When magma rises towards the surface gas bubbles start to form. Whether or not they can escape as the magma is rising affects how explosive the eruption will be.
Red spots suggest Mercury may have formed far away from the sun.
The short answer is that while lava is hot, it's not hot enough to melt the rocks that make up the side of the volcano.
They erupted for billions of years and make Earth's volcanoes look like molehills. Here's what we know and what we don't know about them.