Geology John investigates an extinct volcano

Tucson Mountain Park

70 million years ago, while dinosaurs roamed the earth, Tucson was home to a massive volcano whose remains can be studied in Tucson Mountain Park and Saguaro National Park West. The volcano erupted explosive ash and lava of the type that demolished Mount St Helens in 1980.

Eruption of Mt St Helens 1980
Eruption of Mount St. Helens, May 18, 1980

To see evidence of these cataclysmic events, all you need to do is put on your hiking boots and drive about 6 miles from downtown Tucson to the Sarasota Trailhead. Many well-marked trails are available – ours took us to the summit of 3300 ft Mt Ringtail (about 7 1/2 miles roundtrip).

Geological Background

Calderas form when magma chambers are partially emptied during large eruptions. The land surface subsides and the area above the magma reservoir collapses. The Tucson Mountains Caldera is estimated to have been about 15 miles in diameter – more than double the size of Crater Lake in Oregon.

Diagram of a volcanic caldera
Our hike traversed volcanic breccias and welded tuffs

Our hike took us through a cross-section of Caldera rocks from volcanic breccias to welded tuffs.

Volcanic Breccias

The low-lying ground on our hike is composed of easily eroded volcanic ash deposits many of which contain chunks of rock that cascaded down the slopes of the caldera. Geologists call these inclusions xenoliths (from the Latin/Greek: xeno (foreign) and lith (rock) – ash deposits full of xenoliths are called pyroclastic or volcanic breccias.

Xenolith in volcanic breccia
Volcanic breccia about 1/2 mile north of trailhead

Cavity left by a spherical xenolith in Tucson Mountain Volcanic Breccia.

Early reporters used the very appropriate collective name Tucson Mountain Chaos for this complex mixture of rock types.

Have fun checking for xenoliths of different rock types that tumbled into the caldera.

Welded Tuffs

Hot suspensions of particles and gases flowing rapidly from a volcano are called pyroclastic flows or by the evocative French term nuée ardente (glowing cloud). When the ash and pumice particles are sufficiently hot after settling on the ground they fuse together to form welded tuff.

Welded tuff with flattened pumice
Outcrop of welded tuff from Mt Ringtail.

Welded tuffs (also known as ignimbrites) are hard rocks that resist weathering and form the mountain peaks in Tucson Mountain Park.,

This sample shows ash, pumice, and other lithic fragments flattened and welded together to form a dense solid rock.

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Memories of Snowdonia

As I examined these remarkable rocks, my mind flashed back to the first time I encountered similar welded tuffs, in the bleak landscape of Snowdonia in North Wales, more than six decades ago. I was on a Leeds University geological field trip led by Professor Dennis Wood, who went on to inspire US undergraduates at the University of Illinois.

Our teenage group of aspiring geologists, questioned Dennis’s sanity as he faced the biting cold and horizontal rain. Despite the adverse conditions, we persisted, wiping raindrops from our hand lenses to scrutinize flattened pumice specimens on the windswept outcrops of Mt. Snowdon. (This lesson in geological stoicism was foundational training for an earth scientist).

As darkness fell on Snowdonia, I could really have used one of these illuminated hand lenses now available on Amazon.

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I wish I had one of these when I was a professional geologist.

Similar rocks – different weather

Welded tuffs in Tucson Mountain Park
Similar rocks in Wales but much windier weather!

As a geologist, my professional journey led me across numerous countries, revealing the fundamental truth that rocks transcend political boundaries. The same geological formations found on the snowy peaks of Snowdonia can be seen in the sunny landscapes of Southern Arizona. They were shaped by the same plate tectonic forces separated by thousands of miles and millions of years.

In the realm of geology, there exists a profound realization: we, as earth scientists, are inherently global citizens.

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A Botanical Diversion

Our hike leader, West Virginia Mike, pointed out this example of a crested saguaro.

Cristate or “crested” saguaros form when the cells in the stem start dividing outward instead of in the typical circular pattern of a regular cactus. It’s a fascinating mutation that leads to the growth of a distinctive fan-shaped crest at the top of a saguaro’s main stem or arms.

The cause remains a mystery, but one theory is the role of frost, particularly since crested saguaros seem to be more prevalent in the northernmost areas of their habitat. However, there’s still not enough evidence to confirm this hypothesis.

Stay tuned for more Geological Adventures

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I was an exploration geologist and University Professor working in Botswana, Zambia, Brazil, Saudi Arabia, the Netherlands, Portugal, and other countries before opening Artistry in Glass in 1986. In my more than 35 years of experience, I have brought my technical abilities as a scientist to the trade of glasswork. During this time I have become an industry expert in glass and glass-related skills. Watch out for special insider tips developed from my detailed knowledge of the glass business.

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