Stop 4: Greenstone
These rocks underlie the 50 million year old volcanic rock sequence filling the down-faulted Republic graben. They formed over 250 million years ago, in and around a chain of volcanoes built out in a deep ocean basin.
These rocks represent underwater lava eruptions and accumulations of fine-grained sediment that occurred on an oceanic crustal plate that lay to the west of the North American continent. Between 180 and 100 million years ago, motion between the earth’s crustal plates pushed these oceanic rocks up over the edge of the North American continent at a convergent plate boundary. These “foreign” rocks that were added to the older North American continent are part of an accreted terrane known as Quesnellia, named after rocks exposed near the town of Quesnel in British Columbia. As the rocks were shoved onto North America, they became slightly metamorphosed into a greenstone and were crisscrossed by faults and fractures. The green colors are from minerals characteristic of low-temperature metamorphism.
Toward the north and south ends of the outcrop, there are feeder dikes of the younger volcanic rock sequence. Dikes are solidified volcanic rock formed by magma that has forced its way through cracks in existing rock.
The group takes a closer look at the rock formations.