While much of the western half of North America is covered by rank after rank of rugged mountains, the geologic history of the region is long, rich and complex. Although they may resemble one another in form and majesty, each mountain range contains within its shapes and the rocks that form them a record of the forces that shaped the landscape.
Take, for example, the ranges that bound this vast terrain on the east and the west, respectively the Rocky Mountains and the Cascade Range. Even on their surface, the two mountain chains are significantly different: the Cascades, snuggled up against the Pacific coast, are a rain-drenched land of thick moss and temperate rainforests on the west, dry lands on the east. Well to the east, the Rockies lie entirely within an land of windswept crags and high deserts. That is only at the surface, however.
Dig below the surface of the two mountain chains and you will find vast geologic and geographic differences. For starters, the two are significantly different in size: the Rocky Mountains stretch 3,000 miles from the Yukon Territory to New Mexico. The Cascade Range, by comparison, is less than one-fourth the length of the Rockies and considerably narrower. The reason for this difference in size is a result of their differing geologic histories.
The Rockies are considerably older than the Cascades, not only in terms of the rocks exposed in peaks and cliffs; but also in terms of the timing of their orogenesis, the technical term for mountain-building. The Rockies have undergone orogenesis twice, first in the latter part of the Paleozoic Era, about 300 million years ago. Erosion of these Ancestral Rockies shed vast layers of sedimentary rocks, which ultimately buried the peaks. More than 200 million years later, however, the same weaknesses in the Earth’s crust that helped build the Ancestral Rockies allowed a different set of forces to raise a second great chain in the same region. Those old sedimentary rocks are exposed in the red-hued cliffs of chains such as the Maroon Bells and the Sangre de Cristo Mountains. This second episode of mountain-building is known as the Laramide Orogeny, named for the Laramie Range of southeastern Wyoming.
Farther north, the forces that reactivated Ancestral Rocky Mountain weaknesses in places like Colorado and Wyoming formed a different style of mountains in the northern US and Canada. Compressional forces where the western edge of North America collided with the Pacific oceanic plate shoved great sheets of sedimentary rocks inexorably eastward, crumpling them into complex folded patterns. Such thrust faults are similar to what you might see if you were to try to move a rug along the floor by pushing on an edge. Rocks caught up in these thrust sheets range in age from Precambrian to Cretaceous, a span of 500 million years and more in age, and are mainly sedimentary strata with smaller amounts of igneous and metamorphic rocks.
The Rockies, then, have a complex history involving two separate episodes of mountain-building tens to hundreds of millions of years ago. The range’s dominant style of mountain-building is compression, which produced ranges of complexly folded and faulted rocks. Volcanism during the Laramide Orogeny was limited, and centered west of the position of the range in areas such as Idaho and Arizona. There has, however, been post-Laramide volcanic activity in the range related to the opening of the Rio Grande Rift. A prime example of this volcanism is the San Juan Mountains of southwestern Colorado.
In contrast to the Rockies, the Cascade Range is smaller, younger and occurs within a substantially different geologic setting. The Cascades are a magmatic arc, part of the great “Ring of Fire” that surrounds much of the Pacific Ocean. The Cascade Range lies above the edge of a subducting oceanic plate, the Juan de Fuca Plate, which is melting far beneath the surface to create magma that feeds the volcanoes. While the Cascades also occur within a compressional plate-margin setting, the folding that dominates the formation of the Rockies is less prominent in the Cascades.
That the Cascades are much younger is demonstrated by ongoing volcanism within the 700-mile chain such as the 1980 Mount St. Helens eruption. The earliest Cascade volcanism, on the western side of the chain; began well after Laramide orogenesis ceased. This was about 37 million years ago. The chain stretches from the Silverthrone caldera in British Columbia to Lassen Peak in northern California; more than twenty major volcanic peaks including still-active Mounts Rainier, Hood, St. Helens, Shasta and Adams. Their giant, snow-capped summits give rise to the popular image of the Cascade Range as one of isolated peaks looming on the horizon.
Millions of years of volcanism means that the bedrock of the Cascade Range is almost entirely igneous. Great swaths of the land around each of the many peaks are coated with thousands of feet of layered basalt flows interleaved with layers of volcanic ash. The rich soil created by weathering (see the differences between erosion and weathering) of these unstable rock types has contributed to regional agriculture, including a large, flourishing wine industry. Sedimentary rocks in the region are rare and tend to be concentrated along the coastline, except for the records of cataclysmic mudflows, such as those observed after the Mount St. Helens eruption.
In comparison, the Cascade Range is younger than the Rocky Mountains and is the result of a different style of mountain-building. The Cascades, because they are restricted to the subduction of a fragment of oceanic crust, are considerably smaller in area and length than the Rockies, which stretch for thousands of miles. The two ranges are also built of different types of rocks – mainly volcanic rocks in the Cascades and mainly sedimentary rocks in the Rockies. Finally, their current positions create significantly different climates: the west side of the Cascade Range is one of the world’s few temperate rainforests. The range casts a vast rain shadow, however, resulting in a dryer climate east of the mountains. The climate of the Rockies is dry along its entire length; enough so that residents are heavily dependent on snowmelt for their water resources.
The steep cliffs and wide valleys that characterize both ranges are a product of alpine glaciation. The flowing rivers of ice and snow carved out typical U-shaped valleys throughout both ranges during the continental glaciation episodes of the Pleistocene. Active alpine glaciers still dot the Cascades and northern US and Canadian Rockies. Although still-active glaciers are less common in the central and southern Rockies, their characteristic landforms can be seen throughout both ranges; especially in the San Juan Mountains and preserved sites such as Rocky Mountain and Glacier National Parks. The combination of rocks, water and ice has shaped both ranges into some of the most dramatic landscapes of North America.
