Here's a breakdown of what happens:
1. The Starting Material: Basalt is a dark-colored, fine-grained volcanic rock that is typically rich in minerals like plagioclase feldspar, pyroxene, and olivine.
2. The Transformation: When basalt is subjected to heat and pressure deep within the Earth's crust or during tectonic plate movement, the minerals within the rock undergo a transformation. The original minerals recrystallize into new minerals, which are more stable under the new conditions.
3. The Resulting Rock: The metamorphic rock that results from metamorphosed basalt can take on several different forms depending on the intensity and type of metamorphism:
* Greenschist: A low-grade metamorphic rock characterized by green minerals like chlorite and actinolite.
* Amphibolite: A medium-grade metamorphic rock containing minerals like hornblende and plagioclase.
* Eclogite: A high-grade metamorphic rock with minerals like garnet and pyroxene, forming a dense and hard rock.
Key Characteristics of Metamorphosed Basalt:
* Foliation: Some types of metamorphosed basalt, like greenschist, can exhibit foliation, a layered appearance caused by the alignment of minerals during metamorphism.
* Increased Density: The minerals in metamorphosed basalt are denser than those in the original basalt.
* Different Mineral Assemblage: Metamorphosed basalt will have a different mineral composition compared to the original basalt, reflecting the new conditions under which it formed.
Examples of Metamorphosed Basalt:
* Greenstone belts: These are ancient belts of metamorphosed basaltic rocks found in many parts of the world.
* Oceanic crust: The lower part of the oceanic crust is made up of metamorphosed basaltic rocks.
Understanding the transformation of basalt into different metamorphic rocks helps us interpret the geological history of the Earth and provides insights into the forces that have shaped our planet.
