To form an igneous rock, you need to melt a parent rock deep within the Earth. The melted rock is called magma. Beneath the oceans this happens primarily at spreading centers (mid-ocean ridges) and above subduction zones (e.g. the Pacific ring of fire). On Expedition 312, are we focusing on spreading centers, where new ocean crust is formed. The parent rock for this magma is peridotite, the main rock type in the Earth's upper mantle. Peridotite is named for peridot, the name for gem quality olivine. It contains two main mineral families, olivine and pyroxene. Olivine is a bright green (olive-colored) mineral in which magnesium (Mg) and iron (Fe) are the cations that provide the glue. Mantle pyroxene is dark green and part of a large family of minerals. In addition to magnesium and iron, calcium also provides the cation glue.
The upper mantle is normally solid. To melt it you need to change one or more of three conditions:
Under normal conditions, the mantle temperature doesn't increase. Water lowers the melting temperature of many minerals. This type of melting occurs as wet ocean crust and sediment descend into subduction zones. But water doesn't contribute to the melting of mantle at spreading centers.
This leaves reducing pressure. At lower pressures, melting can occur at the same temperature. This happens at mid-ocean ridges,, when mantle moves upwards to fill the gap left as the plates move apart. This type of melting is difficult to visualize. It might help to think of Denver, the mile high city. Because Denver is so high, the air pressure there is lower than at sea level. This can make it difficult for people to breathe. It also allows water to boil at a lower temperature. Instead of boiling at 100°C, which it would do at sea level, water boils at 98°C.