High School Earth Science: Rocks - Igneous Rocks — Flashcards

The rock beneath Earth’s surface is sometimes heated to high enough temperatures that it melts and forms magma.    
Different magmas have different compositions. They contain whatever elements were in the rock that melted.    
Magmas also contain gases. The main elements found in magma are the same as the elements that are found in crust. Oxygen and silicon are the most prevalent.    
Whether a given rock melts and forms magma depends on several factors. Temperature is one factor. The temperature has to reach hundreds of degrees Celsius before any minerals melt. Minerals melt at different temperatures, so the mineral composition of rock is an important factor influencing whether it melts. Of common minerals, quartz melts at the lowest temperature (about 650 °C), whereas olivine melts at the highest temperature (about 1400 °C). Therefore, as the temperature of rock rises, the first mineral to melt is quartz (if present) and the last mineral to melt is olivine (if present).

Partial melting occurs when the temperature rises high enough to melt only some of the minerals in rock. Temperature increases with depth below Earth’s surface, so melting is more likely to occur at greater depths.    
Pressure on rock also increases at greater depths. However, increased pressure raises the melting point of rock, so melting is less likely to occur at higher pressures. The addition of water to rock, on the other hand, lowers the melting point of rock, so melting is more likely to occur with added water.