One of the fun things about science is that sometimes, when you look closely at a phenomenon, you find out that what you thought was fairly simple turns out to be not only complex but just flat-out weird. That was my reaction to something I first heard about only a couple of days ago, which (like the topic of yesterday's post) comes from the realm of geology.
Continental slopes are generally pretty straightforward. They represent a sharp boundary between continental crust (usually thick, cold, and relatively old) and oceanic crust (by contrast, thin, hot, and fairly recent). The slopes are steep dropoffs -- the topography of the ocean floor is no gradual decline down toward the abyss -- and the continental shelves, the shallow regions of varying widths that ring the continents, are actually geologically part of the continent. (They just happen to be covered by sea water.)
So the continental slopes shouldn't be that complicated. They're a narrow transitional band separating shallow regions connected to the continental land masses from the very different geological realm of the deep ocean.
But then there's the Sigsbee Escarpment.
The Sigsbee Escarpment is a stretch of the continental slope in the Gulf of Mexico, south of coastal Louisiana, Mississippi, Alabama, and Florida. The first clue that there was something weird going on there is that the continental shelf north of it is a good bit wider than it should be -- certainly wider than a lot of continental shelf regions. This is great for the fishing industry, which thrives in shallow continental shelf regions. The deep ocean has far less in the way of life, largely due to the fact that the depth makes significant vertical mixing difficult, so nutrients that settle to the ocean floor tend to stay there. Any given cubic meter of surface water over the deep ocean is unlikely to have much living in it beyond single-celled organisms.
Most continental shelves are relatively narrow, but the Sigsbee Escarpment sticks way out into the Gulf, and the reason why has to do with the combination of something that happened 150 million years ago and something that happened two thousand kilometers away.
At the beginning of the Jurassic Period, around two hundred million years ago, North and South America were joined (well, everything had been joined not long before; Pangaea had lasted through most of the Triassic Period). Rifting opened up what would eventually become the Gulf of Mexico, letting seawater into a new embayment that initially was quite shallow. The climate was generally hot, so for the next fifty million years, the evaporation rate was high, and this water became extremely saline, leading to the deposition of huge quantities of crystalline salt on the seafloor.
These salt deposits are found all over the southeastern United States, and what are responsible for the Lake Peigneur disaster in November of 1980. Lake Peigneur is a broad, brackish lake near Delcambre in Iberia Parish, Louisiana, which unfortunately is right above a huge salt deposit that had been mined for years by the Diamond Crystal Salt Company. The problem is, the area is also a prime spot for oil drilling -- oil deposits and salt domes are frequently found in the same geological context -- and a Texaco oil rig drilling in the lake floor accidentally punched through into a cavern that had been excavated by the Salt Company. Suddenly the bottom of the lake collapsed, creating a vortex like water going down a bathtub drain as the entire lake drained into the cavern. The sinkhole swallowed the oil rig, eleven barges, a tugboat, hundreds of trees, and 26 hectares of land from the lake edge. Where the lake had been, all that was left was an expanse of salty mud.
But back to the Sigsbee Escarpment. The salient point here is that this same salt deposit, created during the Jurassic Period, extends offshore. And that's where the second factor comes in.
The Laramide Orogeny is a complex series of events that is mostly responsible for raising the Rocky Mountains. What had been relatively flat terrain, from Arizona up to Alberta, was now rapidly increasing in elevation and steepness. Well, there's a general rule in geology that if you increase the angle at which a land surface sits, you increase the rate of erosion from running water; rivers run faster, can carry more suspended debris, and have a greater capacity for abrasion. The raising of the Rocky Mountains meant that as they were lifted, the forces of erosion started tearing them down -- and all of that pulverized rock had to go somewhere.
Ultimately, any of it east of the Continental Divide ended up in the tributaries to the Mississippi River, and was flushed out into the Gulf of Mexico.
This plume of debris -- some of it from thousands of kilometers away -- settled out over the Jurassic salt deposits, and the weight of it started exerting significant downward pressure. And salt -- especially the saturated salt mush that was at the bottom of the sea -- flows when it's compressed. So like toothpaste squeezed from the world's largest tube, the salt domes squished outward, forming the lobes that are on the southern edge of the Sigsbee Escarpment.
Geologist Harry H. Roberts, of Louisiana State University, writes, "This process continues today. As sediments have been continually added to the northern and northwestern Gulf rim, salt has been squeezed seaward in front of a constantly thickening wedge of sediment. Today, the steep transition between the bottom of the continental slope and the deep Gulf floor, called the Sigsbee Escarpment, represents the old Jurassic Louann salt formation being squeezed seaward over much younger sediments."
So what started out seeming simple -- the steep boundary between continental shelf and deep ocean -- turns out not to be that simple after all, and way more interesting.
But that's how science is, isn't it? Answering one question raises a hundred more, but that's the thrill of it. As physicist Brian Greene put it, "Science is a way of life. Science is a perspective. Science is the process that takes us from confusion to understanding in a manner that's precise, predictive and reliable -- a transformation, for those lucky enough to experience it, that is empowering and emotional."
****************************************
 |
No comments:
Post a Comment