How Does Metamorphic Rock Become Igneous Rock?

Once the forces of uplift and erosion have finished carving a landscape, new processes begin again. The remnants of old rocks are exposed to heat and pressure once more, this time in much greater quantities than was possible before. These fiercer conditions create new kinds of rock from the remnants of the older ones. This process is called metamorphism because it involves a transformation from one type of rock into another. These processes happen in stages, but for simplicity’s sake we can divide them into two phases: Intense heat turns sedimentary or igneous rock into what geologists call meta-sedimentary or meta-volcanic rock. Once those rocks have been transformed by intense heat and pressure into what geologists call mafic or ultramafic rock, they can become metagreywacke or even metasedimentary gneiss. Each phase has its unique characteristics based on the different minerals that make up the rocks when they finish going through these changes.

How Does Metamorphic Rock Become Igneous Rock?

1. The rocks are hot enough to melt them (either due to high temperatures or by the rocks being near melting points).
2. The rocks are squeezed and heated (as when a rock moves through a fault or is pushed under pressure).
3. The heat and pressure turn some of the minerals within the rock into new minerals.
4. The minerals that were previously in the rock become new minerals and new textures – porphyritic, quartzo-feldspathic, etc.
5. The rocks are cooled again, but they retain their texture and may still have some of their original minerals in them.
6. After further cooling, there may be crystal growth on the edges of grains – euhedral quartz and feldspar crystals in granite, for example – or there may be no crystals at all on the surface. This process is called autocrystallization. It can also turn out to be an important part of a mineral’s evolution because it can change its appearance during metamorphism; for example, opals form without any crystal growth after crystallization starts but then produce large crystals later on when metamorphism has completed its work on them.
7. In some cases there may not be any cooling and crystal growth may continue for some time. This can happen in metagreywacke and green schist, for example.
8. There may be the formation of smaller crystals that are similar to those that form with crystal growth in step 7 but which don’t result in a visible change to the rock’s appearance. This process is called radiative precipitation or kyanite. Other minerals may form as well – such as carbonates as calcite, aragonite, and dolomite – which then precipitate out on the surface of the rock and make it look different if they form more than one type of mineral within it at once (both aragonite and dolomite together can form a rock similar to limestone).
9. The crystals that do grow eventually leave behind sedimentary or volcanic textures on metamorphic rocks – these are called foliations, which are the steps along their surfaces that result from the formation of these new minerals (such as large ribbons in granites or layers in gneisses).
10. Some of those minerals may crack away from the rock surface and create fractures within it, creating what geologists call ripples formed by hydraulics.
11. If the rock is later eroded, the minerals in it may leave behind – as an example – a variety of kinds of sedimentary rocks, such as mudstones, or sandstones and conglomerates, if the erosion has taken place in an area where there are no pre-existing sedimentary rocks but it is still relatively young (such as in a region where sedimentary rocks are very soft and easily eroded, such as a tropical region). These sediments may then be buried further under other rocks.
12. In other cases, the effect of erosion is to change the shape of a body of rock and in that way to change its texture too; one sees this sort of thing when we look at how landscapes form on Earth: hillslopes build up because they erode into something more pointed (such as headlands on a lake or peninsulas on the sea), whereas valleys tend to be formed by rivers flowing into them at an angle and leaving behind their streams that cut narrow channels that parallel themselves over long periods but eventually diverge and lead down different paths (the run-off from this process can also cause erosion).

Igneous Rock Formation

1. Pre-metamorphic rock is formed from sedimentary or igneous rocks that are still in their original form.
2. Metamorphic rock is formed when the metamorphic conditions are intense enough to change the minerals of the original pre-metamorphic rock, either by heating it up or by pressure.
3. Different minerals are formed at different temperatures and pressures, creating different kinds of meta-sedimentary or meta-volcanic rocks.
4. Some of these changes may be very large, and therefore cause the formation of large masses of rock that is called plutons (plural: plutons).
5. Once the metamorphic conditions are no longer intense enough to change the minerals of the original rock, the remaining original minerals are called metasediments.
6. The metasediments can then become meta-gneiss or even meta-sedimentary rocks if they are exposed to intense heat and pressure again, which is called metamorphism.

Sedimentary Rock Formation

1. Pre-metamorphic rock is formed from sedimentary or igneous rocks that are still in their original form.
2. Metamorphic rock is formed when the metamorphic conditions are intense enough to change the minerals of the original pre-metamorphic rock, either by heating it up or by pressure.
3. Different minerals are formed at different temperatures and pressures, creating different kinds of meta-sedimentary or meta-volcanic rocks.
4. Some of these changes may be very large and therefore cause the formation of large masses of rock that is called plutons (plural: plutons).
5. Once the metamorphic conditions are no longer intense enough to change the minerals of the original rock, there are two remaining outcomes for this kind of rock: mafic or ultramafic rocks and metasediments.
6. Metasediments are mostly composed of quartz, feldspars, and mica. They eventually become metagreywacke or even metasedimentary gneiss.
7. Ultramafic rocks gradually change into mafic rocks – for example, serpentine can be metamorphosed into brownish-green olivine and then into green serpentinite.

Meta-Sedimentary Rock Formation.

1. Pre-metamorphic rock is formed from sedimentary or igneous rocks that are still in their original form.
2. Metamorphic rock is formed when the metamorphic conditions are intense enough to change the minerals of the original pre-metamorphic rock, either by heating it up or by pressure.
3. Different minerals are formed at different temperatures and pressures, creating different kinds of meta-sedimentary or meta-volcanic rocks.
4. Some of these changes may be very large and therefore cause the formation of large masses of rock that is called plutons (plural: plutons).
5. Once the metamorphic conditions are no longer intense enough to change the minerals of the original rock, there are two remaining outcomes for this kind of rock: mafic or ultramafic rocks and metasediments.

The Conclusion

Igneous and sedimentary rocks go through many changes, but they never truly disappear or become something entirely new. They are constantly being broken down and formed again by the ceaselessly moving tectonic plates. Once they have reached the final stage of metamorphism, they can become part of the next rock cycle. The remnants of the old rocks are broken down into smaller pieces, ready to begin the process again when a river erodes the soil and carries it downstream to the sea. As the river water evaporates and condenses into clouds, the dissolved minerals gather together and fall back to the earth as rain. The rainwater flows over the earth, picking up dirt and rock fragments and depositing them in a riverbed or coastal area. There, it will sit and slowly evaporate once more, ready to be recycled into the next rock cycle.