Rock of the Week #6

Anyone who's seen this before in the field will no doubt recognize it instantly. For the South African students though.... I have encouraged them to use google, and a certain locality name is actually written on the rock. We'll see how they fair!

1. Rock type
2. How did it form?

Solution to Rock of the Week #5:

This rock is a Jurassic slate from the Merced Falls Formation in the Sierra Nevada foothills, California, USA. The slates were deposited in a Jurassic-age subduction trench and accreted to the western margin of North America in the late Jurassic. They are very low grade metamorphic (sub-greenschist) and are dominated by the “slaty cleavage” formed by pressure solution. The empty cubic holes are missing diagenetic/metamorphic pyrite phenocrysts which most likely oxidized to goetite and washed away. Astute observers might have seen very small strain shadows preserved around the cubic grains in the form of fibrous quartz. This particular hand sample comes from the axial region of a tight fold. In the axis, the cleavage is perpendicular to bedding (which can be seen on one end of the sample. This causes the rock to break at 90° angles, creating a “pencil structure”.

1. (1 pt) What is rock?
: 1 point given for either shale, mudstone or slate.

2. (1 pt) What is missing from those cubic holes?
: 1 point for either pyrite or phenocrysts.

3. (1 pt) shape:
: 1 point for mentioning “pencil” or “fold axis”.

1 comment:

Ron Schott said...

Looks to me like rhyolite/obsidian from a lava dome, displaying a breadcrust texture. I'd venture a guess of somewhere in the Mono/Inyo Craters region of eastern California, but it could be from any number of other recent rhyolitic volcanic centers.