Lava that hardens on the surface is called an extrusion. The rock layers below an extrusion are always older than the extrusion. ... There, the magma cools and hardens into a mass of igneous rock called an intrusion. An intrusion is always younger than the rock layers around and beneath it.
An intrusion is a rock made up of cool lava inside the crust of the earth.
An extrusion is a rock made up of cool lava above the surface of the earth.
Extrusion happens first and then intrusion.
So, the extrusion cool down first and solidified and the layer of intrusion beneath it cools later, and it's always younger.
The formation of extrusion happens first, then the formation of intrusion.
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