JOHT 88: Slow Pregress
Sep. 25th, 2020 10:00 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
stickmaker
The Joy of High Tech
by
Rodford Edmiston
Being the occasionally interesting ramblings of a major-league technophile.
Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. If you find a mistake, please let me know about it.
Slow Progress
Glaciologists sometimes place a line of poles across a glacier and photograph (and these days even keep track of the positions using GPS) their movement through time. This is largely to show how the center of the ice flow moves more quickly than the edges. Is there a place on Earth where zones of tectonic plates moving with respect to each other could have poles driven into the bedrock to directly and visibly demonstrate the movement?
We would need a location with little overlying debris. This requirement eliminates the Mariana Trench (it's Mariana Trench because there's just one; Marianas Islands because there're more than one) which has arguably the fastest subduction zone on Earth; it has a thick layer of sludge and a vastly thicker layer of water. That's a shame, in some ways, because the area is very interesting due to far more than its depth. The Trench is crescent-shaped and measures about 2,550 kilometers in length and 69 kilometers in width. The maximum known depth is 10,984 meters at the southern end of a small slot-shaped valley in its floor, a place known as the Challenger Deep. This entire Trench is caused by the Pacific Plate sliding under the Mariana Plate. The rate of subduction in the Trench is 39 to 51 millimeters per year. This speed is due to the material at the western edge of the Pacific Plate being some of the oldest oceanic crust on earth (up to 170 million years old). It is, therefore, cooler and denser than the higher-riding (and younger) Mariana Plate. Its greater density literally pulls the Pacific Plate under the Mariana Plate. The "tongue" of the Pacific Plate which extends under the Mariana Plate is extremely long has an unusually deep tip because of this speed; it simply hasn't had time for much of it to melt. This geologic activity has been going on for over fifty million years. However, the location is not easy to visit.
No, for our purposes the location must be easy to reach and clearly visible, as well as having fast (relatively speaking) movement. That is, the subduction is proceeding quickly enough to produce an observable result in a few years. A typical rate of plate boundary movement is measured in millimeters per year with a large spread of variation, so finding a place which is fast and visible enough shouldn't be a major problem. The movement should also be at least somewhat steady on this time scale.
I doubt there would be a scientific justification for this, but it would be cool to watch in time-lapse. As it turns out, there may be just such a place.
The Beaufort Range thrust fault is in the upper part of the Cascadia Thrust System. There's a nicely-exposed section of the fault in a road cut through a hill on highway BC-4, east-northeast of Alberni, Cascadia. There are even concrete subduction-survey markers already in place on either side. (I haven't heard if there is any time-lapse photography of these.) The eastern, overthrust, side is rising at around 11 millimeters per year. Now, this is, indeed, an active fault, and around 300 years have lapsed since the last big rupture, with 58 years since the last significant minor shock. So the strain is definitely accumulating. Which means there could be a sudden movement at any time. For now, though, the movement is fairly smooth.
Eleven millimeters per year is far from a record speed for a fault, even on solid ground. The San Andreas slip-strike fault moves at more than 20 millimeters per year (about as fast as your fingernails grow) and as much as 25 millimeters per year in some places. Neither of those is anywhere near as fast as the fastest glaciers, of course. The middle parts of some of those move in multiple kilometers per year! However, that slow creep on some active faults is still fast enough to observe the relative movement in a human lifetime. Think about that. These faults move fast enough that humans can notice the change in the ground, and they sometimes need to take that movement into account. There are roads and buildings in California - including the storage warehouse for a winery - which need frequent repairs because they are built across a fault which moves different parts of the structures in different directions. There are even faster faults on land than those mentioned above, in various parts of the world. During an actual earthquake, when parts of a fault which have been hung catch up with the rest, movements can be tens of meters for a single event.
Though they're slower than glaciers, faults can move across distances which dwarf typical ice rivers. Which can cause problems when part of a long fault is moving smoothly and another part is hung on something. Which is the situation on the San Andreas Fault, as well as many others.
The Afar Triangle is a geologically active area in eastern Africa (in an area known as the Horn of Africa). It is the result of three tectonic plate junctions (hence the Triangle part of the name). It is a depression, a low spot in the terrain, and contains the lowest point in Africa. Lake Assal is 155 meters below sea level and one of the saltiest bodies of water on Earth. (The water is described as 'syrupy.') This depression is caused by the relative movements of the trio of plates. The area has long been very volcanic, and the floor of the depression is old lava, largely basalt. The ground is moving at about 20 millimeters a year on all three faults. The motion on the faults is mostly horizontal, but the whole area is also sinking.
In 2005, a giant rift opened during an eruption of the Dabbahu volcano. This volcano is on the rift which is part of the fault between the Arabian and African plates, and is caused by them moving apart. This new crack, 500 meters long and 60 meters deep, opened when magma from the erupting volcano flowed underground then cooled. It left a 60 kilometer long, 8 meter wide underground magma dike which formed within days. By the way, the Afar Depression is one of just two places on Earth where a section of what is otherwise mid-ocean ridge can be studied on land (the other being Iceland).
