How Magma turns to Rock

is in the mantle. Hot rock moving toward the surface will have less pressure and will begin to melt.

Flux melting can happen too. This is true for rocks when there is water added. Water will shift the melting curve so that wet mantle melts at lower temperatures compared to dry mantle. This means that all of these components – rock content, temperature, water presence, and pressure – all affect the melting ability of rocks in the mantle of the earth's crust.

Rock partially melts on earth in a variety of situations. Plate tectonics contributes to this in different ways. Mantle plumes are areas where magma gets pushed nearer to the earth's surface. Wet mantle that subducts beneath the upper layers will melt as it gets sucked under the upper crust area. In these regions, partial melting of the silica components will occur as the rock sinks. Some components melt more so than others, so melting is uneven. .

Hot spots are seen with volcanic islands, mid-oceanic ridge areas, subduction zones, and along continental rift zones. These are where the magma rises. Magma plumes often give rise to shield volcanoes full of lava. Silica will always melt earlier than basalt rock. At temperatures above 1300 degrees Celsius, all magma will be liquid. Then as it cools, the silica rock crystallizes earlier than the basalt, forcing the rest of the magma to be thicker in its consistency.

HOW MAGMA TURNS TO ROCK

There is a sequence of reactions that happen when magma cools; these are called the Bowen reaction series. It is what explains the that magma can be liquid but will also have solid crystals in it. Olivine crystallizes first; any silica left over after this has crystallized mixes with the olivine to make pyroxene. Once pyroxene is formed, there will be a further reaction to create amphibole, and then biotite – all provided there is enough silica to finish the transformation.

Pyroxene and plagioclase feldspar crystallize at the same time. Plagioclase has lots of calcium in it but as it cools, it will have more sodium in it. This is a continuous series, starting with calcium-rich plagioclase and becoming sodium-rich. This isn't what

happens with the olivine parts. They are discontinuous, leading to distinctly different substances.

So, what do you get as rock cools? It depends on the rate of cooling. You can get nearly pure muscovite mica, quartz, and potassium feldspar, especially if there is a lot of silica tetrahedrons left after the magma has cooled. Especially rich areas of silica will finally cool this way, sometimes making giant crystals.

What this means is that the amount of silica in the magma makes a big difference in how the rock turns out. It will continue to react this way until all the silica is completely used. There are three major outcomes:

1. Mafic – this is rock that has less silica in it – less than 50 percent.

2. Intermediate – this has slightly more silica in it – about 60 percent.

3. Felsic – this has a great deal of silica in it, about 75 percent.

The oxide concentration also matters a great deal. Aluminum and iron oxides are the most prevalent.

Mafic cools in a specific way that is different from the others. Olivine is made when the silica combines with both magnesium and iron. What's left over goes to make calciumcontaining plagioclase and any after that goes to make pyroxene. This is mostly the extent of it. If it cools underground, it's called gabbro. If it cools more rapidly above ground, it's called basalt. These have the same composition. Figure 26 is basalt that has holes in it because of rapid cooling:

Figure 26.

Felsic magmas are different. They crystallize at cooler temperatures and get to the pyroxene phase before making much olivine. Plagioclase predominates resulting in more sodium-rich plagioclase, leading to potassium feldspar and much more quartz. This will have more white than black and very little mica in it. This is because there isn't a lot of aluminum or hydrogen needed for this mica material. Granite and rhyolite are the end result.

Intermediate magma cools to make diorite or andesite and looks about 50:50 black and white. Intrusive rocks of any igneous rock are those that cool underground. Extrusive rocks are those that cool above ground. Andesite cools above ground while diorite cools below.

There are other processes within the magma chamber to consider. Mafic rock is runny because the crystals form early and settle at the bottom of any magma chamber. This removes this rock out of the melting equation, leading to more felsic rock at the top of the chamber. This interesting process is called fractional crystallization. If the sunken