Geocentric Science

Metamorphic and sedimentary rocks are very different. They form in extremely different environments. See below for some of the differences:

Metamorphic Rocks

• Form deep below the Earth’s surface

• Form under extreme heat and pressure

• Rocks undergo re-crystallization (rocks do not melt – old bonds are broken and new bonds are created)

• Banding may occur (typically alternating of light and dark minerals)

• Metamorphic rocks are dense and typically very heavy for their size

Sedimentary Rocks

• Form at or near the Earth’s surface

• Sediments are deposited and buried. Over time, the sediments are compacted and cemented together

• Low heat and low pressure

• Layers of sediment are often visible
  

• Fossils are preserved (great indicator of the geologic history of the area)

Although layering and banding imply similar concepts, banding ONLY occurs in metamorphic rocks, and layers ONLY occur in sedimentary rocks. Banding results from the intense heat and pressure that a rock is subjected to (this often results in the metamorphic rock Gneiss). The pressure forces the crystals to align themselves. Often we see alternating light and dark colored bands. The minerals separate out due to their density differences (the light colored bands are Felsic minerals, and are less dense, the dark colored minerals are Mafic minerals and are more dense).