1. Texture Changes:
* Original Basalt: Fine-grained, aphanitic (crystals too small to see with the naked eye), often with a glassy texture.
* Metamorphosed Basalt: Can develop various textures depending on the metamorphic grade:
* Low-grade: Becomes slightly coarser-grained with the development of small crystals.
* Medium-grade: Shows a more distinct foliation (layered appearance) due to the alignment of minerals under pressure. This results in textures like schistosity (platy minerals like chlorite and mica align) or gneissosity (banding of light and dark minerals).
* High-grade: Can become very coarse-grained with large crystals, often displaying a granoblastic texture (equigranular and interlocking crystals).
2. Mineralogical Changes:
* Original Basalt: Primarily composed of plagioclase feldspar, pyroxene (augite or pigeonite), and often olivine.
* Metamorphosed Basalt: Minerals transform due to increased pressure, temperature, and/or the introduction of fluids. Common metamorphic minerals formed from basalt include:
* Chlorite: Forms at low-grade, giving the rock a green color.
* Actinolite: Forms at medium-grade, also green but often fibrous.
* Epidote: Forms at medium to high-grade, giving a yellowish-green color.
* Garnet: Can form at high-grade, often red or brown.
* Amphibole: Replaces pyroxene at higher grades, can be various colors depending on composition.
* Staurolite: Forms at higher metamorphic grades, often in characteristic cross-shaped crystals.
3. Types of Metamorphic Rocks from Basalt:
* Greenschist: Low-grade metamorphism, commonly green due to chlorite and actinolite.
* Amphibolite: Medium to high-grade, often dark-colored with amphibole minerals.
* Eclogite: Very high-grade metamorphism, with characteristic green pyroxene (omphacite) and red garnet.
* Blueschist: Formed under high pressure and relatively low temperature, often with a distinctive blue color due to glaucophane (an amphibole).
4. Factors Influencing Metamorphism:
* Pressure: Increased pressure during metamorphism causes recrystallization of minerals and development of foliation.
* Temperature: Higher temperatures facilitate mineral transformations and growth of larger crystals.
* Fluids: Water and other fluids circulating through the rock can contribute to mineral reactions and metasomatism (change in chemical composition).
In summary, metamorphism significantly alters the characteristics of basalt, resulting in a wide range of metamorphic rocks with distinct textures, mineral assemblages, and physical properties. Understanding these transformations is crucial for interpreting geological processes and the history of Earth's crust.