Probably coincidentally, the Afar Triangle was also home to some of our earliest ancestors and many distant cousins. Fossil bones of multiple species of early hominins are found there, as well as many early stone tools. Some paleontologists think the Afar area was long the cradle of human evolution. Part of the reason the paleontological and archeological pickings are so rich there is that the geologic activity of the Afar Triangle has exposed ancient layers of rocks through much of the area.
The Red Sea and the Gulf of Aden run along the northeast edge of the Triangle. Given that the land in the Triangle is slowly pulling apart and growing lower, it will soon (in geologic time) become an inland sea off the Indian ocean. There are already lakes in the area, as implied above. Dry salt beds are also found in some parts of the Triangle, indicating the area may have been flooded by oceans in the past. However, volcanos rising along the eastern part of the depression have blocked that access. Most of the water in the triangle subsequently dried, leaving the salt beds. The water in streams running into the area today tends to evaporate - on the ground or after entering one of the bodies of water there - adding to the mineral deposits. The Red Sea is expected to eventually wear another path through those eastern volcanos, and the depressed area will subsequently be flooded again in about 10 million years. (Note that the nearby Dead Sea is also in a depressed area caused by a fault system. So are many other bodies of water around the world.)
One of the fastest moving faults is the Denali, in Alaska. Some parts of this are moving at about 50 millimeters a year. New Zealand's Alpine Fault moves as much as 38 millimeters a year. In some places the Pacific Plate is moving at up to 110 millimeters a year, but I believe that movement is in the center, well away from the faults at the plate boundaries. Those locations are also deep under water.
The Teton fault in Yellowstone only moves about 1.3 millimeters per year. However, this movement is almost entirely vertical. Estimates of the age of the fault range from 2 million years to 13 million years. Whatever the actual age of the fault, it has raised the Teton Range, with most of that rise occurring over the past 2 million years. That's long enough for glaciers to have repeatedly impacted the movement of the fault. Note that while this entire area is prone to earthquakes, the Teton Fault - perhaps because its movement is vertical - contributes very little to this activity.
By the way, though Yellowstone is one of the most seismically active places in the continental US, this is mostly due to brittle failures of deep rocks from fault movement, with some being due to moving magma plumes and ground water flows. Not only are there no signs of the sort of seismic activity associated with the magma chamber deep under the park inflating, there's considerable evidence it is actually, if slowly, emptying. So for now there's no sign that the Yellowstone Caldera is heading for another eruption.
There is a long string of extinct volcanoes stretching across the US, with Yellowstone at the northeast end of the string and the only one still active. This has been going on for more than fifteen million years. Because the continent is moving with respect to the magma plume which causes these volcanoes, there is a good chance that - as with the Hawaiian Islands mentioned below - the plume is finished with Yellowstone and will soon (in geological time) surface somewhere else.
The very complicated geology of Yellowstone - of, in fact, the entire continent - is largely due to what we think of as the unified landmass of North America actually being the result of multiple tectonic plates jamming together. None of these plates - which come in a wide variety of sizes and ages - are moving in the exactly the same direction. In fact, we now know that the entire crust of the Earth - every piece of land and the bottoms of the oceans - is made of segments which are all moving with respect to the each other, and with respect to the Earth as a whole. In the long term, what we think of as solid ground is an illusion, as are fixed positions on the surface of the planet. Something which required the development of sensitive equipment to detect, and years of analysis to confirm.
Other worlds are or may be geologically active. Even the Moon may still have a molten core which occasionally burps gasses. However, the Earth is currently the only rocky body we know of which has active plate tectonics. One reason Olympus Mons on Mars is so large is that the lack of active tectonic plate movement on that planet means that huge volcano is stuck over a magma plume. The Hawaiian Islands on Earth have some huge volcanoes (including the tallest mountain on Earth, if you measure from the seabed) but they aren't nearly that big. Plate tectonics is slowly moving the ocean floor across the magma plume which is responsible for the volcanos which make up the islands in this chain. The Hawaiian volcanoes grow for a while, then move too far from the plume, the volcanoes go extinct, and others form. These build new islands from the ocean floor up.
Some of the solid bodies in the outer solar system - including Pluto - may have similar heat-driven processes. However, those involve water ice - and maybe nitrogen ice - rather than what we think of as rock. Remember, on Pluto water ice is effectively rock, and is far more solid there than it is even at the south pole on Earth.
Shakespeare wrote of an inconstant Moon. As it turns out, much of the Earth is also inconstant. As may be many other supposedly solid bodies in the universe.
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Date: 2020-09-25 10:49 pm (UTC)Perhaps that should be Alberni, BC?
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Date: 2020-09-25 11:03 pm (UTC)Great. Now I can't find the reference I used, or any Google info on either.
Maybe there's more than one Alberni?
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Date: 2020-09-26 05:19 am (UTC)Tried to google BC-4, but that got too many hits on things totally unrelated to the BC road net.
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Date: 2020-09-26 04:46 pm (UTC)I think you know Mevlanen (SP?). She's the one who told me about this highway cut across a fault.
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Date: 2020-09-26 09:37 pm (UTC)no subject
Date: 2020-09-26 09:41 pm (UTC)https://en.wikipedia.org/wiki/British_Columbia_Highway_4
It's on Vancouver Island. So that should help narrow things down. Cross-referencing it with a fault map should pin it down exactly.