“Geological time is not money.”
– Mark Twain, Mark Twain’s Notebook
Dr. Michael Stewart leads the way up Hughes Mountain. The rest of his “History of the Earth System” class—over 30 students—follow him in earnest.
Hughes Mountain is part of the St. Francois Mountains. Together with the surrounding Ozark Plateau, they comprise the largest mountainous region between the Appalachians and the Rockies.
Beside the trail, mayapples dominate the forest floor. New poison ivy shoots wait in the shadows, preparing to take their summer throne. Fresh, drooping leaves hang from the oaks. Erin, the teaching assistant for the class and an ecologist at heart, spots a turtle in the underbrush.
Just as I start to think the hike is longer than I remember, the trees give way to a clearing. Here at the top of Hughes Mountain, the rocks are an exotic shade of reddish purple.
Though the students have seen similar geology today, there’s something unique about this place in particular.
“Take a look at these rocks!” Stewart exclaims. “World class,” he tells the students. “This is a world class exposure.”
The class gathers round, expecting a lecture. “Be sure,” Stewart continues, “to identify the rock type and look for any interesting…structures.” He grins. “In the meantime, I’ll be relaxing just beyond that nearby grove of trees.”
Stewart has been ardently guiding the students in their education all day. Now it’s time to let them figure things out on their own.
“What’s the big deal?” one students asks me. “It’s just more rhyolite.”
The Granite-Rhyolite Province underlies an enormous portion of North America, stretching southwest from Ontario all the way to New Mexico. Its part of what geologists call “basement rock,” the igneous or metamorphic rocks that make up the roots of continents.
In most places the Granite-Rhyolite Province is buried. In parts of the Illinois Basin, you’d need to drill through over two miles of glacial till and sedimentary rock to find it.
In the St. Francois Mountains, the uplift of the Ozark Dome puts it at the surface.
“Take a closer look at the shape of the exposure,” another teaching assistant suggests.
The student inspects a rock. “You mean the fiamme?”
“Fiamme” is the Italian word for flames. They’re bits of pumice—volcanic glass with a pitted surface—that were buried and compacted, taking on a lens-shaped, somewhat flame-like appearance. The rock they’re within is called an ignimbrite, which literally means “rock formed by fire rain,” the product of hot ash and rock fragments violently ejected from a volcano.
The rocks here are similar to what one might find near Yellowstone caldera, the largest volcanic system in North America.
The difference is these rocks are much, much older.
Before revealing just how ancient the Granite-Rhyolite Province is, I must mention something: Geologists have a distorted view of time. Or maybe regular folks have narrow view. Either way, it’s difficult to get a geologist to admit that something is truly old. The Ordovician limestones I saw last week were 450 million years young.
The rocks here in the St. Francois Mountains are pushing 1.5 billion years.
Since these rocks were laid down, continents have broken apart, drifted thousands of miles across the globe at rates of just millimeters per year, and collided to form Himalaya-scale mountains—on several separate occasions.
In that amount of time, life could have evolved lungs, put on shoes, and crawled from the ocean three times over, ready to recite its own variation of Hamlet.
The western Roman Empire could have built itself and crumbled three million times.
A few students look to me, hoping for confirmation.
“Yes,” I tell them. “They’re amazing fiamme. But how about you all take a step back.”
Another teaching assistant chimes in. “Look at the rocks you were just sitting on. Why did they make such good seats?”
The unusual rock formation at the top of Hughes Mountain is called Devil’s Honeycomb. It is an example of columnar jointing, and perhaps the best geology of its kind in the Midwest. Usually formed by contraction during the cooling of a lava flow, columnar jointing is most famous in association with other pieces of the Devil’s handiwork, such as Devil’s Tower in Wyoming or Devil’s Postpile in California. It’s what gives these rock formations their bizarre mathematical appearance.
And because math seems unnatural to many young university students, such demonic names are no surprise to the class.
The students rack their brains for a bit. Stewart gathers them all together before their heads erupt.
Then, in a classic example of geologic thinking, he explains why the rhyolite at the top of Hughes Mountain is so special:
“When did the columnar jointing form?” he begins.
“It had to be after the fiamme, right? The joints cut right through them, so by the principle of cross-cutting relationships, they’re younger.
“But the whole thing is weird. The fiamme indicate that this was a huge, explosive eruption. To make fiamme, you need a lot of material to bury and compact the pumice when everything’s still hot.
“However, columnar joints form when a volcanic body’s thickness is small relative to its lateral extent, causing it to lose heat primarily from the top and bottom.
“And the joints grow inward from the cooling surface, meaning the rhyolite couldn’t have been buried by hot volcanic material when it cooled. But again, it had to be buried to make the fiamme.
“So, fiamme and columnar joints in the same rock. Seems like a contradiction.”
Stewart’s eyes scan the crowd. “Any ideas?”
A few students offer up their hypotheses.
He shrugs in response. “Maybe!” he declares. “Who knows?”
The students stare at him, unsatisfied, waiting for a textbook answer.
“Geology,” he elaborates, “is a science. But, as for all other sciences, that doesn’t mean we know everything. There are still questions we don’t have the answers to. Maybe the overburden that created the fiamme was somehow removed before the rhyolite cooled.”
He turns away from the group, taking in the view from the summit. Round, isolated hills rise above flat, green valleys. Rhyolite, resistant to erosion, forms the hilltops, while the valleys are buried by younger sandstone and dolostone.
Five-hundred million years ago, these hills were rugged islands in an ancient sea, waves piling sand against their shores. At least, that’s the current published interpretation.
Stewart twists back around, a hint of a smirk crossing his face.
“Maybe,” he repeats. “Maybe one of you will figure it out someday.”