Relative age dating geologic cross sections
Sedimentary rocks are secondary in formation, being the product of precursor rocks (of any type).
Igneous rocks form from molten material, and are further subdivided into two main categories, the volcanic rocks which form from lava extruded at or near the surface; and plutonic rocks which form from magma, deep within the crust.
Plutonic rocks on the other hand cool very slowly, on the order of a million years or more for some deeply buried and insulated magmas.
The mineral grains in these rocks can grow very large and are readily distinguished in hand samples.
Granites, for example, have more than 10% quartz and abundant potassium feldspar.
The haloes are caused by radiation damage to the host mineral's crystalline structure.
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In metamorphic rocks, new minerals form that are more stable at higher temperatures and pressures.
Sometimes the minerals segregate into distinct bands.
Note that the halo has the highest intensity of discoloration near the source, gradually fading with distance in the host mineral to a "fuzzy" edge.