Author’s note: This post is the first in a series of great Earth history moments. Stay tuned for a new post every other week.
Around 6 million years ago, the Mediterranean Sea became separated from the Atlantic. Cut off from the world’s oceans, it began to evaporate. By 5.3 million years ago, there was literally no sea left. 1000 years later, it was refilled in a geologic instant.
A number of discoveries led to the conclusion that the Mediterranean dried out completely sometime in the past. The first came in the 1960s, when seismic studies of the floor of the Mediterranean revealed a unique layer – dubbed the M reflector – across the whole basin. Scientists interpreted it to be a large layer of salt distributed evenly across the seafloor.
Later, in 1970, a leg of the Deep Sea Drilling Project cored deep into the Mediterranean seabed. They found what the seismic data predicted: a hard layer of evaporites – rocks composed of salts.
The only way to get evaporite rocks at the base of a sea is to evaporate water until it becomes so concentrated with salts that they can no longer be dissolved. This forces them to precipitate into a solid form.
Just as enigmatic as the salt layer, engineers mapping the base of the Nile River in preparation for the construction of the Aswan Dam around this time found that carved deep beneath the silty floor of the Nile was a canyon whose ancient base was well below sea level.
The only way for a canyon to be carved into bedrock is for a river to flow through it. But a river won’t cut lower than sea level. This deep canyon meant that Medteranian sea level must have been dramatically lower in the past.
In 1972, Kenneth Hsu, the primary investigator on the Deep Sea Drilling Leg that cored the Mediterranean, authored a paper in Nature concluding that the sea must have evaporated nearly completely to produce such an anomalous layer of evaporite minerals and to have cut canyons so deep. In the paper he admitted it was a “preposterous idea,” but stated that no other explanation presented itself. Read the rest of this entry »
The Adirondacks are something of a paradox. Made from some of the oldest rocks on Earth, they are one of the youngest mountain ranges in existence. Pushing their way through the younger rocks of the Appalachians, this jagged, deformed mess of ancient rock, once trapped deep in the crust, has been rising for the past 15-20 million years. And nobody really knows why.
Over a billion years ago, standing high above the lifeless lowlands of the supercontinent Rodinia, a massive mountain range known as the Grenville Orogen extended from coast to coast – one of the largest and longest lived ranges our planet has ever known. Formed when prehistoric continents collided to form a single and massive landmass, its rocks have since fallen deep into fractured valleys and risen once more. They have formed the floors of ancient oceans, and they have withstood the extreme heat of deep burial. These are the rocks that are forcing their way to the surface as the Adirondacks. This complex history makes them unlike any other mountain range – a lesson I learned the hard way.
As a young and somewhat naive hiker in my freshmen year at Skidmore College, I had my heart set on climbing as many of the Adirondack ‘high peaks’ as possible, those peaks that are higher than 4000 feet. I picked up a map of the high peak region and quickly identified what I felt was a surefire way to conquer as many mountains in one trip as possible – I would traverse the Great Range in two days, allowing myself nine peaks in one trip. I was familiar with the ridges of the White Mountains in nearby New Hampshire, and felt assured that it would be similar to those experiences. There I was able to climb to the highest point of a ridge and slowly descend it, making only slight climbs to ascend the other peaks as I moved forward.
The trip was a categorical failure. Two peaks into the trip, my hiking buddy and I were woefully behind schedule and dangerously exhausted. After finishing only the second mountain of what was supposed to be many more that day, I was both dehydrated and incoherent from the effects of mild hypothermia. (Though the trip was late May, there was still three feet of snow on the ground.) Slurring my words, I explained to my friend that I thought we might have set our sights a bit too high.
Unfortunately we were too high to set up camp – it would have been both illegal and too cold. Returning to camp was not easy though. There were two mountains on either side of us, requiring a significant hike before we could get to a lower elevation. Forced to climb, we ascended both Basin and Saddleback mountains, some of the most challenging hikes in the Adirondacks. One of the most terrifying and beautiful sights I have ever seen as a hiker was the sun setting while we were on top of this final mountain, miles from any safe campsite. Beaten by the mountains, we did make it to camp that night, but ended our trip a day early.
We were entirely unprepared for the conditions, and had no business hiking at that time of year. These issues aside, though, there was a more central problem at hand. The Adirondacks are not like the White Mountains, nor are they like any other mountain range on our planet. The ridges that characterize so many mountain ranges, formed by the fault lines of colliding land, do not exist in the Adirondacks. To tackle all the peaks of the Great Range, a hiker must ascend and descend each peak nearly in full, finding no benefit in a raised line of topography.
This difference is rooted in how mountains form in the first place. The White Mountains, for example, are part of the larger Appalachian mountain range. (The Adirondacks are technically considered part of the Appalachians as well, but only because they are close to the other ranges.) The formation of the Appalachians is typical of most mountain ranges. These mountains trace their origins to a time many hundreds of millions of years after the great Grenville Mountains. Rodinia, the supercontinent which held the Grenville Orogen, began to rift apart about 800 million year ago. The process that destroyed those mountains created the Iapetus Ocean – named after the Greek father of Atlantis.
Around 500 million years ago, the Iapetus Ocean began to close. As it closed, landmasses within the Iapetus crashed into the eastern side of what is now North America. As seafloor was forced under North America, volcanoes formed, erupting through land and forming islands that eventually crashed into the continent as well. This process continued for many millions of years, until 250 million years ago, when the super continent Pangea was formed. As this myriad of landmasses hit the North American continent, they formed long ridges – reminiscent of the ridges of a car’s hood after a head-on crash. They are beautifully clear if you get a chance to fly over them, and they make for easy hiking, as peaks connected by a ridge require less descent and ascent.
The Adirondacks, however, are like a giant wart, pushing its way through the beautifully ordered structure of the Appalachians. A giant dome, the Adirondacks look misplaced on even the simplest of maps. The reason for this is unclear. What is known is that for about 15-20 million years the crust under the Adirondacks has been rising, forcing the younger, more typical Appalachian mountains above to erode away. As they erode and the crust continues to rise, the deepest, oldest rocks are exposed – the Grenville ones. Because these rocks have been subject to one billion years of torture, they have a jagged and disordered topography, making the typical ridges I was used to hiking non-existent.
How fast they are rising is the subject of much debate. Some say they are rising nearly as fast as the Himalayas, thought to be the fastest rising mountain range today. Others say they may not be rising much at all. Even more enigmatic is why they are rising. “Both the existence of current uplift and its modus operandi remain a mystery,” states an official 1995 United States Geologic Survey report on the Adirondacks. The mystery remains unsolved.
The most popular idea is that there is a hotspot under the Adirondacks, creating a pocket of relatively less dense mantle, which, forced to rise, pushes the crust above, and ultimately the Adirondacks, to the surface. This would explain why the Adirondacks are dome shaped, but the hypothesis is hard to test.
What was not hard to test was how different the Adirondacks were to other mountain ranges I had climbed. The disconnected peaks of the Adirondacks are a completely different world compared with the ridge-connected peaks of the rest of the Appalachians. Exceedingly beautiful and unique, they remain my favorite mountains of the many I have visited, but they taught me a cruel geologic lesson. Know the history of your mountains, as enigmatic as it may be, before you try to conquer them.