How the Never Summer Range Formed: Rocky Mountain's Youngest Rocks

The Never Summer Range, on the west side of Rocky Mountain National Park. NPS-RMNP.

Most of Rocky Mountain National Park is granite and gneiss 1.4 to 1.7 billion years old. The Never Summer Range is the exception: it holds the park's youngest bedrock and its only volcanic rock, formed 27 to 29 million years ago. It runs from iron- and magnesium-rich (mafic) to silica- and aluminium-rich (silicic) rocks: basaltic andesite, rhyolite lava and ash-flow tuff that erupted at the surface, and igneous rock that cooled underground.

Geologists study its chemistry to understand the recent evolution of the continent. The Never Summer Igneous Complex is the easternmost trace of the "ignimbrite flare-up", a puzzling burst of volcanism that swept much of western North America in the mid-Tertiary, 40 to 20 million years ago.

The study

Katherine Knox of the University of Colorado at Boulder sampled both the erupted and the underground rock units. She measured their chemistry and their neodymium, strontium and lead isotopes, identified their minerals under a polarising microscope, and dated them by 40Ar/39Ar dating, which uses the radioactive decay of argon in minerals. Combining those ages with the volumes of the erupted rocks, mapped with ArcGIS, she estimated how fast the volcanoes erupted.

Layered volcanic rocks on Specimen Mountain: (A) tuff (volcanic ash), (B) basaltic andesite, (C) rhyolite. NPS-RMNP.

What it found

• Successive eruptions moved from mafic to silicic rock.
• The rocks are relatively rich in lead and poor in titanium, europium and strontium.
• The magma probably came from melted upper mantle, but mismatches in the isotope ratios suggest that parts of the crust melted and mixed in as it rose.
• Differences between the older mafic and younger silicic rocks suggest that successive eruptions drew molten rock from different depths.

This summary rests on reports available when it was written in 2008 and is not park policy or a definitive account of the research. More on the park's research: nps.gov/romo.

Sources

• Ansel Bubel, Geoscientists-in-the-Park (Geological Society of America), "The Formation of the Never Summer Range," National Park Service, Rocky Mountain National Park, October 15, 2008. https://www.nps.gov/articles/the-formation-of-the-never-summer-range.htm
• Corrected: "salicic" to "silicic", and "molted" to "molten".
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.