D. changes in temperature trends.
There are four major layers, all separated by temperature. Increasing or decreasing temperature of the gases in the layers keeps them separated.
A volcanic bomb is a mass of molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. ... Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight.
I believe a <span>Stratovolcano</span>