There is a large area made of schist. What is the most likely environment that existed before the area was metamorphosed? Choose one of the following and discuss as to why you chose it?

(A) desert dunes

(B) coral reef

(C) ocean floor

(D) volcano

(C) ocean floor

Explanation:

The protolith or parent rocks that forms schists are basically mudstones and shales. These are clastic sedimentary rocks that are rich in fines (clay sized particles).

Most shales are usually formed in deep ocean floors because larger sized clastic sediments cannot reach these environments. Winds and waves acts as active agents to deposit fines on the ocean floor.

Since shales are predominantly made up of clay minerals which are products of micas that have been chemically altered, we expect these clay minerals which are hydrous (contains water) to revert back to mica under metamorphic conditions.

As metamorphic transformation progresses under intense pressure and temperature and also minerological transformation, our rock moves from slate, to phllylite and then to schists.

Slate is a low grade metamorphic rock that is usually fine grained and foliated. As metamorphism increases, mica begins to grow and a shiny lustre appears in the rock, a phyllite results. Schists have larger mica grains and are foliated also.

Desert dunes are mostly made of sandstones. Their metamorphic transformation yields quartzite.

Coral reefs ar made up of rich calcite deposits and would form a metamorphic rock known as marble.