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USGS Geology in the Parks

North Cascades Geology

Geologists on granitic rock of the Chilliwack batholith
Geologists on granitic rock of the Chilliwack batholith, west ridge of Mount Challenger. The blocky joint pattern and light color is typical of granitic rocks.

Igneous rocks

Igneous rocks form from melted rock, called magma. On the surface of the Earth, igneous rocks usually occur in a cooled state, except where molten rock is erupting from an active volcano. Magma that flows out onto the Earth's surface is called lava. Rocks that crystallize quickly from cooling lava at or near the Earth's surface are called volcanic rocks - no surprise. Black, dense, very fine-grained or even glassy, basalt is the most common volcanic rock and well represented in the North Cascades. A close relative of basalt, andesite, is also a widespread volcanic rock in the North Cascades. Commonly, angular chunks and pieces of volcanic rock debris are cemented together to form volcanic breccia (see Sauk Mtn). Volcanic breccia may form from broken pieces of rock blasted out of a volcano or from rubble eroded from it.

Rocks that crystallize from magma very slowly and at great depths in the Earth's crust are called plutonic rocks. Pluto was the Greek god of the subterranean world of the dead. Granite, with its sparkling and easily-noted crystal faces, is a plutonic rock. Each variety of plutonic rock has its volcanic counterpart. For example, a coarse-grained plutonic granite is chemically equivalent to a fine-grained or glassy volcanic rhyolite . Because the magma which forms plutonic igneous rocks generally rises into or penetrates existing rocks, it is commonly said to be intrusive, and the rock solidified from it is an intrusive igneous rock.

Cross section showing melted rock in magma chamber erupting to form volcanic rock
Cross section showing melted rock in magma chamber erupting to form volcanic rock. When the magma crystallizes at depth, it becomes plutonic rock.

Geologists often refer to a variety of light-colored plutonic igneous rocks as granitic because they look like granite. Granitic rocks, especially the varieties tonalite and granodiorite make up a large part of the North Cascades, and, for that matter, the cores of many major mountain ranges.

An individual body of granitic rock, solidified from one batch of magma, is called a pluton. Very large continuous areas of granitic rocks, commonly made of many plutons, are called batholiths, which literally means "deeply formed rocks."

Ultramafic rocks

Ultramafic rocks are of relatively small extent in the North Cascades, but are geologically significant because they are derived from the Earth's mantle. Ultramafic rocks are dark, heavy, and rich in iron and magnesium minerals. They initially formed from melted rock when the Earth first solidified, billions of years ago. They probably have undergone many transformations during their long residence in their deep earth abode, such as metamorphic recrystallization at very high temperatures and remelting.

Because ultramafic rocks are generally found in relatively small masses--fist- to house-sized--in a variety of other rocks, geologists used to think that the ultramafics invaded the other rocks as magma, that is, that they were intrusive igneous rocks. But throughout much of geologic time, the crust of the Earth was too cold; for the high temperature minerals of ultramafic masses to reach the surface in a melted state. Ultramafic rocks only reach the surface of the earth by way of complex crustal upheavals or as small pieces carried up by other kinds of upwelling magma. By the time most ultramafic materials reach the Earth's surface, they are commonly metamorphosed at lower temperatures to green serpentine minerals, which are iron and magnesium-rich silicate minerals with water bound up in the crystal; lattice. A rock made of serpentine minerals is called serpentinite. Two especially large masses of ultramafic rocks crop out in the North Cascades: the Twin Sisters massif southwest of Mount Baker and an outcrop in the vicinity of Mount Stuart, south of the area covered in this website.

On to Sedimentary Rocks

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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