Water<span> issuing from a </span>hot spring<span> is heated geothermally, that is, with heat produced from the Earth's mantle. ... The rate of temperature increase with depth is known as the geothermal gradient. If </span>water<span> percolates deeply enough into the crust, it will be heated as it comes into contact with </span>hot<span> rocks.</span>
Most organisms either directly or indirectly use energy from the sun to survive, but not all of them. Plants and some microbes use the energy from the sun to perform photosynthesis.
