TPO 9 Lecture 3 Geology
Lecturer: So, continuing our discussion ofdesert lakes, now I want to focus on what's known as the "EmptyQuarter". The "Empty Quarter" is a huge area of sand that coversabout a quarter of the Arabian Peninsula. Today it's pretty desolate, barrenand extremely hot. But there've been times in the past when monsoon rainssoaked the Empty Quarter and turned it from a desert into grassland that wasdotted with lakes and home to various animals. There were actually two periodsof rain and lake formation: the first one began about 37,000 years ago; and thesecond one dates from about 10,000 years ago.
Female Student: Excuse me, Professor. But I'mconfused. Why would lakes form in the desert? It's just sand, after all.
Lecturer: Good question! We know from modernday desert lakes, like Lake Eyre in South Australia, that under the right conditions,lakes do form in the desert. But the Empty Quarter lakes disappeared thousandsof years ago. They left behind their beds or basins as limestone formationsthat we can still see today. They look like low-lying, white or grey buttes,long, narrow hills with flat tops, barely a meter high.
A recent study of some of the formationspresents some new theories about the area's past. Keep in mind though that thisstudy only looked at 19 formations. And about a thousand have been documented.So there's a lot more work to be done.
According to the study, two factors wereimportant for lake formation in the Empty Quarter: first the rains that fellthere were torrential. So it would've been impossible for all the water to soakinto the ground. Second, as you know, sand dunes contain other types ofparticles, besides sand, including clay and silt. Now, when the rain fell,water ran down the sides of the dunes, carrying clay and silt particles withit. And wherever these particles settled, they formed a pan, a layer that watercouldn't penetrate. Once this pan formed, further run-off collected, and formeda lake.
Now, the older lakes, about half theformations, the ones that started forming 37000 years ago, the limestoneformations we see, they're up to a kilometer long, but only a few meters wide,and they're scattered along the desert floor, in valleys between the dunes. So,the theory is, the lakes formed there on the desert floor, in these long narrowvalleys. And we know, because of what we know about similar ancient desertlakes, we know that the lakes didn't last very long, from a few months to a fewyears on average. As for the more recent lakes, the ones from 10000 years ago,well, they seemed to have been smaller, and so may have dried up more quickly.
Another difference, very important today fordistinguishing between older lake beds and newer ones, is the location of thelimestone formations. The more recent beds are high up in the dunes.
Why these differences? Well, there are someideas about that, and they have to do with the shapes of the sand dunes, whenthe lakes were formed. 37000 years ago, the dunes were probably nicely roundedat the top, so the water just ran right down their sides to the desert floor.But there were thousands of years of wind between the two rainy periods,reshaping the dunes. So, during the second rainy period, the dunes were kind ofchopped up at the top, full of hollows and ridges, and these hollows would'vecaptured the rain right there on the top.
Now, in a grassland of Lake Ecosystem, we'dexpect to find fossils from a variety of animals, and numerous fossils havebeen found at least at these particular sites. But, where did these animalscome from? Well, the theory that has been suggested is that they migrated in fromnearby habitats where they were already living. Then as the lakes dried up,they died out.
The study makes a couple ofinteresting points about the fossils, which I hope will be looked at in futurestudies. At older lake sites, there’s fossil remains from hippopotamuses, waterbuffalo, animals that spend much of their lives standing in water, and also,fossils of cattle. However, at the sites of the more recent lakes, there’ sonly cattle fossils, additional evidence for geologists that these lakes wereprobably smaller, shallower, because cattle only use water for drinking. So theysurvive on much less. Interestingly, there are clams and snail shells; but, nofossils of fish. We're not sure why. Maybe there was a problem with the water.Maybe it was too salty. That's certainly true of other desert lakes.
Lecturer: So, continuing our discussion ofdesert lakes, now I want to focus on what's known as the "EmptyQuarter". The "Empty Quarter" is a huge area of sand that coversabout a quarter of the Arabian Peninsula. Today it's pretty desolate, barrenand extremely hot. But there've been times in the past when monsoon rainssoaked the Empty Quarter and turned it from a desert into grassland that wasdotted with lakes and home to various animals. There were actually two periodsof rain and lake formation: the first one began about 37,000 years ago; and thesecond one dates from about 10,000 years ago.
Female Student: Excuse me, Professor. But I'mconfused. Why would lakes form in the desert? It's just sand, after all.
Lecturer: Good question! We know from modernday desert lakes, like Lake Eyre in South Australia, that under the right conditions,lakes do form in the desert. But the Empty Quarter lakes disappeared thousandsof years ago. They left behind their beds or basins as limestone formationsthat we can still see today. They look like low-lying, white or grey buttes,long, narrow hills with flat tops, barely a meter high.
A recent study of some of the formationspresents some new theories about the area's past. Keep in mind though that thisstudy only looked at 19 formations. And about a thousand have been documented.So there's a lot more work to be done.
According to the study, two factors wereimportant for lake formation in the Empty Quarter: first the rains that fellthere were torrential. So it would've been impossible for all the water to soakinto the ground. Second, as you know, sand dunes contain other types ofparticles, besides sand, including clay and silt. Now, when the rain fell,water ran down the sides of the dunes, carrying clay and silt particles withit. And wherever these particles settled, they formed a pan, a layer that watercouldn't penetrate. Once this pan formed, further run-off collected, and formeda lake.
Now, the older lakes, about half theformations, the ones that started forming 37000 years ago, the limestoneformations we see, they're up to a kilometer long, but only a few meters wide,and they're scattered along the desert floor, in valleys between the dunes. So,the theory is, the lakes formed there on the desert floor, in these long narrowvalleys. And we know, because of what we know about similar ancient desertlakes, we know that the lakes didn't last very long, from a few months to a fewyears on average. As for the more recent lakes, the ones from 10000 years ago,well, they seemed to have been smaller, and so may have dried up more quickly.
Another difference, very important today fordistinguishing between older lake beds and newer ones, is the location of thelimestone formations. The more recent beds are high up in the dunes.
Why these differences? Well, there are someideas about that, and they have to do with the shapes of the sand dunes, whenthe lakes were formed. 37000 years ago, the dunes were probably nicely roundedat the top, so the water just ran right down their sides to the desert floor.But there were thousands of years of wind between the two rainy periods,reshaping the dunes. So, during the second rainy period, the dunes were kind ofchopped up at the top, full of hollows and ridges, and these hollows would'vecaptured the rain right there on the top.
Now, in a grassland of Lake Ecosystem, we'dexpect to find fossils from a variety of animals, and numerous fossils havebeen found at least at these particular sites. But, where did these animalscome from? Well, the theory that has been suggested is that they migrated in fromnearby habitats where they were already living. Then as the lakes dried up,they died out.
The study makes a couple ofinteresting points about the fossils, which I hope will be looked at in futurestudies. At older lake sites, there’s fossil remains from hippopotamuses, waterbuffalo, animals that spend much of their lives standing in water, and also,fossils of cattle. However, at the sites of the more recent lakes, there’ sonly cattle fossils, additional evidence for geologists that these lakes wereprobably smaller, shallower, because cattle only use water for drinking. So theysurvive on much less. Interestingly, there are clams and snail shells; but, nofossils of fish. We're not sure why. Maybe there was a problem with the water.Maybe it was too salty. That's certainly true of other desert lakes.
