Strain

One of my absolutely favorite outcroppings in the mountains above Santa Fe is this little stream-polished window into the depths of the Earth’s crust, along Little Tesuque Creek, not far above the Bishop’s Lodge Resort:

Hornblende schist - a kind of amphibolite - along Little Tesuque Creek, Santa Fe, New Mexico

Hornblende schist – a kind of amphibolite – along Little Tesuque Creek, above Santa Fe, New Mexico

The strain displayed by this dark schist is hard to miss. Outcroppings of metamorphic rocks are abundant in the ancient crystalline core of the Santa Fe Range, but few of them exhibit such dramatic stretching as this example. Here’s a close up of a structure known as boudinage:

Boudins of more quartz-rich layers in the schist

Boudins of more quartz-rich layers in the schist

These stony strings of sausages are the result of differing competencies among the mineral components of the schist. The smooth borders attest to the overall ductility of the rock as it was being slowly sheared, deep in the plutonic realm of the middle crust.

These schists are sturdy, and rounded cobbles of this rock are common throughout the foothills and stream beds west of Santa Fe. Most of them glitter in a dark way, as tiny prismatic crystals of hornblende, aligned by shearing strain, send back a little light. Harder to see is their matrix of white plagioclase feldspar, which probably makes up over half the rock’s mineral content.