When I created a country I called it Fire Realms. You could do something based off of that.
The answer is deep sea trenches
The main mechanism through which the layering happened is gravity: it pulled the heavier, denser material to the center, leaving the less dense material on the surface.
The material that was on the outside cooled quicker, but this was the result, not the cause.
The answer is
<span>2) Gravity caused more-dense material to move toward the center of each planet.</span>
Ruler, Key, Compass, Pencil
