English: This is a loose boulder on a gravelly marine shoreline at Yaquina Head, Oregon. Most of the clasts at the site are cobbles. The rocks here have eroded from the Ginkgo Basalt, one of many lava flows in the Miocene-aged Columbia River Flood Basalt.

Flood basalts are vast outpourings of lava from one or more large fractures in the crust. This rare type of volcanism is not occurring on Earth at present, but ancient flood basalt provinces can be examined - for example, the latest Cretaceous-aged Deccan Traps of India and the latest Permian Siberian Traps of Russia. Flood lavas are also known on Mercury, Venus, and the Moon.

The Ginkgo Basalt is a voluminous lava flow that originated from a feeder dike in southeastern Washington State, traveled westward about 550 kilometers, and reached the Pacific Ocean. Yaquina Head, Oregon is the distal-most occurrence of the flow. Rocks in the Ginkgo Basalt include aphanitic basalt, microporphyritic basalt, porphyritic basalt, columnar jointed basalt, pillow basalt, basaltic glass (altered & unaltered), basalt breccia, vesicular basalt, scoriaceous basalt, aa flow tops, and pahoehoe flow tops.

The whitish spots on the rock are encrusting barnacles, which are sessile, benthic, filter-feeding, marine crustaceans. Barnacles are obligate hard substrate encrusters. They are particularly common in intertidal, rocky shore environments. Barnacles can tolerate subaerial exposure during low tides but have to be in water at least occasionally. When submerged, they extend their feathery limbs to filter feed. The barnacle body is enclosed in a small, cinder cone volcano-shaped carapace composed of overlapping calcareous plates. Fossil barnacles first appear in Cambrian rocks.

Classification of barnacles: Animalia, Arthropoda, Crustacea, Maxillopoda, Cirripedia

Rock provenance: Ginkgo Basalt, Frenchman Springs Member, Wanapum Basalt, Columbia River Flood Basalt Group, Middle Miocene, 15.3 to 15.6 Ma

Locality: marine shoreline on the southern side of Yaquina Head, coastal Oregon, USA

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Site-specific info. mostly synthesized from: Ho & Cashman (1997) - Temperature constraints on the Ginkgo Flow of the Columbia River Basalt Group. Geology 25: 403-406. and Mardock (1994) - A geologic overview of Yaquina Head, Oregon. Oregon Geology 56: 27-33.

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See also:

www.usgs.gov/observatories/cvo/columbia-river-basalt-grou...