The Geology of The Grand Tetons

Episode Overview

• The episode presents a chronological journey through the geological history of the Grand Teton mountain range, starting with the formation of its 2.7 billion-year-old Precambrian core.
• It explores key events like the intrusion of massive diabase dikes and a long period of erosion that led to the Great Unconformity, where ancient rock was covered by shallow seas.
• The discussion highlights the modern mountain-building event, explaining how the Teton Fault created the range through extensional forces, thrusting the mountains up and dropping the Jackson Hole valley down.
• Finally, the hosts explain how the iconic, sharp, and jagged appearance of the Tetons was sculpted by massive glaciers during the last ice age, which also formed the famous lakes at the base of the range.

Key Concepts

• Ancient Basement Rocks: The core of the Teton Range is composed of 2.7 billion-year-old metamorphic gneiss and 2.5 billion-year-old granite, which make up nearly all of the high peaks visible today.
• Diabase Dikes: Around 775 million years ago, crustal stretching allowed mafic magma to intrude into cracks, forming massive vertical dikes. The most famous example is the prominent black stripe on Mount Moran.
• The Great Unconformity: After a long period of erosion flattened the landscape, shallow seas flooded the area and deposited sedimentary layers directly on top of the ancient Precambrian rock, creating a major time gap in the geological record.
• Formation of the Teton Fault: The modern Teton Range began forming about 10 million years ago due to extensional forces creating a massive normal fault. This fault has a total vertical displacement of 30,000 feet, thrusting the mountain block up while the valley block (Jackson Hole) dropped down.
• Glacial Sculpting: The final, dramatic shaping of the peaks was accomplished by glaciers during the last ice age. The ice carved the uplifted rock into sharp, angular features like horns, arêtes, and cirques, and deposited moraines that dammed the iconic lakes at the foot of the range.

Quotes

• At 1:57 - "We're going to go in a hot air balloon ride and we're going to have a fast forward button through geological time." - Jesse explains the episode's unique format for exploring the Tetons' history.
• At 4:47 - "Almost all of it. All of the high peaks are this really, really old stuff." - Chris confirms that the dramatic peaks of the Tetons are predominantly made of the ancient 2.5-2.7 billion-year-old rock.
• At 19:40 - "Just the exposed portion of the dyke cutting through Mount Moran, it's enough material, if you melted all of it, it would be enough material to fill in Jenny Lake three times over." - The host provides a powerful visual for the immense volume of the diabase dike.
• At 32:02 - "30,000 feet of displacement... that means the rock unit has been broken and the two pieces separated by 30,000 feet of vertical movement. That's amazing." - The hosts emphasize the incredible scale of movement along the Teton Fault.
• At 39:11 - "Glaciers... they make the Tetons beautiful. You know, glaciers, they make angularity." - The speaker summarizes the crucial role of ice in carving the soft, rounded features of older mountain ranges into the sharp, jagged, and spectacular peaks that define the Teton skyline.

Takeaways

• The iconic peaks of the Tetons are not geologically young; they are carved from some of the oldest rocks in North America, dating back over 2.5 billion years.
• The dramatic rise of the Tetons is a result of a geologically recent process of extensional faulting, where the crust was pulled apart, rather than the more common compressional mountain building.
• The sharp, angular beauty of the Teton skyline is a direct result of glacial erosion, which acted like a sculptor's chisel on the massive, uplifted block of ancient rock.