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
Cave type of solutional cave. They were formed by HS (hydrogen sulfide) gas rising from below, where reservoirs of oil give off sulfurous fumes. This gas mixes with groundwater and forms HSO (sulfuric acid). The acid then dissolves the limestone from below, rather than from above, by acidic water percolating from the surface. Caves formed at the same time as the surrounding rock are called primary caves. Lava tubes are formed through volcanic activity and are the most common primary caves. As lava flows downhill, its surface cools and solidifies. Hot liquid lava continues to flow under that crust, and if