What does the ancient earthquake detector do? it help tell the direction of where it came from and the slightest movement will make the ball fall if you are referring to the ancient china one Edit
Depending on the type of lava flow, the resulting rocks look very different. This was a pahoehoe flow. A lava flow is caused by a volcanic eruption. The lava can flow down the volcano at speeds ranging from 1/24 to 50 meters an hour. They can be 1,300 degrees fahrenheit or more. They are very destructive.
When the magma reaches the Earth's surface, the gas bubble will simply burst, the gas will easily expand to atmospheric pressure, and a non-explosive eruption will occur, usually as a lava flow (Lava is the name we give to a magma when it on the surface of the Earth).