As a volcano extrudes silicic lava, it can form an inflation dome, gradually building up a large, pillow-like structure which cracks, fissures, and may release cooled chunks of rock and rubble.
The top and side margins of an inflating lava dome tend to be covered in fragments of rock, and ash.
Examples of lava dome eruptions include the dome, and successive lava domes of Lava tubes are formed when a flow of relatively fluid lava cools on the upper surface sufficiently to form a crust. Beneath this crust, which being made of rock is an excellent insulator, the lava can continue to flow as a liquid. When this flow occurs over a prolonged period of time the lava conduit can form a tunnel-like aperture or lava tube, which can conduct molten rock many kilometres from the vent without cooling appreciably. Often these lava tubes drain out once the supply of fresh lava has stopped, leaving a considerable length of open tunnel within the lava flow.
Lava tubes are known from the modern day eruptions of Kīlauea, and significant, extensive and open lava tubes of Tertiary age are known from North some extending for 15 kilometres.
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