Beneath a volcano lies magma, which is molten rock found in the Earth's interior. Magma collects in an area known as a magma chamber or a melt pocket, which is located underneath the volcano's surface. When pressure builds up due to an increase in volume or gas content within the magma chamber, volcanic activity can occur. Magma may rise upwards and eventually erupt from the volcano, leading to various types of volcanic eruptions and forming volcanic rock on the Earth's surface.