Snowball Earth is a popular, controversial idea in which Earth is thought to have been completely covered at least once in ice during the Sturtian glaciation of roughly 718-700 Ma. This glaciation episode, bookended by two others, the Kaigas (~785-728 Ma) and the Marinoan (~670-624 Ma), comprise the period of the Late Precambrian known as the Cryogenian Period and are a precursor to the Cambrian Explosion of roughly 530 Ma.
This is proposed as a hypothesis to explain some sediments seen worldwide that some geologists are having trouble characterizing any other way. Glacially-associated sediments are distinct from the more common wind- and water-driven deposits. Because of the viscosity of ice, it is capable of carrying sediments of varying grain size from dust to megaboulder. Some sediments are sorted out during melting into eskers or loess, but other sediments are dropped where they are when the glacier starts to melt. Advance/retreat periods during melting can push up ridges of unsorted sediments at the glacier head known as moraines.
Skeptics of the hypothesis argue that there are problems with the idea of worldwide glaciation, and their arguments are very well-founded. Those of us who see snow every year are aware of how bright the world can get when snow is on the ground and the sun is shining. This is because snow and ice have a high albedo, or ability to reflect light. If light is reflected rather than absorbed, not much energy is available at surface level to melt snow or ice. It's possible that runaway glaciation would be VERY hard to reverse once established simply because of the albedo argument. I believe some models back this up. Others don't. It's controversial, which is what makes it a compelling story of Earth's past.
There are other arguments, but I strongly suggest reading through the Wikipedia article linked at the top of this post. It's fascinating.
Sediments such as these have been observed in paleoequatorial regions. Using paleomagnetic data embedded in rocks (that hasn't been too far altered by subsequent metamorphic processes), it is possible to extrapolate a paleolatitude of a particular rock formation. This is done by isolating the remanent magnetic signature (thought of as a vector) in the native Fe2O3 (hematite), Fe3O4 (magnetite), and FeTiO4 (ilmenite) crystals present in a rock. Alignment may occur in the internal structure of a crystal during cooling and crystallization of an igneous rock (if it has enough time), or during deposition of iron-bearing sedimentary rocks (if the magnetic field has enough influence to align enough of the grains in situ). If it is possible to establish an age for these same rocks, we know when that piece of continent was at that particular latitude (but NOT the longitude, because assuming a perfect magnetic dipole, the magnetic vector is the same at all longitudes at that latitude).
Better yet if you find glacial sediments hanging out in close association with rock types typically found in tropical environments. Uniformitarianism for the win.
That's what scientists have managed to do with a suite of rocks in northwestern Canada. Tropical sediments and presumably paleomagnetic data (not directly mentioned in the article) were used to identify what could be a smoking gun for Snowball Earth.
As a corollary, some glacial sediments are now found at tropical latitudes, but that plus paleomag provides a smoking gun in favor of plate tectonics. Typical glaciations throughout Earth's history were not at the level of the Cryogenian glaciations.
Monday, March 15, 2010
Sunday, March 14, 2010
Daylight Savings Time
. . . Somewhat guilt-inducing, because I wake up thinking I've slept in an hour later than usual. While I haven't in terms of absolute time, social time indicates that I absolutely have!
Well, at least it's Pi Day. I will not be eating pie, or any number approximately equal to 3.14 for that matter, but I will happily ruminate on the philosophical implications of the ratio of a circle's diameter to its radius.
Nevertheless, today requires some level of productivity - finishing my lecture for Monday, then heading into work so I can cap my samples and get the teaching samples ready for tomorrow's lab, as well as hang around in case any students who might be around have some questions.
Well, at least it's Pi Day. I will not be eating pie, or any number approximately equal to 3.14 for that matter, but I will happily ruminate on the philosophical implications of the ratio of a circle's diameter to its radius.
Nevertheless, today requires some level of productivity - finishing my lecture for Monday, then heading into work so I can cap my samples and get the teaching samples ready for tomorrow's lab, as well as hang around in case any students who might be around have some questions.
Saturday, March 13, 2010
Thursday, March 11, 2010
More Big Quakes in Chile
These are probably aftershocks, but I got these e-mails from the USGS about an hour ago:
Magnitude 7.2, Libertador O'Higgins, Chile 3/11/10 14:39:48 UTC
[This has since been revised down to magnitude 6.9.]
Magnitude 6.9, Libertador O'Higgins, Chile 3/11/10 14:55:30 UTC
[This has since been revised down to magnitude 6.7.]
Magnitude 6.0, Libertador O'Higgins, Chile 3/11/10 15:06:03 UTC
Aftershocks may continue for years after a quake in some cases, but these are strong. I couldn't say for sure whether these are indeed aftershocks or this is a new section of the fault breaking. Not my area of expertise, but I'll post a link if I can find anything on that after my class this morning.
What I'm finding strange about these earthquakes (aside from the third, which doesn't have first motion data and hence no focal mechanisms/moment tensors yet) is that they are not compressional, as you'd expect from the proximity to the subduction zone off of the Andes mountains. They are extensional. I'll have to look into this more after class as well.
