Ok....I'll "Try" to play teacher here....
Taylor...your partially right....it is a form of man made Lava.
It's known as Slag....1600 deg. F Slag, and it's a by-product of the process of making Elemental Phosphorus using "Lots" of electricity.
(enough electricity to power a city of about 500,000 people every second)
We are the very last electric phos plant left in the U.S. and it's also where I've worked for the last 28 years.....
Three ingredients are put into the top of the plants three "Huge" electric furnaces and melted.....
Phosphorus Ore, Carbon Coke, and Silica.....
What comes out the top is Elemental Phosphorus as a gas that is then cooled to a liquid...
Side Note Here....
Elemental Phos burns violently when it hits the air so it has to be kept under water.....
What comes out the bottom is Slag and Metal.
The slag is "Tapped Out" of each furnace every few minutes...hauled to the slag hill and dumped.
The metal is "Tapped Out" twice a day into specially set up pots with a thick "Skull" in them so the metal doesn't melt through the pot....
(A Skull is a thick layer of slag with no cracks etc in it)
It is then allowed to cool to a solid....broken up...and sold.
The Phos is pumped into rail road cars...sent to many different other plants that make phos salts out of it....
Phosphorus is used in many different things....
Toothpaste, Coke/Pepsi, Food, Fertilizer, and many, many other things.....
Most of ours now goes into making Round-Up Herbicide though as the company I work for owns both....but we are also the only ones that can make "Food Grade" phos....
Next time you have a Coke/Pepsi look on the side of the can....you will see phosphoric acid as one of the ingredients....it's what gives these there Bite....