Thursday, March 21, 2013

How Old Is This Rock?

Whenever I go hiking with a group of friends everyone expects me to know everything about every rock we see. “Hey, you’re the geologist!” they all say. “Well, yeah, but…” I hedge, diffidently twirling my booted toe in the sand. 

These folks can sure ask a lot of questions!

Often I know the answers right off and I can dazzle my audience with my extensive knowledge of southwestern Utah geologic trivia. Occasionally they must be content with some sputtered response along the lines of “Oh I knew this five years ago when I was in school!” In this latter case I usually end up winging it. 

It frequently surprises folks to learn that southern Utah is considered a very volcanically active area. There are cinder cones all over the place and so many lava flows that the sheer numbers can make your head spin. It definitely makes my head spin to wonder how those who study such things can keep track of all the flows, eruptions, and dates. 

Looking for lunch on a basaltcovered hillside

Enjoying the rocks with a view
The Broken Mesa Rim trail in the Red Cliffs Desert Reserve tracks through one such lava flow. When I led a hike out there recently I had a general idea of the probable date of the rocks. But I had neglected to do the requisite detailed research the night before the hike. When asked the inevitable age question by my adoring listeners I had to wing it and sputter one of my standard responses. 

I’m not sure. There are so many!”
Twin Peaks lava flow covers the ground – Pine Valley Mountain in distance

After getting home I naturally consulted my trusty geologic map. Within five minutes I discovered we had been hiking through the Twin Peaks lava flow. Coincidently enough this flow had erupted from vents on extensively eroded cinder cones at nearby Twin Peaks. Several dates for the flow are given and they cluster around 2.4 – 2.2 million years ago. 

And it’s a basalt trachyandesite.  Aren’t you glad you asked? 

Now if that’s not a dazzling bit of geologic trivia, I’ll hand over my hand lens and relinquish my rock hammer.


  1. I can often tell the type of rock but not all the ages because I struggle with numbers.

  2. Gaelyn - I have several key rock dates that I make it a point to remember, like the Early Jurassic Navajo Sandstone (190-180 million years ago) and the Lower/Early Permian rocks (290-256 million years ago) which include the Kaibab Limestone. It's what I see most often and so it has just sunk in.

    But it seems I'm always researching dates anyway, checking the geologic time scale, to be sure. The data is easy enough to find so that helps!