What is Andesite Breccia?

Posted by Alexandra Matiella Novak Dec 3, 2008

On a recent trip to Nevada, my husband and I decided to take a tour of the Hoover dam. As always when we are exploring new areas, my husband challenged me to identify the rocks around us. From the car - also known as "road-side geology" - it was hard to tell. But as we toured the inside of the dam, I was able to get right up to the bedrock. I could tell at least one thing - it was definitely some kind of pyroclastic flow, meaning that it had originated from an exploding volcano. I could also see lapilli (individual fragments of rock, including pumice) in the rock.

Later, while watching a show on the History Channel about Hoover Dam, I heard the name "andesite breccia" to describe the type of rock that the Hoover Dam was built into. The "andesite" part made sense to me - andesite is a type of lava rock. The "breccia" part, however, did not make sense to me. I'd always associated the term "breccia" with sedimentary rocks. A breccia is a type of conglomerate that is made up of angular clasts.

I decided I needed to refer to my trusty rock bible, "Rocks and Rock Minerals", by Richard Dietrich and Brian Skinner. This is the book that was assigned in my first Geology class, called Mineralogy and Petrology. I looked up "breccia" in the index, under which there were many kinds of breccia, including sedimentary breccia and, there it was, pyroclastic breccia.

Pyroclastic breccia forms as a result of magma being extruded violently during an eruption and, in the process, ripping out chunks of the magma chamber that are solid rock. This was the lapilli that I could see in the rock. Since the andesite itself contained angular-shaped lapilli, geologists named it "andesite breccia".

This is why being a geologist is so fun - a simple tour of Hoover Dam turns into a quest to find out exactly what kind of rock the dam is built into. Or, as my fellow geology student from Minnesota used to say, "You betcha, breccia!"