The colony of Harbour Seals seen in the picture above are basking on a remnant of a wave-cut basalt platform or bench. They are almost the first thing you see when you get out of the car at the Strawberry Hill Wayside on the Oregon Coast and start to walk down to the beach below. The path winds down over the sandy and pebbly raised beach or terrace, left stranded in the Pleistocene when sea levels began to retreat to lower levels, then follows the step-like contours of a basalt-filled dike to the shore (Photo 3).
From the viewpoint afforded by a stop half-way down the path (Photo 4), you can see the whole of the embayment with isolated outcrops of volcanic rock scattered across the otherwise sandy beach, and more extensive areas of rock towards the headlands – looking like low-lying islands as the tide comes in. It is on one of these outcrops that the seals frequently congregate.
The first impression of the rocks on the beach is how “messy” they look (Photo 5). Although all the rocks on the beach were formed at approximately the same time in geological terms, and they are all generically volcanic basalts, they vary a great deal on colour, consistency, and shape, depending on how exactly they were formed. Most of the rocks are made up from unsorted fragments cemented together in a matrix. This is fragmental basalt. See Photos 5 & 6.
Fragmental basalt can form in several ways. For example, when a volcano explosively erupts (a pyroclastic eruption), pieces of shattered rock can be flung in to air, and the molten lava can solidify as rock in the air, before it lands on the surrounding area, blanketing the ground with small solid fragments. Or, the surface of molten lava flow running down the sides of a volcano can cool and solidify like a crust which then breaks up into pieces because of the continuing movement of the still liquid lava below it. Or the hot lava from the volcano may flow into or erupt beneath the sea where the contact with the cold water with the newly solidified lava causes it to fragment. The rocks on the beach here are the result of a series of volcanic events, and I am not able to say which fragmental rock was formed in which way.
Although the majority of the rocks on Strawberry Hill beach are this softer and variously-coloured fragmental type of basalt, there are areas where the basalt is darker, and harder, with an even texture, and few, if any, inclusions. This can be seen in the dikes which are a series of linear features generally running across the shore towards the sea (Photos 7 & 8). Here, molten lava has oozed into rifts or cracks in the fragmental basalt where it slowly cooled and solidified. When molten lava hardens gradually this way it can form a characteristic angular, possibly columnar, fracture pattern (as in the Giant’s Causeway in Ireland or on the south coast of Grand Manan in the Bay of Fundy). Some larger dark basalt outcrops on the beach show this angular, almost geometrical, fracturing well – see Photos 9 and 12 in the gallery below.
For more images of basalt rock textures at this location see the earlier blog post Basalt on the Beach at Strawberry Hill.
Coastal landforms between Florence and Yachats, Oregon by Ernest H. Lund, February 1971, The ORE BIN, Volume 33, No. 2, pp 21-44.
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