Geology
A Brief Geologic History
Today you can see mountains in every direction from anywhere on the Forest. Not surprisingly these mountains weren’t always here, and what we see today across the varying landscapes of the Rio Grande National Forest is very different from what it looked like millions of years ago. The mountains and valleys have seen drastic changes over time from intense volcanic activity to the slow deposition of sediments that created new rock. Mountains were built through uplifts and volcanos, then worn down by glaciers and erosion.
The San Juan Mountains and La Garita Caldera
The San Juan Mountain Range was created by numerous volcanic eruptions starting about 35 million years ago. Hundreds of volcanos covered the landscape, some over two miles tall. The most impressive of them all was the La Garita supervolcano. It erupted about 28 million years ago creating the La Garita caldera. The La Garita caldera is one of the largest calderas and was one of the largest eruptions in the world.
While we don’t have active volcanoes in the area anymore, we can see the remnants of them. A caldera is a large depression left behind by a volcano. When a volcano erupts, magma and gases leave the chamber below creating an empty space. The top of the volcano then collapses inward creating a depression. Crater Lake, in Oregon, is an example where you can clearly see the depression left from the volcano. More locally, looking south from the hills above Creede you can see a dome ringed by a moat-like valley. This is the remnants of the Creede caldera. Snowshoe Mountain sits in the middle of this caldera.
While not impressive to the eye today, the La Garita caldera is impressive in numbers. It’s at least 25 miles wide by 62 miles long. The moat that surrounded the La Garita caldera was partially filled in by other volcanic activity and erosion and so is not as distinct as the Creede caldera. When the La Gartia supervolcano erupted, it spewed 1,200 cubic miles of ash over 9,000 square miles in a layer 100-650 feet thick. This ash is now known as the Fish Canyon tuff. For comparison, the Mount St. Helens eruption in 1980 produced 0.25 cubic miles of ash and the Mt. Vesuvius eruption that covered Pompeii produced about 1 cubic mile of ash.
The Fish Canyon tuff from the La Garita volcano is visible at Penitente Canyon and Elephant Rocks. Wheeler Geologic Area provides a fantastic display of volcanic rocks called tuff from three different volcanos that erupted. In some areas it is easy to distinguish between the different layers of ash as this volcanic tuff continues to erode into spires, fins, and hoodoos.
Other Volcanoes
The Rio Grande National Forest has many more remnants of volcanos, including the Platoro caldera, Summitville caldera, Bonanza caldera, and the Bachelor caldera. These names may sound familiar as they were once prominent mining towns in the area. The vast amount of volcanic activity provided the heat to dissolve minerals from the rocks in water. These minerals later became deposited in cracks in the rock leaving veins of silver, copper, gold, and other minerals.
Overtime these ancient volcanos have been eroded by rains and snows and gouged by glaciers leaving the mountains we see today. Soon ice and snow will cover all the rocky peaks, and the force of freezing water will crack another rock and the spring snow melt will carve just a tiny bit further through the mountains.
The Sangre de Cristo Range
The Sangre de Cristo range was formed through uplift rather than volcanism. These mountains rise straight out of the valley with no foothills. There are major fault lines running on both sides of the mountain. The Sangre De Cristos were pushed up about 5 million years ago and this uplift was accompanied by many earthquakes.
Glaciation further sculpted the mountains on the RGNF. Glacial features including u-shaped valleys, moraines, and other deposits of glacial sediments may be visible to the trained eye but are not as prominent as they may be in other ranges or areas.
Types of Rocks on the Forest
Metamorphic
Metamorphic rocks are a group of rocks that have been changed by heat or pressure. Originally these rocks may have been sedimentary or igneous but then got heated, often by being close to the core of the earth or in proximity to volcanic activity. They may have also been under immense pressure from many layers of rocks being on top. The structure of the crystals change, changing the rock type. Common metamorphic rocks include gneiss, schist, quartzite, and marble.
Sedimentary
Sedimentary rocks are formed from pieces of existing rocks or organic material. Sandstone is formed when loose sand eventually becomes so tightly compacted it turns to rock. Other rocks like coal come from organic materials that were compressed into rocks. Sedimentary rocks are the type of rocks where fossils are found. Conglomerate rocks, like those found in the Sangre de Cristo Ranger are made of many different sized rocks all bound together by a natural type of cement.
Igneous
Igneous rocks are formed by molten rock cooling. Lava, molten rock on the surface of the earth, forms extrusive igneous rocks. Lava generally cools very quickly which doesn't give crystals much time to grow so they are very find grained. Examples of extrusive igneous rocks are basalt, tuff, and obsidian. Intrusive igneous rocks are formed when magma deep in the earth cools slowly. These rocks often have much larger crystals because they have longer to grow as the rock cools slowly. Examples of intrusive igneous rocks include granite, and diorite.
Places to View Geologic Features
Historic Mining Towns
- Creede, Bachelor Loop
- Platoro
- Summitville
- Bonanza
Other Geologic Features
- Chimney Rock, Saguache Park
- Hellgate Canyon, County Road 41G
- Wheeler Geologic Area
- Penitente Canyon (BLM)
- Elephant Rocks, County Road 33
- Creede Caldera, visible from most of the hills/mountains around Creede