Today we’re looking at big things. That’s to say, the smallest thing covered in this digest is something that would have enormous regional impacts and the largest is on a scale of millions of years. So sit back, forget about the small stuff and open your mind to some very big concepts indeed.
Tidal Cycles and the Oceans
In our Earth’s history, cycles don’t get much longer than hundreds of millions of years and physical scales (for want of a better phrase) don’t get any larger than the supercontinental. So this is where we start — with the continental cycle.
Most people with a passing interest in the Earth have at least a nodding acquaintance with the concept of plate tectonics. The planet’s internal convection system drives the process of continental drift. Crust is created at constructive plate margins, from which plates move apart, and consumed at destructive margins, where plates come together.
This process is a slow one (the most-quoted indicator is ‘as fast as your fingernails grow’) but the cumulative effect of moving plates is massive. Over time the continents have regularly assembled into what we call supercontinents, when almost all of the land is crammed together, before breaking up, drifting off and eventually reassembling over a period of around 4-600 million years. (If you’re interested, we’re around a third of the way through such a cycle right now.)
And of course, such reconfigurations will have other impacts. This week a fascinating study appeared in the journal Geophysical Research Letters, proposing that the changing topography and its associated changes to bathymetry (the shape of the sea bed) will have changes on the tidal cycle, and that supercontinents are associated with supertides.
Why does this matter? Well, for us it probably doesn’t — but in terms of scientific knowledge, the study concludes that the process: “affects the dissipation of tidal energy in the oceans, with consequences for the evolution of the Earth‐Moon system, ocean circulation and climate, and implications for the ocean’s capacity of hosting and evolving life”.
Erosion and Greenhouse Gases
From supercontinents, we move down a level, to mere continents. When I was studying geography and, later, geoscience, the cycle of continental uplift and erosion was regarded as a key part of the carbon cycle — a mechanism for locking up (‘sequestering’) carbon dioxide, removing it from the atmosphere and controlling global temperature.
The concept is that weathering releases carbon which is then taken up by organisms and eventually deposited in the sea as organic matter such as coal, and carbonate minerals such as chalk, removing them from the atmosphere.
This week a study from a team at Harvard University has quantified a process in which certain types of bacteria consume the sequestered carbon and release it to the atmosphere. Apparently this isn’t a new idea, though it’s the first time I’ve cone across it.
And it rather turns the traditional understanding of the carbon cycle on its head, given that in the study area: “the team estimates that the mountain belt releases roughly 6.1 to 18.6 tons of carbon per square kilometer each year through this mechanism—double or more the amount of carbon estimated to be sucked out of the atmosphere by traditional weathering”.
This is, of course, only one study, in one particular area, and covering one particular type of rock. But what it does do is demonstrate the enormous complexity of the Earth system, and how much more we have yet to learn before we can understand it fully.
The Original Big One
Today (18 April) is the anniversary of the San Francisco earthquake of 1906. Most people know the basic facts — the USGS summary of it indicates that the San Andreas Fault ruptured along a length of 300 miles. First buildings collapse and then fires in the rubble cost the lives of over 3,000 people, leaving hundreds of thousands homeless.
Since then San Francisco has grown and grown. The city itself has a population of under a million, but the San Francisco Bay area is home to over seven million. And the San Andreas Fault is still there, still moving, and will, at some stage in the future, rupture again.
Our understanding of earthquakes has taught us how to construct buildings that are earthquake resistant, but they aren’t earthquake-proof. A study from the USGS back in 2007 analysed the possible consequences of a repeat of the 2006 event:
“…it would …cost between $90 and $120 billion to repair or replace the more than 90,000 damaged buildings and their contents. As many as 10,000 commercial buildings would sustain major structural damage and between 160,000 and 250,000 households would be displaced from damaged residences. Depending upon whether the earthquake occurs during the day or night, building collapses would cause 800 to 3,400 deaths… A conflagration similar in scale to the 1906 fire is possible and could cause an immense loss.”
Something Windy this Way Comes
I don’t wish to steal Jon Plotkin’s thunder (pun intended) and I hope that Decoded’s meteorologist will pick up on the subject of how accurately the hurricane season is forecast.
Research on the upcoming hurricane season forecasts: “a slightly above-average season, with 14 tropical storms in 2018. Seven are expected to become hurricanes, which have a wind speed of at least 74 mph. Three of those seven are expected to be major hurricanes, Category 3 or higher, with winds reaching a minimum of 111 mph”.
This is a calmer season that 2017, apparently, but it nevertheless suggests that coastal communities might be in for another rough ride.