Episode 15 Meta - morphic

When we look around at the rocks that crop out high in the Sangre de Cristo Mountains, we not only find granite, but also the rocks into which the granitic magmas were intruded. 

These rocks seem to be extremely distorted. Many show stratification, but the rocks themselves glitter with interlocking crystals, and the layers are stretched into sheets and bands. The rocks have compositions that indicate a former existence as sedimentary or volcanic rock, but they certainly don't have the same dull earthy or porous textures of those rock families. And it's clear that they once shared the same hot, subterranean environment of the granites that surround them.

Rocks like these are called metamorphic rocks, a term coined in the early 19th Century to account for rocks so distorted by textural changes driven by heat, that their fossil content was destroyed. Metamorphic rocks form the third great family of rocks, after the sedimentary and igneous groups.

The way we classify metamorphic rocks is - surprise! - by their distinctive textures.

The most characteristic single feature of these textures is recrystallization: the enlargement of old mineral grains and the growth of new ones with unmelted rock.

Limestone becomes luminous marble, glowing with crystals of calcite.

Shale becomes schist, glittering with mica.

Sandstone become sugary quartzite, with quartz grains set together like a stained-glass window.

It requires the addition of heat and the release of volatile elements like water to facilitate these changes. And it follows that this alchemy must take place in an environment of tremendous confining pressure to keep the superheated fluids in place to do the work of rearranging elements. Metamorphic rocks bring as much news from the realm of Pluto as the granitic rocks.

The brightly visible crystals of the metamorphic and the plutonic rocks is the reason these rocks are commonly grouped together as the crystalline rocks.