How do rocks that are cooled in plutons at considerable depths in the crust, or that have been buried and metamorphosed, get exposed at the surface? I’ll use the Skagit Gneiss in northern Cascade mountains as a case history to help explain the basics of the uplift theory. This is an excerpt from my upcoming book, Geology Underfoot in Western Washington (‘GUWW’; Mountain Press Publishing, 2014).
The Cascade Range is eroded only a short way into the North American continental plate; the highest non-volcanic peaks rise less than 2 miles above sea level, the deepest valleys are only a few hundred feet above sea level. The total thickness of the continental plate beneath the mountains is around 24 miles (40 km). Yet we find rocks from deep within the crust exposed at the surface. How? The answer is ‘uplift’, the balance between the rate of magma intrusion into the crust, erosion, and the relative densities of the continental crust and the mantle.
The Skagit Gneiss is easy to examine, as it forms the wall of the Skagit Gorge along Washington Highway 20 in the between Newhalem and Diablo. A variety of the gneiss called migmatite is also exposed in a big road cut at the Diablo Lake Overlook. Studies of the pressures needed to generate the metamorphic minerals in the Skagit Gneiss indicate that rocks that were initially very near the surface were somehow buried as deep as 18 miles (30 km) in the crust. There are at least two models that attempt to explain how that happened, but you’ll have to wait for the book for that part.
Hot, semi-molten magma generated by subduction rises upward into the solid continental crust because the hot magma is lower density and more buoyant than the crust it intrudes. A small portion of the magma avoids solidifying in the crust and erupts at volcanoes but by far the greater volume cools within the crust as granitic plutons. These add to the volume and thickness of the continental crust. Because of these lighter rocks, continental plates float on denser rock in the mantle, just as a block of Douglas fir (density 0.53 gm/cubic cm) floats on water (density 1 gm/ cubic cm). Erosion removes surface rocks and sediment washes out to sea, where much of it eventually leaves the continental plate and is carried offshore onto the oceanic plate by turbidity currents (see the Beach 4 field trip). But, at the same time, magma is constantly rising out of the mantle and into the continental crust. If erosion and intrusion rates are balanced, the crust maintains a constant thickness. But the depth to any given bit of rock decreases due to erosion above. That bit of rock does not move upward through the crust relative to neighboring rocks. Rather, the contents of the crust as a whole migrate upward as the crust is replenished from below with low density plutons. If the rate of magma emplacement exceeds erosion, the crust thickens and rises ever higher above the mantle, and the mountains on the continent grow higher and higher. Another analogy is an iceberg. We’ve all heard that only 10% of an iceberg is above the water. An iceberg that is 300 feet thick will protrude 30 feet into the air, but a thicker iceberg, say one that is 600 feet thick, will be twice as high.The crust beneath the worlds highest peaks, the Himalayas, is around 33 miles (55 km), vs. 24 miles (40 km) at the northern Cascades.
Figure 7 GUWW Skagit uplift: The star represents deep rock at an arbitrary depth in the crust— let’s say 20 km (12 mi). Rock at this depth migrates to the surface through combined uplift and erosion. A) Continental crust, consisting of relatively low density rock, floats on denser rocks in the mantle. Magma rises out of the mantle into the crust due to subduction. Erosion eats away at the surface, sediment reaches the sea and turbidity currents carry it far out onto the subducting ocean plate. B) Erosion has removed a few km of surface rock, and rocks have migrated higher as the continental plate is uplifted. The plate remains the same thickness, assuming magma underplating and intrusion are balanced with erosion. C) With time, deep rocks reach the surface.
The result of this balancing act is ‘uplift’, which gives rise to mountain ranges such as the Cascades. Understanding uplift is essential to grasping ‘The Rock Cycle’: the big picture of how rocks move upward in the crust to the surface, are eroded into the sea as sediment, eventually to be accreted during subduction or carried into the mantle to melt.
How fast does erosion strip away the surface? The Icy Peak pluton, east of Mount Shuksan, cooled 2 kilometers below the surface 3.36 million years ago. Those granitic rocks are now exposed at the summit of Icy Peak, elevation 7077 feet (2157 m). In that interval, an estimated 1.2 miles (2 km) of the surface was eroded away. That is a rate of around 0.6 km per million years, or 0.02 inch per year. In the context of geologic time, this is very fast.
This ‘unroofing’ is the mechanism that exposes deep rocks like the Skagit Gneiss at the surface. Subduction will continue to supply new rock to the core of these mountains until the Juan de Fuca plate has been entirely consumed beneath North America, some 20 million years from now. Then, erosion will outpace uplift, and, barring rearrangement of today’s plates, the North Cascades will be ground down to low hills and finally to a broad plain, the geography that prevailed in pre-Cascade time when Chuckanut Formation sediments were deposited by rivers flowing all the way from the Rockies (Vignettes X honeycomb and Y WWU museum).
There is a great website (http://www.smate.wwu.edu/smate/pdf/isostasy.swf )that demonstrates the buoyancy of a block floating on a liquid. You can adjust the densities of each, and the thickness of the block. Start by using the densities for the block of wood and water given above, then try it for crust and mantle densities. Reduce the block’s thickness to see how rock at a given depth eventually reaches the surface as erosion strips the surface and the crustal block continues to rise.
Figure 8 GUWW Skagit isostasy: A. Wood blocks of the same density but different thicknesses float at different heights in denser substance, such as water. B. Crustal rocks behave similarly. Rocks in both oceanic and continental plates are less dense than mantle rocks, so they float. Ocean plates (basalt) are denser than continental plates (granite, etc), so continental plates ride over oceanic plates in subduction zones. Thinner continental crust does not rise as high as thicker continental crust (e.g. Cascades vs. Pacific sea floor).