A looming thunderstorm casts a magical light on the lush and colorful wildflowers in the Rocky Mountains near Silverton, Colorado. From Red Mountain Pass, the view sweeps across the swampy meadows of Mineral Creek and uphill toward the Black Bear Mountains. The lightning bolt and streaks of heavy rainfall bring an end to a three-week period during which the summer sun had shone continuously from a steel-blue sky.
This was one of the typical and prolonged monsoon breaks in the southwestern USA. During the warm and sunny growing season from June to August, the alpine wildflowers of the high mountains of Colorado blossom into an enchanting spectacle of color. The red Indian paintbrush (Castilleja linariifolia) contrasts with the yellow leafy cinquefoil (Drymocallis fissa) and the white-and-purple Colorado blue columbine (Aquilegia caerulea). The purple alpine aster (Aster alpinus) grows alongside the white Richardson’s geranium (Geranium richardsonii) and the blue Barbey’s larkspur (Delphinium barbeyi). The widely visible aspen sunflower (Helianthella quinquenervis) and the impressively large white-flowering cow parsnip (Heracleum sphondylium) are particularly beautiful.
The water so urgently needed by the wildflowers is initially supplied by snowmelt stored in the soil. By midsummer, however, they increasingly depend on rainfall. This rain arrives as short and heavy showers whenever a monsoon storm rolls across the Rocky Mountains. This happens at irregular intervals whenever humid maritime air from the Gulf of Mexico or the Gulf of California reaches far north. This air first rises over the Colorado Plateau. On its further journey northward, it must pass over the lofty peaks of the Rocky Mountains. As a result, the air mass cools considerably and develops towering storm clouds, which discharge their rain load over the mountains in powerful thunderstorms. As soon as the winds of the large-scale airflow return to westerly directions, the monsoon wave ends and gives way to sunshine again during the next monsoon break.
The geology of the Rocky Mountains around Silverton is unusual because these San Juan Mountains are largely composed of volcanic rocks. The view across this landscape reveals the rim of a huge circular collapse basin. This caldera formed in much the same way as the famous Crater Lake volcano in Oregon. However, the Silverton Caldera is significantly older and formed 30 million years ago when an explosive volcanic eruption partly emptied the magma chamber beneath the mountain, causing it to collapse. Since then, the re-growing lava dome, glaciers of the last ice ages, and erosion by water and weather have eroded the ring structure of the caldera to such an extent that today it can only be recognized by ring-shaped fractures in the bedrock.
During the slow cooling of the magma chamber at depth and of magma in the volcanic vents near the surface, groundwater circulated through the ring-shaped fractures of the superheated volcanic rock. During this process, the hot water also came into contact with the extremely old bedrock surrounding the magma chamber. The heat of the adjacent magma chamber chemically altered this bedrock and deformed the ductile rock. The superheated hydrothermal waters were thus able to dissolve and transport large quantities of metals and minerals toward the surface. There, they were deposited in pockets along the veins and faults in the ring around the caldera. These deposits are rich in gold, silver, copper, lead, zinc, sulfur, and iron. Their discovery in 1870 triggered the Silverton gold rush, which led to conflicts with the Ute Indians, whose hunting grounds this region had long been. Iron and sulfur in particular oxidized the leached rock, creating intense hues of red and yellow and giving Red Mountain Pass its name. How did these volcanoes in the middle of the Rocky Mountains originate?
The most plausible answer to this question has global dimensions. As the American continent collides with the Pacific plate, the heavier oceanic plate subducts beneath the west coast of North America. This creates the active volcanoes of the Cascade Range, including Crater Lake and Mount St. Helens. In this ongoing collision, the East Pacific Rise, a submarine volcanic mountain chain, also descended beneath the continent.
This bulge in the Earth’s crust collided inland at a depth of about 100 km (60 miles) with the deep-lying roots of the Rocky Mountains. This exerted enormous pressure on both plates, which move against each other at a speed of a few centimeters (about one inch) per year. At the surface, this lifted the terrain and created the Colorado Plateau while the Rocky Mountains grew taller. The tremendous pressure caused deep fractures in the Earth’s crust, along which magma could rise to the surface. The San Juan Mountains mark only one of many such fractures in the southwestern USA.
The chronological sequence of the volcanic rocks and eruptions in this region fits into this picture. About 36 million years ago, it began with catastrophic episodes of explosive volcanism. Such violent eruptions, with pyroclastic flows and powerful ash clouds that were welded into tuff during their extremely hot deposition on the ground, are associated with viscous, gas-rich lava, the so-called rhyolites. Such lavas form whenever magma ascends through continental crust. This is exactly what happened when the magma began its ascent during the deep collision of the two plates. About 25 million years ago, the explosive eruptions evolved into increasingly quieter effusive eruptions and finally ended in a deluge of fluid lava, the basalt. This sequence shows that by this time, the pathways of the magma to the surface had become unobstructed, and the magma no longer interacted with the bedrock of the continent.
The three weeks of immaculate blue skies during the monsoon break were ideal for exploring as many mountain trails as possible in search of the place with the most splendid wildflowers. When the clouds of the next monsoon wave finally rolled in, it was only a matter of time before one of the numerous thunderstorm cells with strong lightning activity moved over the ridge at sunset. Only one minute after this picture was taken, the rain set in and lightning struck in the immediate vicinity. For a short time, even hail fell, followed by torrential rain. The thunderstorm darkened the sky so much that the onset of night went unnoticed. When the rain and rolling thunder finally cleared, it was already night, and the stars were twinkling in the sky.
