The core of the earth’s crust, about 1,800 miles below the surface, is the hottest part of the planet earth. Most of that heat is generated by a continuous decay of radioactive isotopes. The heat generated may cause core temperatures to rise above 5,000 degrees Celsius, radiating outwards and leading to considerable warming of gas, water, rocks and other geological materials.
As the heat radiates from its source at the core towards the earth’s surface, the warming effect gradually reduces, creating a geothermal gradient with a temperature difference of about 25 degrees Celsius for every kilometer of depth.
If underground rock formations become heated to temperatures between 700 and 1,300 degrees Celsius, they become partly melted. When mixed with gas and gas bubbles they form magma, which exists in the lower crusts, but whenever the magma bubbles to the earth’s surface as lava, a volcanic eruption is said to occur.
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