Geology Reference
In-Depth Information
The long plod out of the canyon still rang in my head as I returned the topic to the shelf
and stepped back outside. I savored the view and my day immersed in geologic time. Read-
ing about earth history is one thing; to see and feel it for oneself is another.
I thought back to the beginning of my day, just after dawn. The towering rock walls rising
above the bottom of the canyon baked in the early morning light as they've done for count-
less years. My knees still ached from the hike down two days ago; and the trail rising a
vertical mile ahead promised another brutal hike under the Arizona sun. There was no al-
ternative. I was committed to climbing out of one of the deepest holes in the world, passing
through time from the dawn of life in the depths of the canyon to the modern desert at the
top.
I approached the Colorado River, the clear turquoise water marking the start of the trail
back up to the canyon rim. Watching the river flow beneath me as I crossed the footbridge,
it dawned on me that the sediment-trapping Glen Canyon dam almost a hundred miles up-
river robbed the river of the sand and erosive power that together cut a narrow slot into the
hard rock exposed along the canyon floor.
Halfway across the river, at the far side of the bridge I saw a tunnel enter the rock wall
rising from the river's edge. I entered it and felt like I'd stepped back into deep time.
In the smooth rock walls I saw the signature of abrasive sand-charged floods surging
down the canyon. The surface of the hard, crystalline Vishnu Schist was a polished face
made of intergrown quartz, feldspar, and mica stretched and folded at high temperature
and pressure, deformed into great swirling patterns. Deep within the earth, below a now-
vanished mountain range, the schist in front of me had crystallized long before dinosaurs,
about a third of the way back through geologic time. But it didn't start out as hard rock.
Ghost beds of sand lie preserved as light-colored, quartz-and-feldspar-rich layers sand-
wiched between dark layers of ancient mud now baked into aluminum-rich mica and gar-
net. This layering is a telltale sign that the schist formed when the sand and mud of an an-
cient seabed were buried deep enough to recrystallize and deform like melting ice cream.
To get hard rock to flow requires both extreme heat and high pressure. Recrystallizing
and deforming the particular combination of minerals in the Vishnu Schist takes temperat-
ures of 900-1300°F and more than three thousand times atmospheric pressure. Geologists
know from temperatures measured at the bottom of deep drill holes that it gets 104-122°F
