Which geologic principle is the reason rock layers typically form parallel to each other?

The geologic principle that is the reason rock layers typically form parallel to each other is Original Horizontality. It is important to know that the principle of Original Horizontality states that due to the gravitational action, the layers of sediment are deposited horizontally. It is actually a technique of relative dating. 

Magma is outside the volcano  and lava is inside the volcano.