Instead, the venusian mantle likes to push the surface vertically. In some areas, this vertical shove has created highlands. Miranda, one of the moons of Uranus, appears to have a mantle made of ice, rather than of rock. The urge of the mantle to lose heat can also cause rock to bubble up toward the surface in blobs. Lava lamps work on the same principle.
Heat causes blobs to form and float up.
If you turn a syrup bottle upside down, you can see blobs of air slowly rise upwards. The same basic thing happens inside planets, except that the blobs are made of hot rock, instead of air. In fact, scientists have watched bubbles in syrup in order to understand the effect of rock blobs on the surface of planets. Because blobs carry hot rock from the interior of the planet toward the surface, they supply volcanoes with lava. This is what happens in the volcanoes of Hawaii and East Africa.