How Mole Hill in Virginia became a mountain is an old story, but not as old as some geologists think. The reason for that has to do with volcanoes—and may help explain why the U.S. East Coast, so long removed from geologic upheaval compared with the West, still suffers from relatively powerful earthquakes like the one that shook Mineral, Va., and much of the East Coast, in 2011.
Five years ago or so, newly minted professor of geology Elizabeth Johnson needed something for her undergraduate students at James Madison University to study on field trips. Locals suggested the unusual geology of Mole Hill, just a few kilometers from campus, where one could find black obsidian (a superhard rock glass formed when magma cools quickly) or rocks that when cracked open looked as pure white as newfallen snow thanks to the carbonate minerals inside.
When Johnson and her students started to poke around through the dense vegetation swathing Mole Hill, the very texture of the volcanic rock appeared unusual. The igneous rock was fine-grained with small crystals of various kinds, except every once in a while where a relative giant crystal—1 centimeter or more across—intruded. Intrigued, Johnson studied up on the local geology, finding that this is not the first time these interesting igneous rocks had been spotted. As far back as 1899, such obsidian and minerals had been reported in this area. Most other geologists simply assumed they were much older.
But Johnson had other suspicions after having taken the most careful look to date. So she passed the rocks to an expert on magma, geochemist Esteban Gazel of Virginia Tech University. By measuring the abundance of an isotope of the noble gas argon in the rock or its crystals, Gazel and his colleague Michael Kunk of the U.S. Geological Survey found that the magma was much younger than the last known volcanic event on the East Coast—which occurred when the supercontinent of Pangaea slowly pulled apart into North America, Africa and South America some 200 million years ago, forming the Atlantic Ocean in the process. In fact the argon dates suggested that roughly 50 million years ago in the Eocene—when a warmer world of forests stretched from pole to pole and the ancestors of mammals such as bats, elephants and primates first evolved—cinder cone volcanoes dotted Virginia for a million years or two. "After 200 million years ago, you are not supposed to have any magma on the East Coast," Gazel says. But Johnson had found it.
The conventional geologic story renders the East Coast of North America as a so-called passive margin, an area free of volcanoes and earthquakes due to its position within the fractured plates that make up the surface of the globe. Volcanoes are most common at sites where one such plate slips under another, such as the U.S. West Coast or Central America, according to this theory.
But the new research suggests the East Coast should be thought of as more of a passive-aggressive margin. By measuring the levels of magnesium in magma itself, Gazel can tell at what temperature and pressures it formed—and therefore where it came from and what type of volcano was most likely to produce it. The Mole Hill lava shows high levels of magnesium, which is typical of not-too-hot, not-too-cold volcanoes in the Basin and Range out West—roughly 1,400 degrees Celsius molten rock. Mole Hill itself is the remnant of the neck of a nearly 50-million-year-old volcano that has now eroded away. In other words, volcanism made a mountain out of Mole Hill—and Mole Hill is just one of a swarm of such volcanoes in the region, such as Trimble Knob.
But where did the volcano swarm come from? One idea is that there might have been a hot spot, such as the one still forming the Hawaiian island chain, which North America slowly slid over. The hot spot started somewhere in Missouri, migrated toward the coast as the continent shifted and then moved up the East Coast and off Maine in the present day. But there the trail grows cold as no new volcanic islands like Hawaii are known to be forming off the coast of Maine. Plus, the magma itself does not seem to have been hot enough to have created a plume like the ones in Hawaii or Yellowstone. Such plume-related volcanoes require magma hotter than 1,500 degrees C.
Another idea is that, for reasons yet unexplained, there is simply a thinner layer of continental crust shielding the surface from volcanism that runs through the North American continent from roughly New Madrid in Missouri to the coast in Virginia. Perhaps a rift got started, thinned the crust but then failed to fully pull apart. Yet magma in rifting zones is typically around 1,350 degrees C, so this theory also does not quite fit Mole Hill's profile.
The not-too-cold, not-too-hot theory favored by Johnson and Gazel suggests that the roots of the Appalachian Mountains themselves may be to blame. Essentially the remnants of the plate that slipped under what is now the East Coast and shoved up the Appalachians 480 million years ago or so had been heating slowly over all those long years. Roughly 50 million years ago, some of these deep roots beneath the mountains in modern day Virginia simply dripped off the underside of the crust and into the mantle 40 kilometers below the surface, displacing a burst of magma. The relatively young volcanoes like Mole Hill are thus the result of magma that seeped up through the cracks in the overlying rock, in essence further weakening pre-existing faults on the way up. "Magma is really lazy," Gazel explains. "If there is not an easy way to get to the surface, it won't come out."
That hypothesis also has the benefit of explaining why the Appalachian Mountains in Virginia are higher than they should be, rather than having more fully weathered away over the last 480 million years. These mountains may have gotten a rejuvenation treatment late in life. Johnson and Gazel's initial findings have been submitted to Geology.
The presence of such young volcanic rocks has modern-day implications. Such rocks may prove good news for efforts to combat climate change because basalt reacts with carbon dioxide to form carbonate, locking a greenhouse gas in a carbonate mineral that often appears as white as snow. But the finding also has repercussions for modern seismology.
The after-effects of this volcanic outburst are still being felt today, as earthquakes like the 5.8 magnitude quake centered near Mineral, Va. in 2011 still rumble through the faults weakened by the volcanoes. More such earthquakes are extremely likely. And there's no reason that more of the heavy roots of the Appalachians could not drop off at any time, spurring a recurrence of volcanism on the East Coast and an outbreak of lava in the most densely populated regions of the U.S. But don't worry too much. As Johnson says: "If something hasn't erupted for the last 47 million years, you can call it completely extinct."