Intrusive rock formed from magma that cools and solidifies within the crust of the planet. In contrast, an "extrusion" consists of extrusive rock; rock formed above the surface of the crust. Large bodies of magma that solidify underground before they reach the surface of the crust are called plutons. Plutonic rocks form 7% of the Earth's current land surface. Coarse-grained intrusive igneous rocks that form at depth within the earth are called "abyssal" while those that form near the surface are called subvolcanic or "hypabyssal". Intrusive structures are often classified according to whether or not they are parallel to the bedding planes
Lunar lava tube Lunar lava tube Lunar lava tubes are sub-surface tunnels on the Moon that are believed to have formed during basaltic lava flows. When the surface of a lava tube cools, it forms a hardened lid that contains the ongoing lava flow beneath the surface in a conduit-shaped passage. Once the flow of lava diminishes, the tunnel may become drained, forming a hollow void. Lunar lava tubes are formed on surfaces that have a slope that ranges in angle from 0.4° to 6.5°. Lunar lava tubes may be as wide as before they become unstable against gravitational collapse. However, stable tubes