Trip Reports

Don't Know Much About Petrology (14-Oct-2002-20-03):
8:00 PM local time, Monday, October 14 (1800 Oct. 14 UTC) 36 09 N 005 21 W. Temp. 75, Humidity 68%, Cloud Cover 0%. At a berth in Gibraltar.

Greetings from the crew of Maverick.

And now about that limestone that we've been finding everywhere. I wouldn't discuss it further were it not for the great number of requests we've had to fill out the geological picture we began drawing back in Eritrea. And is there a greater number than the number one, the mother of all numbers? The person who was inspired to ask for more geology discussions was none other than David Tolmie, an old rockhound in every way. I humor him at the risk of boring our other readers, as he would effortlessly do himself were he here. To get more data for Dave I once again phoned my friend at the University of Antalya, Hassan Ozturk, Professor of Geophysics. [Here it is our duty to remind our readership that Professor Ozturk is a product of the Captain's rich, and at times troubling, fantasy life. Unfortunately, however, Mr. Tolmie is a real person. -Ed.] The Professor was out, but fortunately the maintenance man was able to provide the following information.

When Gondwanaland, one of the best-named continents of all time, began to separate itself from Laurasia in the early Jurassic, around 180 million years ago, breaking up Pangaea, a sea sloshed into the rift between them, that the folks back then named the Tethys sea. It got bigger and bigger and wider and wider and then, wouldn't you know it, Africa and Arabia, now joined by India, changed their minds, as old rocks are as fickle as Mr. Shrode's dining preferences, and decided they missed the comforts of home. So they all headed back to sweet Laurasia and rammed her with all their considerable might, squishing her so passionately that those big mountains in Turkey are the result. And not just them, cuz the Alps and the Himalayas resulted from the same hug.

But while they were away spreading their wild oats, stuff was sinking to the bottom of the Tethys and dirtying up the floor. There are, broadly speaking, three types of sedimentary rocks, rocks that start out as mud or sand and over time harden into stone. These are sandstone, shale or mudstone, and limestone. Real scientists break these down further and add a few others but this level of detail will satisfy the likes of Dave. Sandstone and shale are terrigenous, meaning they come from the land. In California, for example, granite weathers and erodes in the Sierra and is transported to sea by rivers that grind and sort the stones by size, and in effect, by hardness. The white sand on Ocean Beach in San Francisco, the same stuff that inhabits the "potato patch," a river bar outside the Golden Gate that can create dangerous conditions for sailors, comes from the white specks, the silica, in the granite in the mountains. (The specks occur because the minerals of granite, which are really the same stuff as makes up basalt, harden before they reach the surface at different rates, kind of like a hot snow-cone.) The grains of silica are brought down by the Merced, the Feather, and the American rivers, etc. to the San Joaquin and the Sacramento, and out the Golden Gate, where currents and wave action transport them to the beaches. Poor Rodeo Beach in Marin has sand that is weathered from neighborhood rocks, and it's dark in color since we have too little silica in our local Franciscan formation to create the white beaches many of our discriminating readers prefer. Mudstone or shale comes from silt in the river, which, if it isn't dropped earlier, like in the Bay or the Delta where river velocity slows, is transported further out to sea than the sand before it is deposited. It's smaller and lighter and stays in suspension longer. The entire state of Louisiana is mud ripped off from the Rockies or the hills of Minnesota and deposited by the Mississippi, and in the old days ships would sell their ballast of rocks in New Orleans for a tidy profit to be used for pavers or building stone, since there was no other local source

But beyond the sand and mud, beyond the deltas (a word coined by Herodotus for the Nile delta because it's shaped like the Greek "D"), the mud on the bottom comes not from the land but from another source, an eternal rain of the shells of one-celled foraminifera and sometimes other larger animals who make the hard parts of their bodies from the calcium carbonate in the water. This happens primarily in warm, shallow seas like those around the Bahamas. It's difficult to imagine the time involved--since every little guy that that dies after living a full life is so small and the mud on the bottom gets very compressed--in creating a thousand-foot-thick layer of limestone that's a hundred miles wide.

There can be another kind of rain, too, from two different kinds of one-celled animals, radiolarians and diatoms. These make their skeletons or shells from silica instead of calcium carbonate, and the mud formed by their remains becomes chert, a stone that looks a little like milky quartz, as it's made of the same stuff. Chert could also be precipitated directly from seawater and the kind of ribbon chert visible along the road cuts of the Marin Headlands where friends watched as Maverick sailed off into the welter may be of this sort. The origin of bedded cherts is not agreed upon. In any event, cherts generally are found even further out to sea, and in deeper water, than the limestones. But I found a bunch one evening on a sunset walk on Salamis.

It is very likely then, although I'm not going to give your money back if I'm wrong, that the sandstone we saw in Eritrea, and the limestone we saw in the pyramids and high in the mountains of Turkey and in the Greek Islands and the Corinth Canal and the Rock of Gibraltar, the marble-metamorphosed limestone--used in monuments, and the chert I found on Salamis, were originally deposited on the bottom or shores of the Tethys Sea during a period between approximately 180 million years ago and 40 million years ago, at which time the collision between those continents mentioned above and Eurasia, who had lost some weight and changed her name, started to close the sea. Then the collision itself produced and continues to cause all sorts of cool things, such as earthquakes like the ones you see on TV in Turkey and the uplifting of mountain ranges and wrecked ski lifts on Mt. Etna and dead people in Pompeii. And it raised all that limestone from the depths up to the mountain peaks.

But there's another chapter in the story. Before six million years ago the collision, which I should say is drastically oversimplified above to fit into the mind of Dave, and if you want something better consult a professional, the collision, as I say, had cut off the Mediterranean from the Atlantic Ocean. Since the evaporation rate in the Med is more than twice the rate of influx from the rivers and rain that flow into it, it dried up. Perhaps it didn't completely dry up and perhaps it dried up many times and the details are sketchy but at times there were a few basins with deep and very salty lakes. It would have been a desert-like environment much more severe, and much deeper, than our Death Valley, as some parts were well over a mile below sea level. And then sometime around five million years ago the dam broke at Gibraltar and the Atlantic ran into the Med full blast, with the flow that has been estimated by geologist K. J. Hsu at a thousand times grander than Niagra falls, far more spectacular than any on earth today. Even so, it would have taken more than one hundred years to fill up the Mediterranean basin like a big bathtub.

Because of that evaporation rate, even today there is a net flow of water from the Atlantic through the Strait of Gibraltar into the Med, keeping it full, and against this current Maverick will have to sail if she ever wants to get home and see that crusty old geologist, Dave Tolmie.

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