Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface. It takes place at temperatures in excess of 150°C (300°F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation.

Metamorphism is a process where existing rocks undergo transformations due to exposure to high temperature and pressure conditions without any actual melting. This leads to changes in mineral composition or texture within the rocks. The result can be either metamorphic rocks with different characteristics from their parent material (protolith), such as marble formed from limestone, or new minerals may form through chemical reactions between existing components of rock.

In this context, let's consider an example where a mafic rock undergoes changes due to metamorphism:

1. The starting point is typically basaltic basalt which contains high levels of iron and magnesium along with some other elements such as silicon dioxide (SiO2), aluminum oxide (Al2O3) etc.

2. As these rocks are subjected to increasing pressure & temperature conditions over millions or billions years, certain chemical reactions start occurring within them leading eventually towards formation new minerals like hornblende , actinol Waistline chalcopyrite pyroxenites ; this entire process is known as metamorphism since no melting occurs during transformation phase itself but only rearrangement takes place among already present atoms ions inside solid rock matrix until finally we get final product - greenschist facies rocks rich in minerals such as chlorite epidote quartz etcetera depending upon exact conditions under which they formed.

Some common examples of metamorphic reactions include:

1. Serpentine formation from peridotites: This reaction occurs when ultramafic rocks like serpentinized peridotites are subjected to low temperatures and high water pressure, leading to the formation of serpentinite minerals such as antigorite, lizardite, or chrysotile.

2. Kaolinization of feldspar-rich rocks: In this process, feldspars in igneous or sedimentary rocks undergo alteration due to weathering under acidic conditions, resulting in kaolinites and other clay minerals like montmorillonites or illites depending on the specific chemical environment present during alteration.

3. Metamorphic facies series:

a) Sedimentary Facies: These are formed by lithification of sediments that settle at low temperatures without much cementation.

b) Lower Meta-Facies (Green Schist): Formed from metamorphism of shale, argillite or sandstone under increasing pressure and temperature conditions than those in the sedimentary facies but still below regional metamorphic grade.

c) Middle-Upper Meta-facieses (Amphibolites & Gneisses):

i. Amphibolites: These are formed through metamorphosis of mafic igneous rocks like basalts, basaltic andesites under conditions intermediate between lower meta facies and migmatite facies.

ii. Migmatitic gneisses: Formed due to partial melting (migmatization) within amphibolite or other metapelites during regional metamorphism.

d) Upper Meta-Facies (Garnet Schists & Granulites):

i. Garnet schist formation occurs when mica-rich rocks like muscovite-biotite schists undergo high-grade metamorphic transformation under pressure and temperature conditions exceeding those of amphibolitic facies but below granulitic grade.

ii. Granulites: These are formed through complete melting (anorthosisis) followed by solidification at very low pressures compared to other types mentioned above, resulting in rocks rich mainly in feldspar with minor amounts of quartz and/or garnet crystals embedded within them.

In summary, metamorphism refers to the transformation or alteration of existing rock materials due primarily to changes caused by heat stress rather than any actual melting taking place during this process itself; thus allowing us distinguish between different types based upon degree intensity at which these transformations occur depending largely upon factors such as temperature variations pressure gradients available within specific geological settings being considered.

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