SW Arizona News

From January 30, 2023 post.

Coronado Cave - The Backstory

Every year, thousands of visitors make the steep hike up to Coronado Cave. They usually do this without ranger assistance or much by way of information. Have you been there? It is a great experience - just you, your flashlight, and 600 feet of darkness.

Did you ever wonder how the cave formed? Let’s talk.

First off, the geology of our little park, including Coronado Cave is complicated. So many geologic events over millions of years have left a record that even geologists have a hard time interpreting it. 

The basic story is that 300 million years ago this area was periodically inundated by the sea. Where we are now was near the equator then. It was a warm sea, full of sea creatures like coral, crinoids, fish, shellfish, and sharks. As these died, they left more and more bones, shells, and teeth on the floor of the ocean. With time, the heat of the earth underneath and the weight of the water and accumulated debris above compressed the remains to form a layer of limestone, limestone rich in the calcium from the bones, shells, and teeth.

Roughly 30 million years ago, the crust of the earth in this area was being stretched. Fault lines appeared and blocks of crust tipped down into the faults, creating valleys to the southwest and mountain ranges to the northeast - our mountains in this area, the Sky Island Ranges of the Greater Southwest, including the Huachucas.

There was also volcanic activity at different times, too complex to address in detail here. One result of the vulcanism was that the layers of limestone were pushed up and broken. Huge pieces of limestone were embedded in volcanic rock and became part of a strange landscape of ash flows, granite domes, and volcanic materials that contained older components of limestone. 

The vulcanism heated and deformed the limestone, destroying most fossils, and changing the sequence of layers. Fossils and stratigraphy are used by geologists to identify and date formations. Their lack in the park’s limestone makes it difficult to definitively identify the limestone chunks scattered around, badly out of any sort of stratigraphic order.

Attached are some pictures of the limestone layer near the Cave. See the captions.

So, that gave us the limestone we need for a cave. We will continue this post tomorrow when we will address how the cave was created in the limestone.

Original source can be found here.