Simple or compound sentence
1. Earth’s surface seems calm, but its interior seethes with energy.
2. Pressure and heat inside the earth melt rock.
3. Molten rock is lighter than its surroundings; it rises to the surface.
4. Molten rock inside the earth is magma; magma on the earth’s surface is lava.
5. A volcano is formed from magma.
6. Some volcanoes erupt with great power; others are less violent.