Rocks in which minerals and texture have changed while remaining solid after exposure to high-temperature and high-pressure conditions are called metamorphic rocks. Understanding types of metamorphism and the rocks they produce helps us understand Earth's interior.
Basic Knowledge
Contact metamorphism (thermal metamorphism) occurs when surrounding rocks are altered by heat from an intrusion of magma. It occurs under high-temperature and low-pressure conditions. Typical metamorphic rocks include hornfels (sandstone and mudstone metamorphosed by contact with magma; dense and hard) and crystalline limestone (marble) (contact-metamorphosed limestone with recrystallized calcite). A metamorphic zone forms around an intrusive body as an aureole several cm to several km wide.
Regional metamorphism occurs when high-pressure and high-temperature conditions develop over a wide area in convergent plate boundaries (subduction zones and collision zones). There are high-pressure types (blueschist facies: low temperature and high pressure; subduction zones) and high-temperature types (sillimanite facies and amphibolite facies: high temperature and high pressure; collision zones). Typical metamorphic rocks include schist (platy texture, with foliation and parallel alignment of flaky minerals), gneiss (banded texture, high temperature and high pressure), and eclogite (garnet + omphacite; ultrahigh-pressure metamorphism).
In Japan, the Sambagawa metamorphic belt (high-pressure, low-temperature type caused by subduction) and the Ryoke metamorphic belt (high-temperature, low-pressure type caused by magmatic heat) occur as a pair, forming the characteristic structure called a paired metamorphic belt.
What rock forms when limestone undergoes contact metamorphism? What do sandstone and mudstone become when they undergo contact metamorphism?
Answer: Limestone becomes crystalline limestone (marble) as calcite recrystallizes into coarser grains due to heat from magma. Sandstone and mudstone become hornfels, a dense and hard rock, as their minerals rearrange and fuse because of heat.
- Contact metamorphism: heat from magma, local, high temperature and low pressure
- Hornfels: product of contact metamorphism of sandstone and mudstone
- Marble (crystalline limestone): product of contact metamorphism of limestone
- Regional metamorphism: convergent plate boundaries, widespread, high pressure to high temperature and high pressure
- Schist: platy texture (foliation), representative of regional metamorphism
- Gneiss: banded texture, high-temperature and high-pressure regional metamorphism
- Paired metamorphic belts in Japan: Sambagawa (high pressure and low temperature) + Ryoke (high temperature and low pressure)
Points to Note
① Metamorphic rocks change while remaining solid; if they melt, they become magma. ② Marble is a metamorphic rock formed from limestone and differs from limestone, which is a sedimentary rock. ③ Schist and gneiss are similar in having foliation, but gneiss has undergone a higher degree of metamorphism.
Practice
- What rock forms when limestone undergoes contact metamorphism?
- State the geological environment in which regional metamorphism is likely to occur.
- Name the two metamorphic belts that make up Japan's paired metamorphic belt.