Geology Reference
In-Depth Information
Because of this, all the
206
Pb in zircon had to have come from the decay of
238
U, since
the mineral cooled. Geologists use a mass spectrometer to measure the amount of
206
Pb
and
238
U in a grain of zircon and determine the age of the rock from the ratio of the two
isotopes.
Continuing along the trail, thin bands of pink granite rise like fossilized stripes and
cut through the swirling Vishnu Schist. These narrow lines of granite, called dikes, have
uranium-lead ages of up to almost 1.7 billion years and neatly truncate the banding in the
schist, adding a geometric flourish to the fluid forms crystallized in the inner canyon's rock
wall. I could tell the schist is even older because the granite dikes cooled in cracks within
it. The schist was already there when the granite cooled.
I felt like an ant crawling along the narrow trail as I snaked my way out of the inner
gorge. Once I gained enough elevation, I could look down on the river and across to the
other side. There, I could see a sequence of tilted rock layers on top of the Vishnu Schist
and tucked in below the flat-lying Tapeats Sandstone that defined the upper lip of the inner
gorge. Pitched up at a jaunty ten- or twelve-degree slant, this stack of limestone, shale, and
quartzite (a hard rock made when sandstone is heated deep within the earth) records the
changing depth of water in an ancient sea, deep-water limestone on the bottom giving way
first to shale made from offshore mud and then to sandstone from a fossilized shoreline.
Whether eroded by rivers, wind, or waves, the truncated upper surface of the schist, to-
gether with the hardened marine sediment sitting right on top of it, is evidence that rocks
once buried miles underground were brought to the surface, exposed to the elements, and
then buried again deep below the bed of an ancient sea. After all this, the whole package got
tilted up and planed off by erosion for a second time before being capped by the sediment
composing the still flat-lying rocks rising far above. I could see it all laid out in the cliff,
right across the river—two rounds of uplift and erosion buried beneath a three-thousand-
foot-high wall of rock. An ancient story only unveiled because the Colorado River carved
the Grand Canyon.
The striking outcrops of the inner gorge illustrate several simple rules geologists have
used for centuries to read the story of rocks the world over. The first is that layers of sedi-
mentary rock that accumulate in depositional environments, like sandstone and shale, rep-
