File:Sea arch (Yaquina Head, Oregon, USA).jpg

English: Tholeiitic basalt in the Miocene of Oregon, USA.

This is Yaquina Head, along the Pacific Ocean in northwestern Oregon. The rocks here are part of the Ginkgo Basalt, one of many lava flows in the Miocene-aged Columbia River Flood Basalt Group. 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.

At the center of the picture is a sea arch, a type of rock arch developed along marine coastlines. Other types of rock arches include natural arches, natural bridges, blind arches, and lake arches.

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

Locality: 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...