Caves, those deep dark tunnels going down into the depths of the earth. They at one time sheltered man from the elements as well as stirred his imagination, harboring dragons and riches alike. Even to this day are still both terrifying and fascinating to many. How did they form?
Most caves are formed in limestone formations by water dissolving minerals containing calcium carbonates from surrounding rocks. This occurs when the slightly acidic water dissolves out the carbonate compounds (To see a more rapid example, put an egg in vinegar and watch. The eggshell is almost pure calcium carbonate and the vinegar is a mild acid), leaving an opening beneath the earth. This is why most caves are associated with water, streams dissolve out the water and then carry it off to the sea. All of the larger cave systems have significant rivers running through them and are found in limestone formations.
It is inside these caves that you have might stalactites (holding tight to the ceiling) and stalagmites (they rise mighty from the floor), "God's Cup" cold, clear water and hundreds of different types of crystal and mineral formations. The acidic water makes interesting shapes and can concentrate elements with get left behind when the water evaporates. But are these the only types of caves found around the world?
There are also volcanic caves or lava tubes.
Shield Volcanoes are comprised of hot explosive gases and ash which can blow out suddenly. They also have large pockets of pumice. The gases collect in sizable pockets and can then explode out, or sometimes leak out slowly, this can cause small, circular caves or chambers. The pumice is a very soft, brittle rock that can erode quickly due to wind or rain. The pumice gets broken up over time and physically carried out as silt, in the Caribbean it collects black sand beaches. These are usually smoother with little or no extra crystal formations. The lava tubes form from when the lava has a hot center and then cools, shrinking back and creating smooth, hard tunnels. Are either of these true caves?
The limestone caverns are certainly more spectacular, with their columns hanging from the ceiling and rising up from the floor, thus are where most spelunkers go. They are deeper, more extensive and more varied. But the volcanic ones are natural geological formation and definitely look like caves to most! While they are different, both are caves!