Great Hall Fireplace

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The Great Hall fireplace is made up of rocks representing the Olympic Mountains. The Olympics are composed of ocean-bottom rocks that were scraped off the oceanic plate as it slid beneath Western Washington.

The oldest Olympic rock is basalt that originally formed from lava erupting out of crustal fissures into water at the bottom of the ocean. 

The other Olympic rocks were sediments deposited on the ocean floor, buried, and then hardened into sandstone and shale. Later, they were jammed against the basalt, pushing it backward and upward until it now stands vertical at the eastern end of the mountains.

Olympic Mountain Diagram.png

Diagrammatic cross section of the Olympics. As the Juan de Fuca plate moves under North America, young ocean sediments are forced under previous slabs of sediment along curved thrust faults – note the small arrows for the relative motion along these faults. This motion of the Juan de Fuca plate generates large earthquakes along the Washington coast.


Basalt is typically dark gray, but here we see mostly orange rust stains on the surface caused by oxidation of iron-bearing minerals in the basalt. The lighter colored rock across most of the fireplace is sandstone, representing the sedimentary rocks in the Olympics. Some of the Olympic rocks were dragged down deep enough to be partly metamorphosed by heat and pressure – these are represented by the band of dark bluish-green altered basalt or serpentinite.

Pdf.png PDF version of the poster that describes this fireplace.