Metamorphic Rocks vs Sedimentary Rocks
Rocks in the earth’s crust can be broadly categorized in to three types. Those major rocks types are igneous rocks, sedimentary rocks, and metamorphic rocks. Geologist made this classification based on the geological process, which formed the given rocks. Igneous rocks are formed when melted rock cool and solidify. Sedimentary rocks are formed when sediments get solidified. Metamorphic rocks are rocks that have changed from igneous rocks or metamorphic rocks. Like water cycle, there exist rock cycle (geological cycle) in geology. Rock cycle means the process by which rocks are formed, degraded and reformed by the internal geological processes like plutonism, volcanism, uplift etc and/or by external geological process like erosion, weathering, deposition, etc. According to rock cycle one rock type can be changed into another (either of other two types). Out of the volume of the outer 16kms of the earth’s crust, 95% is igneous rocks and 5% is made up of sedimentary rocks. Note that here the metamorphic rocks are included either of the category based on their original rock type, that is, if it is from igneous origin then that is considered under igneous rocks
Rocks are broken up in to small pieces due to weathering agents like wind, water, etc. Those small particles are known as sediments. These sediments get deposited by various mechanisms. These sediments form very thin layers. Then these layers become harder over a long period of time. Those hardened layers of sediments are called sedimentary rocks. Texture of sedimentary rocks reflects the mode of sediment deposition and subsequent weathering. Sedimentary rocks are easy to identify as layers are visible. Most sedimentary rocks are formed under the water (sea). Sedimentary rocks normally have pores as they formed from sediments. Shale, sandstone, limestone, conglomerate, and coal are some of the examples for sedimentary rocks. These rocks are usually rich in fossils. Fossils are the remains of the animals and plants, being preserved in rocks. Sedimentary rocks are found in a variety of colours.
Metamorphic rocks are formed due to metamorphism from existing igneous or sedimentary rocks or even from existing metamorphic rocks. When existing rocks undergo changes due to high pressure and/or high temperature and/or high shearing stresses, metamorphic rocks are formed. Usually metamorphic rocks are formed deep in the earth. Heat comes from magma, while pressure comes from the layer of rocks on top of the other layers. Metamorphic rocks are classified based on foliation as foliated rocks and non-foliated rocks. Foliation means the existence of series of parallel surface. These rocks usually contain crystal. Gneiss, slate, marble, and quartzite are some of the metamorphic rocks.
What is the difference between Metamorphic Rocks and Sedimentary Rocks?
Sedimentary rocks and metamorphic rocks have some differences between them.
- Formation of metamorphic rocks might involve with heat from magma, while it is not so with sedimentary rocks.
- Sedimentary rocks are formed in the earth surface, while metamorphic rocks are formed deep in the earth.
- Sedimentary rocks often contain fossils, while metamorphic rocks rarely have fossils.
- Sedimentary rocks usually have pores between pieces, but metamorphic rocks rarely have pores or openings.
- Metamorphic rocks may have bent or curved foliation, while sedimentary rocks often have layers.
- Metamorphic rocks are harder than sedimentary rocks.