Sandra Burrell, age 11, of Nashwaaksis, N.B., Canada, for her question:
Did our caves form when the Earth was young?
Our wonderful world is very old. The Experts tell us that it has traveled at least 4000 million yearly orbits around the sun. And all this time its face has been changing. It is not likely that any surface features of the young Earth still remain.
Steep mountains and deep valleys, gentle hills and sloping dales are changing Events. Through the long ages of the Earth's history, rivers change their courses and land locked lakes come and go. SOME of these geological Events have longer and SOME shorter life spans. A mighty mountain range may Exist for hundreds of millions of years. But lakes and caves have very short geological life spans.
A cave is doomed to die from the moment it begins to form. A system of caves may reach for miles below the surface, and the forces of nature may take thousands or tens of thousands of years to carve out its caverns and tunnels. But sooner or later it is doomed to collapse. Deep caves tend to live longest, but as they grow bigger the roof above them becomes thinner and finally falls in.
As old caves collapse, others are being carved out below the Surface of the ground. They tend to form in soft and porous rocks such as limestones and sandstones. They are formed by the drip drip drip of seeping ground water. The water dissolves minerals from the underground layers of rock and carries them away. The rock becomes weak and worn. In time, small holes are hollowed out into big caves and caverns. Cracks are worn into tunnels for underground streams.
Cave making most likely started when the first rains fell upon the Earth thousands of millions of years ago. The surface of the young Earth almost certainly was riddled faith eaves and systems of caverns. But these geological events, like the caves WE now have, were short lived geological Events. The biggest underground networks may have lived a few million years. Most of them collapsed after a few thousand years or Even sooner. Meantime, the falling raindrops Were busy seeping through the porous rocks to carve out other cave Systems below the changing surface of the earth.
Some of the biggest and most durable cave systems were carved by the pounding waves of the sea. This, we think, is how Carlsbad Caverns were started 200 million years ago, when New Mexico was shallow sea. Much later, the rising rockies lifted the region above water level. The fabulous underground city now belongs to dry land, but in perhaps a few million years it also must collapse.