Different factors play into the composition of a magma and the rock it produces.
Composition of the Original Rock
The rock beneath the Earth’s surface is sometimes heated to high enough temperatures that it melts to create magma. Different magmas have different composition and contain whatever elements were in the rock or rocks that melted. Magmas also contain gases. The main elements are the same as the elements found in the crust. Table below lists the abundance of elements found in the Earth’s crust and in magma. The remaining 1.5% is made up of many other elements that are present in tiny quantities.
How Rocks Melt
Whether rock melts to create magma depends on:
Temperature: Temperature increases with depth, so melting is more likely to occur at greater depths.
Pressure: Pressure increases with depth, but increased pressure raises the melting temperature, so melting is less likely to occur at higher pressures.
Water: The addition of water changes the melting point of rock. As the amount of water increases, the melting point decreases.
Rock composition: Minerals melt at different temperatures, so the temperature must be high enough to melt at least some minerals in the rock. The first mineral to melt from a rock will be quartz (if present) and the last will be olivine (if present).
The different geologic settings that produce varying conditions under which rocks melt will be discussed in the chapter Plate Tectonics.
What Melts and What Crystallizes:
As a rock heats up, the minerals that melt at the lowest temperatures melt first. Partial melting occurs when the temperature on a rock is high enough to melt only some of the minerals in the rock. The minerals that will melt will be those that melt at lower temperatures. Fractional crystallization is the opposite of partial melting. This process describes the crystallization of different minerals as magma cools.
If the liquid separates from the solids at any time in partial melting or fractional crystallization, the chemical composition of the liquid and solid will be different. When that liquid crystallizes, the resulting igneous rock will have a different composition from the parent rock.
Melting of an existing rock to create magma depends on that rock’s composition and on the temperature, pressure, and water content found in that environment.
Bowen’s Reaction Series indicates the temperatures at which minerals crystallize from a magma or melt from a rock.
Since minerals melt at different temperatures, a rock in which some minerals have melted has undergone partial melting; the opposite process, in which some minerals crystallize out of a magma, is fractional crystallization.
INTRUSIVE IGNEOUS ROCKS:
Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intrusive rocks form plutons and so are also called plutonic. A pluton is an igneous intrusive rock body that has cooled in the crust. When magma cools within the Earth, the cooling proceeds slowly. Slow cooling allows time for large crystals to form, so intrusive igneous rocks have visible crystals. Granite is the most common intrusive igneous rock
Igneous rocks make up most of the rocks on Earth. Most igneous rocks are buried below the surface and covered with sedimentary rock, or are buried beneath the ocean water. In some places, geological processes have brought igneous rocks to the surface.
Extrusive Igneous Rocks:
Igneous rocks are called extrusive when they cool and solidify above the surface. These rocks usually form from a volcano, so they are also called volcanic rocks:
Extrusive igneous rocks cool much more rapidly than intrusive rocks. There is little time for crystals to form, so extrusive igneous rocks have tiny crystals
Some volcanic rocks have a different texture. The rock has large crystals set within a matrix of tiny crystals. In this case, the magma cooled enough to form some crystals before erupting. Once erupted, the rest of the lava cooled rapidly. This is called porphyritic texture.
Intrusive igneous rocks cool from magma slowly because they are buried beneath the surface, so they have large crystals.
Extrusive igneous rocks cool from lava rapidly because they form at the surface, so they have small crystals.
Texture reflects how an igneous rock formed.