Intrusive

October 1 2017

Sophia’s birthday fell on a Wednesday this year, so we celebrated the weekend before by going camping in the Mineral King Valley of the southern Sierra Nevada. Mineral King is a bit of a hidden gem which, similar to Hetch Hetchy, belongs to a National Park but has an isolated ingress/egress point. I’d been about ten years ago, but it was exciting to try some new trails and hike the west side of the valley.

While on a day hike to Eagle Lake we spotted this interesting rock in what is normally a sea of granite. What make this rock so cool is it’s only mostly granitic! The dalmatian like quality of this rock can to be attributed to a sprinkling of gabbroic rocks. Gabbro, like granite, is an intrusive igneous rock type. Unlike granite however, it’s composition is mafic which in turn yields a darker composition.

Given that both these rock types are the result of crystallized magma it’s likely these rocks formed at different times. It seems unlikely that small mafic pockets of magma would freeze in isolated chambers surrounded by a felsic flow but I’ve been wrong before.

Composition aside, what adds further intrigue to this rock is the fact that it’s been sheared flat by an extinct glacier. If the glacial striations aren't a dead give away, the handy park guide mentions that Eagle Lake is a tarn carved out by the late glacier. RIP good buddy.