#blueschist

The Franciscan Formation

The geology in California today is a consequence of hundreds of millions of years of interaction between a continent and oceanic plates. One of the most widespread consequences of that interaction found today are the rocks you see here, the rocks cities like San Francisco are built on; the Franciscan formation.

The story of the Franciscan formation starts over 500 million years ago when a piece of North America rifted away from what is today California. An ocean basin, like that found today off of the East coast of the U.S., formed. Sediments piled up on this plate; worn down pieces of North America, washed to the ocean by rivers.

When oceanic plates get a couple hundred million years old, they get cold and they get dense. They get so dense that they start sinking into the mantle; they start to subduct. Sometime in the Jurassic period, the oceanic crust off California started subducting.

When oceanic plates subduct, the oceanic crust sinks into the mantle, and the pile of sediments on top has to go somewhere. The density of the oceanic crust is what drives subduction, but the sediments aren’t nearly as dense so they don’t go down easily. Whether they go down depends on whether the oceanic plate is dense enough to drag them down with it.

At a subduction zone like the one in California, the dense oceanic plate subducts beneath the continental plate, and the continental plate can work just like a rubber kitchen scraper. Any rocks sitting loosely on the oceanic plate can be scraped off into a giant pile at the edge of the continental plate.

These scraped-off rocks form a thick, wedge shaped, faulted pile with a tip towards the subduction zone. This shape is called an accretionary wedge or an accretionary prism.

The rocks that make up an accretionary wedge have a pretty rough life, all things considered. They’re scraped off of the oceanic plate, faulted and fractured, but the subduction zone keeps trying to drag them downwards.

This creates a unique situation. These rocks are dragged to high pressures in the upper portion of the mantle, but they’re also colder than the other rocks at that depth because they were sitting at the surface and haven’t been heated very much. This set of conditions is appropriate for a certain type of metamorphism that creates a rock known as a “blueschist”.

Some blueschist rocks are actually blue because of specific minerals, but not all of them, including the rocks you’re looking at here. The rocks of the Franciscan formation are mostly blueschists even though they’re not all blue.

Between about 200 million years ago and 30 million years ago, the Farallon plate subducted beneath California. As that happened, a mixture of sediments sitting on that plate and the occasional broken piece of the Farallon plate were scraped off by the North American plate, forming a thick accretionary wedge. Many of those sediments were pushed to high pressures, causing metamorphism and the growth of new minerals.

The subduction zone wiped out the rocks that made up the crust of California beforehand. Since subduction stopped, the rocks that made up the accretionary wedge have been broken up and moved by new faults; most notably the San Andreas.

The Franciscan Formation is this subduction complex. Much of California outboard of the Sierra Nevada is made up of rocks like these, scraped off of the Farallon plate and smashed together by the forces of subduction.

-JBB

i am so so so so so sorry but for some reason I accidentally deleted them, so I’ll just explain everything again

blueschist and mixite, another couple of onyx’s component gems

blueschist was a judge who saved mixite from as unfair trial by fusing with her as agardite (which has already been posted), and soon they escaped to digilitis and found nepheline

agardite and nepheline built a relationship which became a romantic one, what leads to them eventually fusing into euchroite (which has already been posted)

agardite, mixite and blueschist’s permafusion 

her powers include: summoning anything she wants (doenst work for magical itens), minor hydrokinesis, and she’s a heck of a good singer

Both Blueschist and Greenschist are metamorphic rocks, often called Metamorphic Facies, where rocks change from heat and pressure. Their “ingredients” recombine into new minerals suited for their environment. While Blueschist is considered a schist, Greenschist isn’t. Schist is a term used to describe metamorphic rocks that’s formed by dynamic metamorphism at high temperatures and pressures that aligns grains of mica, hornblende and other flat or elongated minerals into thin layers (by the way, schist is an ancient Greek word meaning “split” or “to split”). 

The two rocks are formed within a high pressured and low temperature environments, being the sea floor or marine crust. They undergo a subduction, where sediments are carried beneath a continental plate and kneaded by changing tectonic motions while sodium-rich fluids marinate the rocks. Greenschists are formed under the lowest temperatures possible and pressures produced by regional metamorphism. With Blueschist being a schist, all traces of the original structure as well as the minerals, as wiped away, leaving its layers imposed. Greenschist’s composition comes from a combination of green minerals, such as  chlorite, serpentine, and epidote while Blueschist gets its blue color from minerals like basalt and gabbro. Sometimes, according to other minerals present in the rock, Greenschist may not always appear green. Here’s a good list of other differences and similarities between the two rocks.