A spring is a place where groundwater
comes to the surface. The earth's crust
can have many layers of
, some porous
and some impervious
. When a layer of impervious rock lies underneath
a porous layer, water that has soaked in to the porous part will flow, underground, along the
top of the impervious layer. If the
level of the ground
drops so that the impervious layer is exposed, this water will come out of the ground as a
There are several kinds of springs.
Sometimes water is expelled from the earth under pressure, because of the shape of rock layers and
channels; in that case it's called an artesian spring. Geysers are another kind of spring. Hot springs occur
when groundwater is heated by geothermal activity before coming to the surface. Temperature of the Earth's
hot springs varies a great deal -- anywhere from lukewarm to life-threateningly boiling.