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Home Archived Aug 4, 2016

USGS Geology in the Parks

Domains of the North Cascades

FIELD TRIP STOP 2 - North Cascade Highway (State Route 20)

Cliffs of orthogneiss rise above Newhalem


Dating ancient magma using the zircon radiometric clock

The buildings of the town of Newhalem cluster in an orderly fashion on well-groomed lawns across the flat floor of the Skagit River valley. This fastidious landscape contrasts with the wild mountainsides that abruptly tower some 3,000 to 4,000 feet above it. Across the Skagit River, which bounds the town on the southeast, is the North Cascades National Park Visitor Center. Rocks holding up the cliffs surrounding the town and Visitor Center are metamorphosed igneous plutons (orthogneiss) in the Skagit Gneiss Complex .


Minerals to see in tonalite orthogneiss.
Minerals to see in tonalite orthogneiss.

For a good close look at the orthogneiss, leave the grassy park area under the power lines, west of the highway, and walk to the nearest clean granitic cliff. The rock is rough, and its crystals are visibly faceted. With the unaided eye in good light the petrophile can see glassy quartz, milky feldspar with flat faces, black, shiny biotite flakes, and prismatic greenish-black hornblende. The dark minerals are somewhat aligned, like fish in the current of a stream. This orthogneiss, made from a tonalite by metamorphic squeezing and recrystallization, is typical of many metamorphosed plutons in the Skagit Gneiss Complex. Geologists actually know the approximate age of the orthogneiss here thanks to modern technology and the hard work of geochronologists.


Zircon crystal greatly enlarged.
Zircon crystal greatly enlarged.

In order to determine the age of the rock, the geochronologist grinds up 20 to 100 pounds of rock, and, using various methods, some similar to panning for gold, separates out tiny but relatively heavy zircon crystals from the other minerals. When large and pure, zircon crystals are used in jewelry, but most crystals common in many granitic rocks are so tiny that only a flea, or maybe an ant, would value them for decoration.

Zircon (zirconium silicate) is a hard mineral and is particularly useful in dating rocks because it resists breakage, melting, and recrystallization during metamorphism. Its real clockwork virtue is that the radioactive element uranium tends to follow the zirconium into the crystal lattice and most zircon crystals thus contain a little uranium. Some atoms ( isotopes) of uranium are unstable and gradually decay radioactively at a known rate, changing into various isotopes of lead. After a zircon crystallizes from a magma, lead isotopes from the decaying uranium are trapped in the crystal. By measuring the relative amounts of uranium isotopes and lead isotopes in the crystal, the a geochronologist can calculate how long ago the crystal formed. The zircon radiometric clock from the orthogneiss at Newhalem has been running about 70 million years . (That is to say, the rock first crystallized from a magma in the Cretaceous).

On to the Skagit Gorge

Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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