Igneous rock as extrusive rocks. Igneous rock may form with crystallization to form granular, crystalline rocks, or without crystallization to form natural glasses. Igneous and metamorphic rocks make up 90–95% of the top 16 km of the Earth's crust by volume. Igneous rocks form about 15% of the Earth's current land surface. Most of the Earth's oceanic crust is made of igneous rock. Igneous rocks are also geologically important because: In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic and hypabyssal) or extrusive (volcanic). Intrusive igneous rocks are formed from magma that cools and solidifies within the crust
Lunar lava tube would give ready access to elevated regions for communications, basaltic plains for landing sites and regolith harvesting, and underground mineral resources. 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