much slower rates than at their boundaries. No matter where we are, the crust under our feet is always under stress. In the eastern US, the main source of horizontal stress is “ridge push” from the mid-Atlantic ridge, which has a roughly NE-SW direction.

North American plate showing ridge push from mid Atlantic ridge on eastern boundary
Direction of ridge push force from mid-Atlantic ridge on the North American plate. Modified from original on Wikipedia.

Normally, the interior of a plate is strong enough to take this stress, and transmit it onwards. But not always! The exceptions mostly occur in continental crust, which hangs around accumulating tectonic damage – faults & fractures – that make it weaker. Several times in the past billion years or so, this part of North America has been much closer to a plate boundary than it is now. Most obviously, the Appalachians are a testament to a continental collision about 350 million years ago.

Satellite image of a series of ridges associated with a mountain range that spans the picture from bottom left to top right.
Satellite view of the Appalachians, centred on Pennsylvania. Source: NASA Earth Observatory

But after the Appalachians formed, this region was stretched by the rifting that eventually formed the Atlantic Ocean; and the older rocks that the Appalachians is built from record earlier collisions & rifts. These past brushes with plate boundaries have left plenty of faults that could be reactivated by the forces currently stressing the tectonic plates.

So we have stress, and structures that can potentially accommodate that stress. The focal mechanism indicates NE-SW compression on a reverse fault.

Modified from focal mechanism on USGS summary page

The direction of compression is consistent with the expected stress direction in this region due to ridge push. But as most of the old faults and structural boundaries in this region are also oriented NE-SW – the trend of the Appalachians – we might expect strike-slip faulting on a reactivated fault trending parallel to the stress direction, which seems to be a common type of earthquake in this region, rather than thrusting at right angles to the general regional trend.

So yesterday’s earthquake was a bit of an outlier. One possibility comes from the fact that this is not a perfect double couple mechanism. It could be due to a problem with data coverage, or it could indicate that something more complex, involving multiple faults with different orientations, is going on. Zooming out, however, it seems that compressional intraplate faulting is actually quite common to the east of the Appalachians- see the red region in the figure below.

Map of North America showing variations in direction and magnitude of horizontal stresses in the crust, and types of faulting accomodating them.
Source: Nature

The basins created in this region by continental rifting as the Atlantic started to open in this region were linked together by NW-SE trending transform faults and lineaments (at least in Pennsylvania – I’m less familiar with subsurface structures in North Carolina). Perhaps one of these is being reactivated – at least to me, this seems more likely than entirely new faults forming.

Anyway, to sum up:

  • we expect earthquakes like this every so often.
  • this one is at least broadly reflecting the known regional stresses.
  • it is a little bit weird, but intraplate earthquakes tend to be an idiosyncratic bunch.

[this post was collated from this twitter thread]

Categories: earthquakes, geology, structures, tectonics