What is Contact Metamorphism and Its Examples
Contact metamorphism is a geological process that occurs when rocks come into contact with molten magma or hot fluids. The heat from these sources causes changes in the composition, texture, and mineralogy of the surrounding rocks, resulting in the formation of new minerals and rocks. This metamorphic process primarily happens in the upper few kilometers of the Earth’s crust.
Examples of Contact Metamorphism:
1. Aureole around an igneous intrusion: When a plutonic or volcanic body of magma intrudes into the surrounding rocks, it heats them up, causing contact metamorphism. The altered rocks create a distinct zone known as an aureole.
2. Skarn deposits: Skarns develop when hydrothermal fluids associated with intrusive rock deposits minerals onto the walls of fractures, often forming economic mineral deposits. These deposits commonly contain minerals like garnets, pyroxenes, and vesuvianite.
3. Hornfels: Hornfels is a fine-grained metamorphic rock formed by the contact metamorphism of siliceous sedimentary rocks or volcanic rocks. It typically has a smooth, splintery texture and can exhibit colors like gray, green, or brown.
4. Marble: Limestone or dolomite rocks that come into contact with hot magma or fluids undergo contact metamorphism and transform into marble. Marble is prized for its aesthetic value and is used in sculptures and architectural applications.
5. Quartzite: Quartz-rich sandstones or chert rocks can be transformed into quartzite through contact metamorphism. Quartzite is a hard, durable, and heat-resistant rock that is often used in construction and road-building materials.
6. Hornblende hornfels: Hornblende hornfels is a dark-colored, fine-grained metamorphic rock that develops from the contact metamorphism of basaltic or andesitic rocks. It consists mainly of hornblende minerals, with or without small amounts of other minerals.
7. Epidote hornfels: Epidote hornfels is a metamorphic rock formed during contact metamorphism of basic igneous rocks. It contains the mineral epidote, which gives it its characteristic green color.
8. Nicolas Andalusite: Nicolas Andalusite is a variety of andalusite that forms as a result of contact metamorphism. It is named after the French geologist Georges-Antoine Nicolas and displays unique aesthetic patterns consisting of light and dark bands.
9. Cordierite hornfels: Cordierite hornfels is a metamorphic rock formed from the contact metamorphism of pelitic rocks. It is composed primarily of cordierite minerals and has a characteristic bluish-gray color.
10. Thermal aureole in limestone: Limestone rocks that come into contact with hot igneous intrusions develop a thermal aureole. This zone of metamorphism alters the limestone, resulting in the formation of new minerals such as garnets, wollastonite, and diopside.
11. Chlorite hornfels: Chlorite hornfels is a green-colored metamorphic rock formed during the contact metamorphism of mafic rocks, particularly basalts.
12. Thermal metamorphism of shale: Shale rocks that are subjected to contact metamorphism due to the intrusion of hot magma or fluids undergo significant changes. This can result in the formation of rocks like hornfels or the development of minerals such as biotite or cordierite.
13. Almandine garnet in schist: Almandine garnet is a mineral that often forms as a result of contact metamorphism of pelitic rocks, leading to the formation of garnet-rich schists.
14. Contact metamorphism of coal: Coal beds that come into contact with hot igneous intrusions result in the formation of anthracite coal. The heat from the contact metamorphism drives off volatile components, leaving behind a carbon-rich, hard coal.
15. Contact metamorphism of limestone to form marble: Limestone rocks exposed to heat from nearby igneous intrusions undergo changes, leading to the formation of marble, a metamorphic rock composed of recrystallized calcite.
16. Contact metamorphism of sandstone to form quartzite: Sandstone rocks subjected to the heat from magma or fluids undergo contact metamorphism, transforming into quartzite. Quartz grains in the sandstone recrystallize, resulting in a harder, more durable rock.
17. Contact metamorphism of shale to form hornfels: Shale rocks exposed to high temperatures during contact metamorphism undergo a complete recrystallization process, resulting in the formation of hornfels. Hornfels is typically fine-grained and dense.
