Thursday, December 28, 2023

Haystack Rock and Invasive Columbia River Basalt Group

Haystack Rock at Cannon Beach is one of the iconic images of Oregon. Cannon Beach has a very broad sandy beach both north and south and between the town and the rock. The site is a popular destination and is only about 80 miles from Portland. 

Haystack Rock consists of basalt that is more resistant than the generally very soft marine sediments that make up much of the coast in this area. The basalt is Columbia River Basalt Group magma that erupted as huge flood basalts in northeast Oregon and southeast Washington. The lava flows extended down the ancestral route of the Columbia River all the way to the Pacific Ocean about 15 million years ago. 

The basalts along the Oregon Coast were thought to be of local derivation as there are numerous intrusive dikes and sills within the local sedimentary rocks. However, as detailed chemical analysis and age dates of the basalts were obtained it turned out that these north Oregon coast basalt matched the chemistry and the age of the basalt of the Columbia River Basalt Group. This presented a challenging problem: How could the upper mantle produce virtually identical sequences of magma in these widely separated and tectonically dissimilar regions and yield them for eruption at the same time?  

Beeson, Perttu and Perttu (1979) proposed that the Oregon Coast basalts were the results of lava flows that flowed into the soft sediment along the coast as invasive lava flows that sank down through the muds and sands of the soft sediments. Hence, the dikes and sills of basalt within the soft sediments along north coast of Oregon are not from local volcanic centers but are from lava flows that sank down into the soft sediment intruding or invading the soft sediment mud. 

The haystack rocks and cliff south of Cannon Beach are good sites for observing this phenomenon.

A challenge is to get to the area at low tide. Given that my trips to Cannon Beach have been in the winter when daytime tides were high and coincided with large storms, my observation time was limited or restricted entirely. 

Not much of beach with waves reaching the base of the bluffs.

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