Identified: the Shorecrest Big Rock is a metamorphic Greenstone

Emma Agosta has taught geology at Shoreline Community College for the past ten years. She was able to get close enough to the Big Rock recently uncovered at Shorecrest High School during site preparation for the new high school, to provide us with this information. (see previous article) 

By Emanuela "Emma" Agosta

Professor of Geology

Shoreline Community College

The rock recently recovered during construction at Shorecrest High School in Shoreline, is a “glacial erratic.” These are very common occurrences in the Puget Sound region, which was covered by continental ice-sheets during the last age up to about 12,000 years ago. 

Massive ice-sheets moving down from what is now British Columbia ripped off rocks of various origin and composition along their path and embedded them in the ice. When the ice-sheets melted these erratic were strewn across the landscape on top of local sediments and rocks and often embedded into them.

They are called “erratic” because their composition is foreign compared to the local rocks. They come in many different sizes and compositions. Some are huge, like the recently discovered Lake Stevens erratic (very likely the largest known erratic by dimensions in the state of Washington), some are just very large boulders. Many if not most of these erratics are made of granite or “granitoid rock compositions” as this is a very common rock in the North Cascades and in British Columbia.

For more information about local glacial erratics, go to NorthWest Geology Field Trips, a website maintained by Dave Tucker, professor of Geology at Western Washington University and local expert on Western Washington erratics.

Glacial erratic excavated from Shorecrest site by construction crew.  Photo by Emma Acosta.

This particular erratic is roughly 7 ft. long, 4 ft wide and 4.5 feet tall. Looking at it from a distance, it appears gray and striped (see large-scale picture) but actually the gray color is mostly from dirt and sediments it was embedded in and the stripes are actually excavation marks from the machinery that took it out of the ground during construction. 

Upon closer examination (I was able to see it closely only from one side as the area is fenced off), it is a dark green, hard and smooth rock (see close-up picture). 

Close up of greenstone.  Photo by Emma Acosta.

While I cannot provide a conclusive identification, given the minimal contact I was able to have with the rock, I would say that it is most likely a greenstone, a metamorphic rock that forms when basaltic lava is subjected to low-grade metamorphism. 

The basalt was originally erupted on the seafloor of the Pacific Ocean, then subducted beneath the North American plate. Heat and pressure associated with the process of subduction chemically altered the original minerals in basalt and transformed them into new minerals such as chlorite that confer the rock its characteristic green color. Greenstones are exposed in various coastal areas north of Seattle, such as Whidbey Island, Fidalgo Island and Deception Pass. It is possible that this boulder may indeed come from this region where more conspicuous outcrops of greenstones are exposed.

The rock is actually quite beautiful and if properly cleaned could be used as a landscaping rock in the newly renovated school grounds.