This is the explanation I have come across in Lunarnetworks blog...This is soo simple and understandable in a go..I just loved it...See why volcanism and How volcanism in a few lines
It is necessary to understand some elementary facts about planetary geology. Planets generate heat and this heat must be dissipated. Typically, the heat generated from both the original energy release during formation (accretion) and from the decay of radioactive elements (e.g., uranium) melts the interiors of planets, forming bodies of liquid rock called magma. This magma is usually less dense than the rocks from which it forms and thus, rises upwards towards the surface. Sometimes, the molten rock cannot ascend any higher from the deep locations where it comes from and freezes in place – geologists call this type of frozen rock body an intrusion, because it intrudes into pre-existing rock as a liquid and then solidifies by crystallizing. When magma actually reaches the surface of a planet, it can erupt onto its surface as lava; this activity is called extrusive because the molten rock extrudes onto the surface and then solidifies as lava flows.
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