Source of geothermal energy

Geothermal energy is generated in the earth's core, about 5,000 kilometres below the surface. Temperatures hotter than the sun's surface are continuously produced inside the earth by the slow decay of radioactive particles, a process that happens in all rocks.
The earth has a number of different layers:

• The core itself has two layers: a solid iron core and an outer core made of very hot melted rock, called magma.
• The mantle which surrounds the core is about 3,000 kilometres thick. It is made up of magma and rock.
• The crust is the outermost layer of the earth, the land that forms the continents and ocean floors. It can be 5 to 8 kilometres thick under the oceans and 15 to 50 kilometres thick on the continents.
   

The earth's crust is broken into pieces called plates. Magma comes close to the earth's surface near the edges of these plates, this is where volcanoes occur. The lava that erupts from volcanoes is partly magma. Deep underground, the rocks and water absorb the heat from this magma. The temperature of the rocks and water get hotter as you go deeper underground.
The temperature increases by 30 Celsius each kilometre from the surface to the magma but in certain areas the increase is even higher. In Hungary the temperature gets higher by 50-60 Celsius each kilometre and it often exceeds 100 Celsius 2 kilometres deep. This is the geothermal energy provided by the heat of the rocks. In Hungary the geothermal energy is easily available and there is an abundance of underground waters. 

Where is geothermal energy found?

Most geothermal reservoirs are deep underground with no visible clues showing above ground. Geothermal energy can sometimes find its way to the surface in the form of volcanoes, hot springs and geysers. The most active geothermal resources are usually found along major plate boundaries where earthquakes and volcanoes are concentrated.
When magma comes close to the surface it heats ground water found trapped in porous rock or water running along fractured rock surfaces and faults.   Such hydrothermal resources have two common ingredients water and heat.  Naturally occurring large areas of hydrothermal resources are called geothermal reservoirs.  Geologists use different methods to look for geothermal reservoirs. Drilling a well and testing the temperature deep underground is the only way to be sure a geothermal reservoir really exists.