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Archive for the ‘natural history’ Category

Farmland Volcanoes

 

The mound you see here is Mole Hill in Northern Virginia, a bit over an hour away from Thomas Jefferson’s Monticello. 48 million years ago it was a volcano.

It has obviously worn down quite a bit since then, but the geological processes that gave it birth are still active. The Journal of Geophysical Research released results of a study that found parts of the mantle below this region are peeling off, weakening and destabilizing the crust above. This is what set off the 2011 earthquake that rattled Washington, DC and a good chunk of the Southeastern U.S.

The area sits close to the middle of the North American plate and because of that it’s normally stable. But the evidence revealed by the study suggests the mantle erosion is continuing and will eventually produce more earthquakes. No timetable on that – it might not even happen in our lifetimes.

But the slow grind of plates and the interaction of mantle and crust will continue, and as sometimes happens in these things, can open up fissures that allow hot magma to rise to the surface – giving birth to a new volcano.

http://blogs.agu.org/geospace/2016/05/03/scientists-find-likely-cause-recent-southeast-u-s-earthquakes/

Scientific American summarizes the history of east coast volcanoes, presented as detective story that takes you through time and shifting landscapes.

http://www.scientificamerican.com/article/recent-east-coast-volcano/

There are two extinct volcanoes in Virginia. Mole Hill is one; Trimble Knob is the other.

Trimble Knob
https://en.wikipedia.org/wiki/Trimble_Knob

Drive-by video of Trimble Knob
https://www.youtube.com/watch?v=TQm8mvBXvPI

Overhead view of Trimble Knob

 

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Written by Influential Prose

May 5, 2016 at 3:26 am

Grenville Rocks

Just outside Baltimore, there’s a trail in a state park that winds down toward a small river. The river descends down a short rapids, gliding over rock. The rock is unique; it’s over a billion years old. It’s known as the Baltimore gneiss. This is what it looks like:

The DC metro area has been through four mountain-building periods in the past billion years, give or take a few hundred million. The first one raised the Grenville mountains. Over eons, they drifted around the Earth aboard a tectonic plate, eroding, worn down by wind, water and ice. There were four global glaciation events between 900 and 600 million years ago. The ice certainly extended as far as the DC area during these events, and may have girdled the entire planet. In any event, the Grenville mountains were scoured down and the roots that remained became the “basement rock” of the east coast.

Later there was a rifting, a sort of splitting of the Grenville rock, with some blocks being dragged out to sea. Magma welled up and spread, then cooled to form a new layer of rock 2,000 feet thick in some areas. These flows formed the Blue Ridge province, and you can see exposures of that rock at Catocin mountain.

Later another plate began converging on the North American continent, with the ocean floor diving beneath the plate. This generated a chain of underwater volcanoes topped by islands running north and south called the Chopawamsic Arc, similar to the arc of islands in southern Alaska. The plate and these volcanoes eventually converged on the eastern North American coast, sweeping up everything on the sea floor in between, carrying it all back onto the continent where it was raised and mashed and mixed with existing rock. This event created the Taconic mountains.

The Taconics wore down. The Acadian mountains formed when continental fragments collided with the east coast. This episode created much larger mountains than the Taconic. To give you a sense of scale, the erosion of the Acadian mountains created pile of sediment to the west. That pile was 9,000 feet high in central Pennsylvania, sloping down to 1,000 ft in Ohio. That was just the stuff left over after the mountains wore down.

The last period of mountain-building was the biggest one. This time, 250 million years ago, Africa and North America collided as all the other continents came together and formed a single supercontinent known as Pangea. This created the Appalachian mountains, and they were as tall as the Rockies and the Alps when fully formed. They’re much smaller now, of course – that’s what 250 million years of constant erosion does to mountains.

 

Throughout all of this, the billion-year-old Grenville basement rock has been squeezed, heated, folded and torn apart. It’s metamorphic rock, with complex layers. Most of it still lies deep beneath layers of other rock. But there are some areas around Baltimore where you can see it, and Dylan and I did that today.

This rock is older than multicellular life, at least 400 million years older. It was here 500 million years before land plants existed. Our solar system, along with the entire Milky Way galaxy, completes one rotation around the galactic center every 250 million years. This rock has made the trip four times.

Dylan found some rock fragments at the site and brought them home. The rock has been in the water, it’s hydrated and its age shows. It’s very brittle and flakes easily. As we were leaving, one of the rocks he collected split in half. In that moment, he was gazing on the surface of rock that no one, anywhere, had ever seen before – formed a billion years ago, and only now exposed to light and a 16-year-old’s wondering eyes.

 

It’s been a good day.

Written by Influential Prose

June 22, 2015 at 7:11 pm

A River Runs Through It

 

 

What you see here is over 300 million years of geological history at Goosenecks State Park in Utah. You’ve seen similar vistas before, such as looking into the Grand Canyon, and there’s a reason for that.

At the bottom of these chasms is the San Juan river, and at a glance you might imagine the river slowly carving its way down through rock over eons. It didn’t happen that way. What actually happened is much more interesting.

You’ve seen photos of rivers that meander back and forth in a form called oxbow tails, such as this one in the Innoko National Wildlife Refuge in Alaska:

 

These develop in areas that have low gradients, i.e., the upriver areas are very slightly higher than downriver, just enough for the water to slowly flow downward. The San Juan developed in that way, over 300 million years ago.

But rivers like these don’t cut into rock. They actually tend to collect and carry sediments, then drop them at turns, which is why it ends up winding back and forth.

The continental crust underlying the San Juan river broke off from the supercontinent Pangea and wandered around the globe. It’s still moving west, at about the speed fingernails grow; if you’re living in North America, you’re along for the ride right now.

And so through eons, about 280 million years, the river placidly plodded along, providing dinosaurs and other animals with drinking and bathing water.

Then the mountain building began, in fits and starts, over millions of years. Today that process is called the Laramide and Sevier orogenies. A thin dense layer of underwater basalt rock that makes up the Pacific plate began diving under North America. The angle of the collision was very shallow, but still powerful enough to squeeze and deform the rocks that make up the continental crust.

That compression created the column of mountains that stretch from Canada down through the US and into Mexico. The average thickness of continental crust runs between 22-25 miles. Imagine, if you can, the epic scale at work here; much of the western basin, an area that was once the bottom of a shallow inland sea, being slowly pried upwards. You can duplicate the pattern on a smaller scale by pushing a piece of tablecloth.

The new mountains transformed the gradient of the surface water’s flow. Upriver was now significantly higher than downriver, which in turn generates faster water flow.

Normally when water runs fast, it flows relatively straight, creating long classic valleys. But this river was already formed. The new, faster flow was strong enough to carry sediments out and further downhill, but not strong enough to alter the shape of the river. So it carved down through the rock…in 20 million years.

So what you see here, in a sense, is a region that has been power washed by an incessant high-flow river running 24/7 for 20 million years, steadily working its way down through 300 million years of layers. This all happened in the last 15% of the river’s existence.

So it doesn’t take 300 million years to carve this deep channel. Twenty million years and fast water is enough to make it happen.

Written by Influential Prose

May 10, 2015 at 10:58 am