The Roots of Pikes Peak

Geoheritage Highlight!

The oldest geologic event at Florissant can be seen today as rock outcrops of the Pikes Peak Granite. This 1.08-billion-year-old rock is more than 30 times older than Florissant’s fossils, and it gives us a perspective about the vastness of time in Earth’s geologic heritage.

 

Beautiful natural boulder sculptures!

NPS/SIP: Mariah Slovacek

The Pikes Peak Granite is famous though you may not know it. The mountain that gives the rock its name is one of Colorado's famous "fourteeners". Most of the monument has this rock supporting the valley, but it is invisible and buried under the Florissant Formation and younger rocks. But the Pikes Peak Granite is the backbone of the monument.

 

The Pikes Peak granite magnified with the key minerals identified.

NPS*

The rock you see here is called the Pikes Peak Granite. Some of the surrounding hills and Pikes Peak itself are also made up of this rock. The Pikes Peak Granite began 1.08 billion years ago as a large, molten, igneous intrusion known as a batholith. Hot magma intruded into pre-existing rocks and then cooled extremely slowly. Large minerals had time to crystalize, forming the coarse-grained rock you see here.

• Minerals of note:
  • K-spar (Potassium Feldspar): Usually pink-colored fedspar that is very common in the Rocky Mountains.
  • Quartz: The most abundent mineral in the Earth's crust.
  • Biotite: A dark variety of mica
  • Hornblende: A common name used for dark amphibole.
  • Plagioclase Fedspar (Sodium-Calcium Feldspar): The second most abundent mineral in the Earth's crust.

 

The Pikes Peak batholith was massive. Its remnants extend from Florissant to Colorado Springs and north toward Denver. Although Pikes Peak itself is not a volcano, the Pikes Peak Granite holds evidence of a possible caldera eruption near Lake George, Colorado, 1.08 billion years ago. Since then, many different volcanic events have occurred through Colorado.

 

Left image
A theoretical schematic of the caldera near Lake George.
Credit: Geocorps: Emily Thorpe

Right image
Modern surface with the exposed subsurface rocks.
Credit: NPS/GIP: Mariah Slovacek

When the Rocky Mountains were uplifted, first 320 to 270 million years ago and again 70-40 million years ago, the rocks above the batholith and part of the batholith itself were eroded away. This erosion erased a billion years of history and continues today.

 

Onion-Skin Weathering

The Pikes Peak Granite often forms rounded and even dome-shaped structures as it erodes. This is due to three main factors: the release of pressure as the rock comes to the surface, ice, and water.

 

Illustration of the buried Pikes Peak Batholith.

NPS/GIP: Mariah Slovacek

Batholiths form and cool under the surface of the earth and are under great pressure from the rock and soil above them.

 

Illustration of the Pikes Peak Batholith unearth by erosion.

NPS/GIP: Mariah Slovacek

As that soil and rock erodes, the pressure is released and the compressed batholith expands and forms tiny cracks within.

 

Illustration of the Pikes Peak Batholith undergoing onion-skin weathering. Insets
show process of chemical formation of clay and freeze-thaw within cracks.

NPS/GIP: Mariah Slovacek

Water penetrates the cracks and undergoes two separate weathering processes. As liquid, the water seeps into the surrounding rock and chemically changes it to clays. The clay further weakens the cracks and makes them prone to break. When the water freezes during the night, it becomes ice and expands in the cracks making them widen a little bit, and form new cracks, each time the water freezes anew. Each time the water thaws it seeps into the new spaces and then repeats the cycle every time it freezes and thaws.

 

Illustration of the Pikes Peak Batholith undergoing onion-skin weathering and the
exfoliation of rock sheets weakened by water and ice.

NPS/GIP: Mariah Slovacek

Over time, the weathering of the rock makes sheets of rock slough off the exposed batholith. As the surface rock is removed, more pressure is also removed from the underlaying batholith and the process begins anew.