As a note, revisions to magnitude are common as more seismograph stations worldwide pick up tremors from an event. The more data we have, the better we can characterize the motions and strength of the event.
Magnitude 7.2, Libertador O'Higgins, Chile 3/11/10 14:39:48 UTC
[This has since been revised down to magnitude 6.9.]
Magnitude 6.9, Libertador O'Higgins, Chile 3/11/10 14:55:30 UTC
[This has since been revised down to magnitude 6.7.]
Magnitude 6.0, Libertador O'Higgins, Chile 3/11/10 15:06:03 UTC
Aftershocks may continue for years after a quake in some cases, but these are strong. I couldn't say for sure whether these are indeed aftershocks or this is a new section of the fault breaking. Not my area of expertise, but I'll post a link if I can find anything on that after my class this morning.
What I'm finding strange about these earthquakes (aside from the third, which doesn't have first motion data and hence no focal mechanisms/moment tensors yet) is that they are not compressional, as you'd expect from the proximity to the subduction zone off of the Andes mountains. They are extensional. I'll have to look into this more after class as well.
As a note, revisions to magnitude are common as more seismograph stations worldwide pick up tremors from an event. The more data we have, the better we can characterize the motions and strength of the event.
Monday, March 8, 2010
More Chilean Earthquake Stuff and Mars
In other news, Concepcion, Chile is now sitting ten feet further west than it did previous to the magnitude 8.8 earthquake.
Only about 3.1 meters displacement, you say? For one event, that is pretty damn significant. That and in the grand scheme of things, there's only a smidgen of crustal shortening and subduction going on there. Amazing to think about how much ocean crust once did exist there before the subduction zone developed. But alas, as the ridge is still actively spreading, the crust must go somewhere!
Something about that whole conservation of mass thing . . .
Then there's this article about Mars. Cool. One of the great questions about Mars is centered around what controlled the erosional and sedimentary features on the planet's surface and what happened to it.
Mars' surface yields some spotty paleomagnetic evidence of possible early plate tectonics (I like to think so, at least. However, it's not universally accepted and other models exist), but it's plainly evident that whatever the tectonic style of the planet, it shut down while Earth was still in the Precambrian Eon (I guess it's considered a "Supereon"?). Mars is just too small to maintain plate tectonics/convection cycles like those of Earth - the heat gradient from core to surface is too steep. I could talk all day about what I think about the formation of the giant shield volcanoes and Tharsis, but that's not what the article's about. Well, it could tie into Tharsis. But it's not directly about it.
Later on, sedimentary features developed - most strikingly, the canyons larger than any found on Earth. Whatever fluids and erosional processes that once controlled this are mostly gone now. It's now being postulated in this article that some of the channel features were carved out by lava.
I can buy this - camped in Snow Canyon State Park in Utah a couple of years ago. It admittedly started as a canyon carved by fluvial (waterflow-driven) processes, but was located proximal to a basaltic magma source as young as 20,000 years old. Eruptions there changed the course of the river - partly by building up topography in some places and carving it out in others. Redirection of waterways carved new, deeper canyons and remnants of older lava flows can be seen near the top of the present day canyon. This is a phenomenon called "inverted topography," where younger rock sits lower than older rock. This is only one way to form inverted topography, but that could be a blog post in and of itself, so maybe I'll save that for another post.
Only about 3.1 meters displacement, you say? For one event, that is pretty damn significant. That and in the grand scheme of things, there's only a smidgen of crustal shortening and subduction going on there. Amazing to think about how much ocean crust once did exist there before the subduction zone developed. But alas, as the ridge is still actively spreading, the crust must go somewhere!
Something about that whole conservation of mass thing . . .
Then there's this article about Mars. Cool. One of the great questions about Mars is centered around what controlled the erosional and sedimentary features on the planet's surface and what happened to it.
Mars' surface yields some spotty paleomagnetic evidence of possible early plate tectonics (I like to think so, at least. However, it's not universally accepted and other models exist), but it's plainly evident that whatever the tectonic style of the planet, it shut down while Earth was still in the Precambrian Eon (I guess it's considered a "Supereon"?). Mars is just too small to maintain plate tectonics/convection cycles like those of Earth - the heat gradient from core to surface is too steep. I could talk all day about what I think about the formation of the giant shield volcanoes and Tharsis, but that's not what the article's about. Well, it could tie into Tharsis. But it's not directly about it.
Later on, sedimentary features developed - most strikingly, the canyons larger than any found on Earth. Whatever fluids and erosional processes that once controlled this are mostly gone now. It's now being postulated in this article that some of the channel features were carved out by lava.
I can buy this - camped in Snow Canyon State Park in Utah a couple of years ago. It admittedly started as a canyon carved by fluvial (waterflow-driven) processes, but was located proximal to a basaltic magma source as young as 20,000 years old. Eruptions there changed the course of the river - partly by building up topography in some places and carving it out in others. Redirection of waterways carved new, deeper canyons and remnants of older lava flows can be seen near the top of the present day canyon. This is a phenomenon called "inverted topography," where younger rock sits lower than older rock. This is only one way to form inverted topography, but that could be a blog post in and of itself, so maybe I'll save that for another post.
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