Friday Headlines, January 10, 2014
THE LATEST IN THE GEOSCIENCES
Mount Sinabung erupts
Earth’s ancient crust was… saggy
When Mole Hill was a mountain, it was a volcano
I admit, when I read this headline out loud, I got hungry. Mount Cinnabun?
But then, I remembered that I’m allergic to cinnamon, got over my disappointment and read on.
Indonesia is an island nation frequently rocked by earthquakes and volcanic eruptions. This makes perfect sense in the plate tectonic scheme of things.
The India plate is being subducted under the Sunda Plate, resulting in the formation of the Sunda Trench and the islands of Indonesia, including Sumatra.
Sumatra would be an example of an island arc that forms due to subduction.
Thus, one would expect Indonesia to be hit hard with earthquakes and with volcanoes because of its plate tectonic setting. And it is.
The crust is the thin outer part of the Earth upon which we reside. Today this surface of the Earth is continually remodeled by the processes of plate tectonics, in which the crust and upper part of the mantle (the lithosphere) move parallel to the Earth’s surface. Modern plate tectonics involves the consumption of old crust in subduction zones, and the formation of new crust at spreading centers.
About 4 billion years ago, in the Archean Eon, the Earth’s crust was different. The mantle was hotter and the crust (which was thicker than modern crust) was mostly formed of magnesium-rich minerals. Unlike modern crust, which is less dense than the underlying mantle, Archean-aged crust was more dense than the mantle.
Being more dense than the mantle caused the Archean crust to frequently sink into the mantle. Thus remodeling and recycling of the crust involved vertical movements, rather than the modern horizontal movements. The Archean crust would sag and melt into the mantle in a process called delamination. Modern continents as we know them did not exist.
Just to be clear, this headline is referring the the east coast of North America. There are plenty of eastern coasts that have volcanoes, but not on North America.
The west coast of North America is part of the so-called “Ring of Fire” that surrounds the Pacific Ocean basin. This is due to subduction of the ocean floor beneath the continents that surround it. These continental margins are marked by massive earthquakes and volcanoes because of the great tectonic activity.
The east coast – or better, the entire coastline of the Atlantic Ocean – is called a passive margin, because earthquakes and volcanoes are notably absent (though we do still have earthquakes from time to time).
But were there ever volcanoes along the East Coast of North America? The answer is yes!
When the continents came together to form the super continent of Pangaea, there was subduction along the east coast of North America, along with volcanoes and earthquakes.
Pangaea began to break up about 200 million years ago leading to the configuration of continents that we know today.
But here’s what’s interesting. The last time there were active volcanoes on the East Coast was about 50 million years ago, well after the breakup of Pangaea and the formation of the modern passive margin.
So how did that happen? And what evidence is there?
There’s this place called Mole Hill in the state of Virginia. At Mole Hill are curious rocks that could only have come from volcanic eruptions. Modern argon dating methods applied to these rocks shows that they’re only about 50 million years old. Mole Hill is just one of several 50-million-year-old volcanoes in that area.
But then, why or how would there be volcanism then?
There are several hypotheses, including a hot spot (similar to the Yellowstone hotspot) that might have moved beneath North America at that time. It’s possible also that the crust is simply thinner in that area for some unknown reason, perhaps by rifting that started but didn’t finish. Or perhaps the lower part of the lithosphere in that region dripped off, allowing magma to make it to the surface.
There’s more research to be done here for sure.