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Home Archived Aug 4, 2016

Geology of the National Parks


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Dungeness River, Royal Creek

STOP 3: Surprise Pass

Surprise Pass, the prominent notch through The Needles, has been eroded out of pulverized basalt along a fault. Movement of the blocks of rock on either side of the pass have ground up the basalt in the fault zone. The high, rugged ridge extending from Little Mystery to The Needles is a major segment of the inner basalt ring (figs. 16, geologic map).

Crags at head of Surprise Basin
Fig. FT 5. Well developed joints in basalt and diabase at the head of Surprise Basin. The snowbank marks the trace of the fault responsible for the weak rock eroded out of the slot.

The lavas that form much of this ridge oozed out on the ocean floor somewhat later than the lavas making up the main basaltic horseshoe. The lava beds in The Needles are upended and their hardness makes them stand high as erosion gnaws away faster at the surrounding softer rocks. The pinnacled aspect of the ridge is due to widely spaced joints (cracks) (fig, FT5).

Erosion tends to work along the joints and the beds, etching out blocks, ribs, and spines (figs FT5, FT6). Although much of the lava is in the pillow form, indicating that it erupted under water, some of the rock of The Needles evidently did not reach the bottom of the ocean but forced its way as a melt between beds of lava already deposited. These intrusive sills (fig. 11) show no pillows. From a distance they look smoother than the pillow lavas, but on freshly broken surfaces can be seen to be composed of interlaced, thin, rectangular, light-colored crystals of feldspar in a black matrix of pyroxene and altered material. The intruded layers, called diabase where fine- grained and gabbro where coarse, were insulated from the chilling sea water so that the crystals had time to grow.

Arrowhead pinnacle
Fig. FT6. Arrowhead pinnacle in surprise basin.

In contrast, the quickly cooled pillow basalt is dark green and finely textured, with few if any visible crystals (fig. FT 7). Pillows commonly contain many small, globular, greenish-white structures ranging from BB to pea size. These are masses of tiny crystals that grew from the glass that formed a rim on the pillow when the molten glob of lava came in touch with sea water. Glass, which does not have the ordered structure of crystals, never lasts long geologically. Its molecular components slowly gather together to become crystals, a more stable form of matter, and the lumps in the basalt are centers of this crystallization. Click on the outlines mineral terms for definitions.

Drawing of basalt and diabase textures
Fig. FT7. Textures of basalt and diabase.

On to Stop 4. Cameron Basin

Feldspar Pyroxene

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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