Hydrogen is the most abundant element in the solar system. There are various configurations of hydrogen on Earth. Hydrogen exists as water vapor in the atmosphere, and water and ice in the ocean and land water, fluids in the magma and volcanoes, minerals in the crust and mantle, and with metallic iron in the core.
Hydrogen and water are important in the dynamics of Earth’s interior. They lower the internal friction of rocks and cause earthquakes. Also, water generates magmas by lowering the melting temperature of silicates in the mantle. Water softens rocks and enhances mantle convection.
Water is degassed to the surface through magmatism and returned to the mantle by subducting slabs. There might be a balance in the flux of water between the input through subducting slabs and output through degassing through arc volcanism to the surface. On the other hand, a third of the water penetrating through subduction is recycled into the mantle, whereas two thirds of this water is degassed through dehydration of the slabs during subduction.
The water contents were estimated by many procedures such as mineral inclusions in diamonds, fluid or glass inclusions in magmas, mineral physics and the phase stability of water, and geophysical observations such as electrical conductivity and seismic observations.