North Cascades Geology
Metamorphic RocksMetamorphic rocks are changed rocks, that is, rocks whose original materials have recrystallized to form new minerals. Generally, this recrystallization, called metamorphism, takes place when deeply buried rocks are subjected to great pressure and high temperature. Most metamorphic rocks have also been squeezed so that their shapes and the shapes of their constituent minerals have changed during metamorphism, resulting in a layered or streaky appearance. Metamorphic rocks in the North Cascades formed from pre-existing rocks of every kind and from all of the major rock groups, igneous, sedimentary, and metamorphic. For example, mica schist, a common metamorphic rock in the North Cascades, recrystallized from shale. Gneiss, which looks like granite that has been squeezed so that the rock looks streaky. Much North Cascades gneiss formed from granitic rocks in exactly that way. Gneiss also forms from schist when the rock remains hot enough for the metamorphic crystals to grow large. Marble is metamorphosed limestone, whose calcite crystals have grown large enough to see without a hand lens. This brief introduction to petrology, or the study of rocks, has touched only on a few of the hundreds of different kinds of rocks found in the North Cascades. We name and describe many of the others as the geologic story unfolds.
An ideal metamorphic sequence: A Rock's Progress from Shale to Gneiss or from Basalt to AmphiboliteIn an ideal metamorphic sequence of increasing heat and pressure, a shale becomes slate and slate becomes phyllite, then schist, and finally gneiss. A shale, when highly squeezed deep in the Earth, first forms slate, as the shale's tiny grains and platy clay minerals slide around under the stress. The rock deforms that is, bends and flows and the minerals align themselves in the direction of yielding, or flow. Slate exhibits cleavage, meaning it can be broken into even, flat chips, slabs, or blackboards (at least in the past). If slate minerals further recrystallize, giving the rocks a shiny appearance, the new rock is phyllite. Phyllite also abounds in the North Cascades
Still further recrystallization produces a flaky crystalline rock called schist, usually mica schist made up of flat mica flakes and other minerals. These rocks exhibit foliation, a tendency to break into thin, curving leaves, or sheets, like the pages of a book (see Diablo Dam). Thorough recrystallization at higher temperature will produce a gneiss, looking rather like granite, but with minerals clearly arranged in a parallel fashion the foliation. In gneiss, the minerals feldspar and quartz have grown to become conspicuous crystals. The rock no longer breaks as easily along the foliation because the feldspar and quartz are more randomly oriented and commonly have interlocking, irregular crystal borders. In progressive metamorphism of a basalt, the course of change is different because the original basalt reacts differently to heat and pressure. In fact, it is so stiff and resistent to the squeezing (unlike wimpy shale) that the first reincarnation as a metamorphic rock is simple recrystallization to a rock called greenstone named because it is made of many green metamorphic minerals. Further squeezing finally overcomes the basalt or greenstone resistance, forming greenschist, which has many of the same minerals as greenstone but with the flaky foliation of all schists. Rising temperature and continued squeezing causes new minerals to crystallize, and what was formerly basalt becomes amphibolite, a rock that looks like a dark gneiss and is rich in hornblende and feldspar, but with very little quartz. To see this progression illustrated, click here. Although all the rocks mentioned in these progressions occur in the North Cascades, ambitious hikers would find few places where they could walk continuously from the shale or basalt into well-metamorphosed gneiss or amphibolite. In a few places geologists have found parts of the progression, such as phyllite, grading into schist, or basalt grading into greenstone. |
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| On to Geologic Time |
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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