Antarctica Once Abutted Death Valley 182
Science News has a story of strange bedfellows. It seems that Antarctica was once adjacent to what is now the American Southwest, some 800 million years ago. Earth's continents then formed a supercontinent called Rodinia, predating Pangaea by some 550 million years. "...the ratios of neodymium isotopes in the ancient sediments in the Transantarctic Mountains are the same as those in what was then Laurentia, says Goodge. Also, the hafnium isotope ratios in the 1.44-billion-year-old zircons found in East Antarctica match those of the zircons found in the distinctive granites now found primarily in North America. Finally, the researchers note, the ratios of various isotopes and elements in a basketball-sized chunk of granite found in East Antarctica — a chunk ripped by a glacier from bedrock now smothered by thick ice, the team speculates — match those of granite found only in what was southwestern Laurentia, which today is the American Southwest."
Re:Death Valley is a bitchin place (Score:1, Informative)
However, anyone who rides an unmuffled motorcycle and is proud of its noise is, by definition, at least an inconsiderate jerk.
The phrase is "loud pipes save lives." That's why they add speakers with artificial noise to electric and hydrogen powered bikes. A lot of bikers die when some idiot doesn't bother to look closely, doesn't see them, and switches lanes right into them. The noise alerts drivers and prevents accidents, many of which are fatal. Some bike owners overdo it in my opinion, but like most topics, this one has two sides and you seem uninformed about one of them.
Re:How many supercontinents were there? (Score:5, Informative)
The oldest one proposed is called Columbia, existing from 1.8 - 1.5 bya.
The next widely accepted was Rodinia, existing from 1.3 bya - 800 mya
The next possible was Pannotia, but it didn't last long, only from 600 - 550 mya.
The last one was Pangaea, from 250 mya to 150 mya.
The earliest ones are deduced mainly from paleomagnetism, so there may have been earlier supercontinents that we do not know about due to a lack of rocks that old.
Re:but wait... (Score:3, Informative)
Re:but wait... (Score:2, Informative)
Re:How many supercontinents were there? (Score:4, Informative)
Wikipedia talks about Vaalbara, Ur and Kenorland predating Columbia, which was then followed by Rodinia, Pannotia, Pangaea, Laurasia and Gondwana.
This was all unknown to me until about ten minutes ago, but I'm pleased to see the Pilbara region of Australia (my country) is one of the oldest places on Earth, stretching back 3.6 billion years (the other's in South Africa).
I guess that if you can date the geology, you can talk about the continents, but their shape must be a bit of a mystery.
Re:but wait... (Score:5, Informative)
Why so black and white? Just because we don't know every detail does not mean we have no way to figure out how the earths crust has changed over time. What you are missing is that to a large degree the continents sit in the middle of tectonic plates while the edges of the plates move over and under each other, coastline can change dramatically with the level of the oceans but this has nothing to do with the movement of plates or the location of the continent. Where continents do sit meet the edge of the plates you get mountain ranges. These together with ocean trenches mark the edges of ancient/modern collisions and seperations. Add evidence from fossils, the current motion of the plates, geological features, etc, and it gives you a resonable idea (ie: not a precise map) of what bits have moved where over time. The only thing that I know of where the gross features would be impossible to reconstruct are the land masses that have been subsumed back into the mantle, AFAIK this occurs mainly in deep ocean trenches and not in the middle of a continental land mass (eg: The bedrock in central Australia is ~4 billion years old, The Hawaian islands are an example of a long lived volcano in the middle of a plate).
BTW: Tropical glaciers still exist today but only at very high altitudes.
Re:but wait... (Score:3, Informative)
"Continental drift maps are usually drawn by moving around the outlines of the modern continents for the most part"
actually, no, what is primarily used is geological core examination, where they look at all the layers of rock, at the atomic decay of various isotopes, etc, etc, the idea came from someone looking at the continents, and saying it looks like Africa and south America fit together like a jigsaw piece. just the appearance alone, wasn't enough to 'scientifically' prove or date when areas were pieced together, but it did keep some scientist going, until they could prove that the continents were once pieced together..
oh hey, and if the continental plates move apart at a rate of 3 inches a year within 500 million years a single plate would travel the entire circumference of the earth.
Re:but wait... (Score:4, Informative)
IIRC it's not all that odd at all. Russians are allowed to have a few scientists going around naming things if they want. Wikipedia (which may be my original source) agrees with me: "In geology, Rodinia (from the Russian [rodina], or 'motherland') ..."
Re:but wait... (Score:5, Informative)
As others have pointed out, the maps tend to be drawn with modern features in place to help with orientation and recognition. In point of fact, the East Coast of Laurentia probably didn't exist until 600-700 million years ago (evidence about the exact date is a bit contradictory) when one of the fractures in Rodinia separated Laurentia from Gondwanaland by opening up an ocean called the Iapetus Sea. The Iapetus subsequently closed in a complicated series of events starting about 460 million years ago and then opened up again on a sort of parallel line in the Triassic forming the modern Atlantic. We (think) we know where the East coast of Laurentia was because there is a quite distinctive geologic boundary called Emmon's (Logan's) Line that can be traced from Newfoundland to Georgia where Iapetus sea sediments were pushed up into/onto Laurentia as the Iapetus Sea closed. There are a couple of zigs and zags in Emmons line -- one NW of New York city and one SE of Montreal -- but mostly it follows the course of the Appalachian mountains and lies a bit West of the Easternmost range of the mountains.
Re:Umm... Shouldn't This Be Obvious? (Score:5, Informative)
Actually, the continents are different from the sea floors, and not just because of the water. They do in fact "float around like rafts" over the sea floors, creating new sea floor behind them and pushing it down below them in front into trenches. The sea floors are much younger than land surfaces for that reason.
Re:but wait... (Score:4, Informative)
The Pilbara region of NW Australia is one of two (the other's in South Africa) that dates back to 3.6 billion years or so. There are a few places left with intact geology, but they're far between.
Re:but wait... (Score:3, Informative)
But back to your point about how they knew what it was called, I have a related question. How do they know that Eastern Laurentia had crinkle cut coastlines like Canada? Weren't they formed by glacial activity? How does that happen at the equator?
Most likely, they don't know that, or even think that it did. Continental drift maps are usually drawn by moving around the outlines of the modern continents for the most part, probably because that best communicates which parts went where, rather than amorphous blobs labeled things like "p.s. this is actually Canada".
My understanding would be that the actual outline of the old continents looked nothing like that and we have no way to figure out what they actually did look like.
Actually, it seems to be quite a bit more complicated than just moving things around to see where they match. David Morgan-Mar had a nice rundown of one case as an annotation in irregular webcomic here [irregularwebcomic.net] (He must be really bored sometimes).
In this case, two separate places have geological and biological features that match despite being on opposite sides of the atlantic ocean, so you can well guess those features existed before separating.