18. Contact metamorphism of basalt to form amphibolite: Basaltic rocks that come into contact with hot magma or fluids metamorphose into amphibolite. Amphibolite is a coarse-grained rock composed mainly of amphibole minerals.
19. Contact metamorphism of limestone to form skarn: When limestone interacts with igneous intrusions, it undergoes metasomatism, resulting in the formation of skarn. Skarns are characterized by the presence of minerals like garnet, pyroxene, and epidote.
20. Contact metamorphism of granite to form greenschist: Granite rocks in contact with hot fluids may undergo metamorphic changes, altering the minerals present. This can lead to the formation of greenish-colored greenschist, which typically contains minerals such as chlorite and actinolite.
20 Questions and Answers about Contact Metamorphism and Its Examples:
1. What is contact metamorphism?
Contact metamorphism is a geological process in which rocks are altered by the heat from nearby magma or hot fluids.
2. Where does contact metamorphism primarily occur?
Contact metamorphism primarily occurs in the upper few kilometers of the Earth’s crust.
3. What are some examples of contact metamorphism?
Examples of contact metamorphism include the formation of hornfels, marble, quartzite, skarn deposits, and various hornblende, epidote, and chlorite hornfels.
4. How does contact metamorphism change rocks?
Contact metamorphism changes rocks by altering their texture, mineralogy, and composition through the heat from nearby igneous intrusions or hydrothermal fluids.
5. What is an aureole in contact metamorphism?
An aureole is a distinct zone of altered rocks surrounding an igneous intrusion, resulting from contact metamorphism.
6. What are skarn deposits?
Skarn deposits are economic mineral deposits formed from the metasomatization of rocks interacting with hydrothermal fluids associated with an igneous intrusion.
7. How is marble formed through contact metamorphism?
Limestone or dolomite rocks are transformed into marble through contact metamorphism when they come into contact with hot magma or fluids.
8. What are the characteristics of quartzite?
Quartzite is a hard, durable, and heat-resistant rock formed by the contact metamorphism of quartz-rich sandstones or chert rocks.
9. What minerals are commonly found in hornfels?
Hornfels can contain various minerals depending on the parent rock, but commonly include feldspars, pyroxenes, garnets, and other silicate minerals.
10. What is cordierite hornfels?
Cordierite hornfels is a metamorphic rock formed from the contact metamorphism of pelitic rocks. It is primarily composed of cordierite minerals and has a bluish-gray color.
11. What is the process behind the formation of skarns?
Skarns are formed through the interaction of hydrothermal fluids with carbonate rocks, resulting in the deposition of minerals such as garnet, pyroxenes, and vesuvianite.
12. How does contact metamorphism affect coal?
Contact metamorphism of coal causes the expulsion of volatile components, resulting in the formation of harder, carbon-rich anthracite coal.
13. How does contact metamorphism affect shale rocks?
Contact metamorphism of shale rocks can lead to the formation of hornfels or the development of minerals such as biotite or cordierite.
14. What is the difference between contact and regional metamorphism?
Contact metamorphism occurs locally due to the heating effect from intruding magma or hot fluids, while regional metamorphism occurs over large areas due to tectonic forces and deep burial.
15. Why do limestone rocks undergo contact metamorphism to form marble?
Limestone undergoes contact metamorphism to form marble due to the heat provided by nearby magma or fluids, causing recrystallization of the calcite minerals.
16. How is quartzite different from sandstone?
Quartzite is different from sandstone because it undergoes recrystallization during contact metamorphism, resulting in a harder, more durable rock.
17. What is the characteristic color of chlorite hornfels?
Chlorite hornfels is characterized by its green color, which is due to the presence of the mineral chlorite.
18. What is the main mineral found in epidote hornfels?
Epidote hornfels contains the mineral epidote, which gives the rock its characteristic green color.
19. How is hornblende hornfels formed?
Hornblende hornfels is formed through the contact metamorphism of basaltic or andesitic rocks. It consists mainly of hornblende minerals.
20. How does contact metamorphism affect the minerals in granite?
Contact metamorphism can alter the minerals in granite, resulting in the formation of rocks like greenschist and changes in mineral composition.
