This week in geoscience, 21-27 March, 2019, I have a couple of connected articles, both of them focussing on continental breakup. Two items on intra-plate earthquake activity (i.e. within a tectonic plate rather than at its margins) caught my eye this week, one of them in Africa and the other in Antarctica. By contrast — and I mean complete contrast — the third item is about a new fossil discovery in China.
M5.5 Earthquake, East Africa
Although this is a geoscience digest these days rather than solely concentrating on earthquakes, I still keep a close eye on the United States Geological Survey’s real time earthquake map. Most significant earthquake activity is associated with plate margins but this week offered us an example of an earthquake of M5.5 — not huge, but certainly significant — which occurred within the African plate.
The cycle by which oceans open and close and continents break up and reassemble (the Wilson cycle) takes place over timescales of hundreds of millions of years and although we aren’t generally aware of it, it’s ongoing on our planet. A look at any topographic map will show Africa’s Rift Valley, extending for thousands of kilometres from Ethiopia down to Mozambique — a feature that can be interpreted as the beginning of continental breakup.
In such locations, the crust is moving apart and the typical feature will be a narrow (in continental terms) rift valley characterised by normal faulting at its margins and by shallow earthquake activity. This week’s M5.5 earthquake broadly meets those criteria.
Kearey et al note that although earthquakes are typically 12-15km depth they can be deeper further away from the front axis and this tremor, located in the uplands above the central rift and at a depth of 22km, fits this description. Unusually, however, the dominant motion is compressional rather than extensional, which is probably a reflection of local tectonics.
Meanwhile, Underneath Antarctica…
Antarctica tends to slip below the radar a lot in terms of seismicity, largely because of its remoteness and the small matter of the vast ice cover and a consequent lack of seismological monitoring. There are scholarly articles on the seismicity of the Antarctic continent but they tend to be limited in scope, and the continent gets only the briefest mention in more general texts.
Robert Yeats, for example, dismisses it in a sentence: “little is known about the crustal tectonics of this region”; but just because earthquakes aren’t recorded here, however, doesn’t mean they don’t occur. It may just mean that we don’t know about them and, as our knowledge increases, it may be that the number of identified earthquakes increases, too.
This was the topic of a research article that caught my eye this week. “The apparent lack of tectonic seismicity was thought to be anomalous relative to other continental interiors”, the paper says, and goes on to report on over two dozen earthquakes which were identified within the continent.
Only the abstract is freely available but an article in Newsweek, extensively quoting expert seismologists, explores the matter further, linking the occurrence of the earthquakes to a subglacial mountain range.
The study’s lead author, Amanda Lough, is quoted in the Newsweek article as saying the earthquakes: “may be caused by tension in the Earth’s crust settling out through scars left by a 100-million-year-old rift valley, where the land stretched and cracked”.
This doesn’t just echo what’s going on in East Africa, but it also illustrates how a rift that develops may not necessarily lead to the break up of a continent. Oh, and as always, it reminds us how much more we have to learn.
New Fossil Discovery Sheds Light on Early Life
Fossils are our window into life hundreds of millions of years ago. New fossil discoveries continually offer us new evidence on the life of known species — often those that our ancestors of some still living, filling the gaps in the evolutionary chain, showing us species that no longer exist and extending the timeline of life on our planet back into the Cambrian period, half a billion years ago, when life on Earth expanded with unprecedented rapidity, and beyond to the origins of life very much earlier.
Large, old, unaltered sedimentary formations such, such as the Burgess Shales in Canada, are legendary among palaeontologists for the range of ancient fossils they contain, but new assemblages are coming to light all the time.
This week scientists are reporting on a new fossil discovery, this time in China, dating from around 518 million years ago. The assemblage, which they have named the Qingjiang biota, is a few million years older than the Burgess Shales and more than half of the species it contains are previously unknown.
Unfortunately this report, too, isn’t available without subscription to the journal, but the abstract and reports in other science media contain enough information to whet the appetite for more and, according to the abstract: “The site rivals previously described Cambrian sites, such as the Burgess Shale, and should help to elucidate biological innovation and diversification during this period”.
More information will surely come from this dig, and I shall be keeping an eye open for it.