Meteor Crater in NH?

Sharp-eyed reader Rogers George was playing with the tornado map I posted, and saw this:

I zoomed out a bit on the map, and I see you have a meteor crater a bit south of there! http://maps.google.com/?ie=UTF8&t=h&ll=43.11151,-71.184196&spn=0.138099,0.363922&z=12

That impressive circular structure sure looks like a meteor crater, doesn't it, but it's the Pawtuckaway Ring Dike. First time I saw it, it fooled me too.

But a ring dike is just as cool: In geology, a Dike is a sheet-like intrusion of rock that cuts through another rocky body. Dikes can form when molten rock oozes up through a weak seam in a shallower rock body, widening the seam and filling it with the molten rock from below. When everything cools, the intrusive rock appears as a band or wall (aka "dike") , surrounded by the original rock.

Here's Wikipedia closeup of a small section of a picture-perfect exposed dike in Alaska: The dark rock flowed upward through cracks in the lighter-colored rock:




The cracking and filling happened when these rocks were far underground, perhaps as part of large magmatic earthquakes. The old surface has worn away and the rocks have broken, so we now see the exposed dike.

Ring dikes are formed when a dome of molten magma rises in a slow-motion bubble from deep within the earth (think of a lava lamp). The solid rocks surrounding and above the rising bubble are stretched and broken to make room for the ascending mass. Magma can then flow upward through the cracks that open up. Because the rising magma bubble is roughly circular, the field of dikes surrounding the bubble will also be roughly circular.

Eventually, the magma bubble cools down. Erosion sets in and lowers the surface down to the level of the previously-buried dikes.

Remember that these dikes are intrusions of one kind of rock rising through a different, older rock. Because the rocks are different, the erode at different rates, which eventually creates and exposes the structure that we call a ring dike.

The Pawtuckaway Ring Dike is one of three enormous ring dikes in New Hampshire, including one of the largest and best-known ring dikes in the world: the Ossipee Mountain dike.

When Earth's original supercontinent, Pangaea, split into separate tectonic plates, it created fractures and weaknesses in the rocky foundations of the newly-forming continents.

During the Jurassic age--- the classic "Age of the Dinosaurs" in popular culture--- what's now New Hampshire either slowly drifted over a magmatic "hot spot" (exactly the way Hawaii does today), or "rifting" stretched the crust thin. Either way, magma approached the surface as giant blooms and bubbles, fracturing and diking the rock above. We see the remnants of those magma blooms as today's Ring Dikes.

The area was pretty active, way back when. There were other even larger magma blooms that didn't approached the surface, but slowly cooled in vast underground lakes--- magma seas, really. The molten minerals cooked, congealed and crystallized into grainy rocks--- "grain-ite," or granite. Erosion has now exposed that once-subterranean rock, and New Hampshire is now known as "The Granite State."

Where the magma headed towards the surface, NH grew ring dikes: The modest one at Pawtuckaway, a smaller one at Gunstock, and that truly enormous one in Ossipee; one of the largest and best-known ring dikes in the world. On maps, it looks like a round-based mountain.

Check this out, and toggle between topo and satellite images. the dike structure is huge!

http://tinyurl.com/5zk438

You probably don't normally think "volcanoes" when you hear "New Hampshire." But until a week ago, you probably didn't think "tornadoes" and NH, either. It's quite a place! :-)

http://www.nhgeology.org/default.htm

http://www.dred.state.nh.us/divisions/forestandlands/bureaus/naturalheritage/Pawtuckaway.htm

http://wikimapia.org/85750/

http://www.google.com/search?q=Ossippee+%22Ring+Dike%22

http://www.google.com/search?q=pawtuckaway+"Ring+Dike"