Black squirrels may increase in a certain area due to natural selection because of the ground below it being dark so the squirrels would adapt to the terrain, making them harder to see by predators.
EXAMPLE: if a light fur colored squirrel is on a dark terrain, an Owl or Large bird could easily see the color differentiation and notice that it is a small land organism (prey).
So the squirrels kin would evolve in the sense that they would have darker fur to match their inhabitance
sand dunes
because it is and sand dunes get deposited
The silica content affects the type of explosion in a volcano: explosive or quiet. depending on hte explosion the lava can take more time to harden. in that time the igneous rock can trap several things in the lava and those things can affect the color.
A river carrying dirt into a lake