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Geology of the National Parks

GEOLOGY OF OLYMPIC NATIONAL PARK:
PART I OLYMPIC GEOLOGY

Basalt and basalt breccia on Klahhane Ridge
Fig. 8. Basalt and basalt breccia of the Crescent Formation on Klahhane Ridge above Port Angeles.

Basalt and its Associated Rocks

Most Olympic basalt is black or dark green, although weathered surfaces may be red or yellowish owing to chemical changes in the iron-bearing minerals. Basalt is hard, dense rock made up mostly of tiny crystals readily seen only with a microscope (fig. 5). Basalt forms from molten rock, called lava, that has erupted on the earth's surface. Bubbles of gas in the molten lava are commonly trapped in the rock, leaving globular holes, or vesicles. Later, these vesicles are commonly filled with zeolite minerals, giving the rock a polka-dot appearance.

Formation of columnar joints
Fig. 9. Coumnar joints form as lava cools and shrinks toward evenly distributed points in the hot mass.

Sometime after sediments began accumulating in the ocean, molten rock poured out on the ocean floor. We know that Olympic basalt had this watery beginning because it is commonly made up of globular masses called pillows, which form primarily when hot lava erupts under water.

The basalt lavas piled up to incredible thicknesses not only as pillows but also as piles of broken rubble called volcanic breccia. The actual thickness of the pile is not known, but on the east side of the Olympics, the lava and breccia beds, now tilted on end, measure more than twelve miles thick. Some thickening results from folding, and some parts of the pile may be repeated on account of faulting. Even so, the pile is enormous by man's standards. This thick pile of basalt is known as the Crescent Formation.

The lavas piled up so high on the ocean floor in some places that they protruded above the surface as islands, even though the floor was probably sinking under the great load. Lava flows erupting on these islands were not chilled as quickly as those in the water, so they did not form pillows. Instead, as they cooled and shrank they developed cracks that broke the lava into polygonal columns (fig. 9). Columnar joints are typical of lavas erupted on land, and they may be seen in roadcuts along Hood Canal.

Also within the basalt pile and in some of the sedimentary rocks are thin layers of basaltic rock that never reached the ocean floor but solidified under a blanket of rock or sediment. These dikes and sills (fig. 10) cooled slowly, and crystals grew larger in them than in the quickly chilled lavas. If the crystals are just large enough to see, we call the rock diabase; if they are coarse enough to rest a little finger on, gabbro.

Interbedded with the pillow lava is red limestone rich in shells of one-celled animals (Foraminifera) that thrived in deep water. Also abundant are the submicroscopic plates of floating animals called plankton. Some limestones appear to be formed entirely of these plates.

Dikes and sills
Fig. 10. Dikes and sills.

Of  special significance to technological man are the manganese and copper-bearing minerals closely associated with the red limestones. Apparently, the metal-bearing minerals formed when volcanic gases and solutions from the lavas reacted with sea water during or shortly after volcanic eruption. Small deposits of copper and manganese were prospected and mined at various times in the early twentieth century around the periphery of Olympic National Park. There was much activity at the Crescent Mine near Lake Crescent, the Elk-horn Mine on the south side of Mount Constance, the Tubal Cain Mine northeast of Buckhorn Mountain, and the Black and White Mine near Mount Cruiser. The occurrences of ore minerals in the Olympics are now more curious than commercial, and man has gone elsewhere for his supplies of manganese and copper.

The entire pile of basalt and its associated rocks has a primitive chemical composition. We can therefore conclude that the lava is derived from material deep in the earth that has never been subjected to the chemical and mechanical sorting processes of the earth's surface. Olympic lavas have much in common with those that make up most of the Pacific Ocean floor and numerous undersea volcanoes active today.


Material in this site has been adapted from Guide to the Geology of Olympic National Park by Rowland W. Tabor, of the USGS. It is published by The Northwest Interpretive Association, Seattle.

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