Geology of Rocky Mountain National Park
The creation of Rocky Mountain National Park has been over a billion years in the making!
Geologic History/Setting
1.7 billion years ago, during the Precambrian Era, the oldest metamorphic rocks (such as schist and gneiss) were being formed. Shortly after that, relatively speaking, at 1.6 billion years ago a large volume of magma pushed into the older rock creating what is known as the Boulder Creek Batholith. This caused regional metamorphism and created the basement igneous and metamorphic rocks found within the park.
During the Paleozoic era (544-245 Ma), inland seas covered much of present-day North, depositing thick layers of marine sediments that would later turn into sandstone and limestone. At about 285 million years ago, a mountain building processes raised the ancient Rocky Mountains. This ancient mountain range was much smaller than the modern Rockies, only reaching up to 2,000 feet high and stretching from Boulder to Steamboat Springs, Colorado. Over the next couple hundred million years the ancient Rockies eroded away, leaving behind sediment and a much less rugged landscape. 100 million years ago the entire state of Colorado and much of middle North America was submerged under the Western Interior seaway. This flooding left behind large amounts of sedimentary deposits, like the Pierre Shale and Fox Hills Formation (sandstone). Starting 75 million years ago and continuing through the Cenozoic era (65-2.6 Ma), the Laramide Orogeny (mountain-building event) began. This process uplifted the modern Rocky Mountains, and was soon followed by extensive volcanism ash falls, and mudflows, which left behind igneous rocks in the Never Summer Range. Another period of uplift and erosion during the Tertiary period raised the Rockies to their present height and removed significant amounts of sedimentary deposits and revealing the much older basement rocks. Over the last 300,000 years there were two major periods of glaciation: The Bull Lake Glaciation period occurred from 300,000-127,000 and the Pinedale Glaciation Period occurred from 30,000-12,000 years ago.
Glaciation
Glaciation is one of the strongest erosional forces on the planet and is responsible for shaping Rocky Mountain National Park as it is today. In the last 700,000 years, there have been at least 6 major glaciation events, with the two most recent (Bull Lake and Pinedale) causing the most easily noticeable alterations to the landscape. Glaciers are massive amounts of ice and snow over land that form in places where more snow accumulates (the accumulation zone) in an area during winter than is lost during the summer (the ablation zone). Glacial erosion is very strong because the massive ice blocks apply a formidable downward force on the rocks beneath them - enough to carve, crack, and push rocks of any size down the mountain (collectively known as till). Ripped up rocks can be picked up and incorporated into the ice and can travel along for the ride within the glacier, scraping lines (striations) into the bedrock as the glaciers travel across the land and leaving behind evidence of the direction the glaciers dragged them along. Rocky Mountain National Park is defined by its many broad U-shaped valleys instead of steep V-shaped valleys which come from rivers and streams carving out steep canyons. At the edges and end of these valleys are depositional features called moraines (lateral moraines along the sides of the glacier and terminal at the end of the glacier) which are the dumping grounds of glaciers, composed of rocks of various sizes and glacial flour that were once trapped in the ice. Moraines indicate the size of the glacier and they show how far the glacier flowed and how high in elevation it reached before the ice melted. Scientists have grouped glaciers into three categories: cirque glaciers, valley glaciers, and continental ice sheets. Valley glaciers typically form at the top of a narrow (stream) valley and slowly spread downward. There are no more valley glaciers in Rocky Mountain National park today but they were abundant about 15,000 years ago. The only remaining type of glacier in Rocky Mountain National Park is a cirque glacier, which is a small glacier (sometimes the remnant of an old valley glacier) that occupies the bowl shape within a small valley. Continental ice sheets are the largest glacier type, up to kilometers thick, and did not exist in this region. Only two continental ice sheets exist on Earth today, in Greenland and Antarctica.
Why are the Rockies weird?
The modern-day Rocky Mountains are considered “weird” by geological standards. Most mountain ranges occur at tectonically active spots where tectonic plates collide (convergent plate boundary), move away from each other (divergent plate boundary), or slide past each other (transform plate boundary), The Rockies, however, are located in the middle of a large, mostly inactive continental interior away from a plate boundary. Furthermore, the mountains that this region would be expected to support would only be about half the size of the mountains we see today. How can this be? After years of research, geologists have a better understanding of their formation by studying ancient plate tectonic movement off the coast of California. At the beginning of the Laramide Orogeny roughly 70 Ma, a small tectonic plate made of more dense oceanic crust began to slide underneath the North American plate very shallowly. This shallow subduction angle meant that the Farallon Plate could have reached farther east under the continental interior before plunging deeper into the mantle, releasing water into the lithosphere above. Water lowers the melting point of rock, so this newly melted magma likely migrated upward into the lithosphere above the sinking Farallon Plate. After burial from sedimentary rocks from the Western interior seaway and then the pyroclastic material from this volcanism the Rocky Mountains were essentially buried. Erosion from glaciers and rivers like the Arkansas and South Platte removed thousands of feet of this less robust sediment, leaving behind the hard basement granites and gneiss that makes up the core of the Rockies.
Figuring out how the Rockies are able to stay standing at their size was another story. For mountains to be stable, there must be a crustal root underneath them that is thick enough to support the weight of the mountains. This mechanism is essentially the buoyancy of the lighter continental crust on top of the dense mantle underneath it. No definitive answer has proven exactly what is keeping the Rockies afloat yet, but it is believed to be a combination of very dense crust underneath the mountains (Pratt isostasy) and hot underlying mantle supporting the range’s weight.
