When sedimentary rocks are subjected to intense heat and pressure, they undergo a process called metamorphism. Metamorphism causes changes in the mineral composition, texture, and structure of the sedimentary rocks. Here are the main changes that occur:
- Recrystallization: The minerals within the sedimentary rocks undergo recrystallization, meaning they rearrange their atomic structure and form new, more stable minerals. This process can lead to the growth of larger mineral crystals and the development of a new mineral assemblage.
- Texture Change: The original texture of the sedimentary rocks, which is often characterized by distinct layers or bedding, may be obliterated during metamorphism. The rocks become more homogeneous and may exhibit a foliated or non-foliated texture, depending on the metamorphic conditions.
- Formation of Foliation: Under directional pressure, some metamorphic rocks develop a foliated texture, which is the alignment of minerals into parallel bands or layers. This foliation is typically seen in rocks like slate, schist, and gneiss.
- Recrystallization of Organic Material: If the sedimentary rocks contain organic material, such as plant remains or fossils, these organic components may undergo metamorphism as well. They can be transformed into new minerals or can be completely replaced by minerals.
- Change in Mineral Composition: With metamorphism, the mineral composition of the rocks may change significantly. New minerals may form due to the metamorphic conditions, and some minerals present in the original sedimentary rocks may recrystallize into different mineral phases.
- Increased Hardness and Density: Metamorphism can result in increased hardness and density of the rocks. The intense heat and pressure cause the minerals to become more tightly packed and compacted, leading to a denser and harder rock.
- Loss of Fossils: The process of metamorphism often involves high temperatures, which can cause the destruction or alteration of fossils present in the original sedimentary rocks. Fossils may be replaced by minerals or may be entirely erased during metamorphic processes.
These changes occur gradually over long periods of time and are influenced by the specific temperature, pressure, and chemical conditions during metamorphism. The resulting rocks, known as metamorphic rocks, exhibit distinct characteristics and can provide valuable information about the geological history and conditions that the rocks have undergone.