Igneous Rock is formed when a magma cools underground and crystallizes or when it erupts unto the surface of the ground, cools and crystallizes. Magma that erupts onto the surface is called lava. When magma cools slowly underground the crystals are large enough to see.
Readings for compression tests usually vary between 175 to 250 PSI (Pounds per Square Inch). A higher reading generally indicates better engine performance; however you should check your vehicle manual to find out what the ideal reading should be for your vehicle.
The compression ratio of an internal-combustion engine or external combustion engine is a value that represents the ratio of the volume of it's combustion chamber; from it's largest capacity to it's smallest capacity. It is a fundamental specification for many common combustion engines. In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke. Picture a cylinder with the piston at the bottom of its stroke containing 1000 cc of air.