What observable characteristic is present with banding of metamorphic rocks that is not present in the layering of clastic sedimentary rocks?
The observable characteristic present with banding of metamorphic rocks that is not present in the layering of clastic sedimentary rocks is the presence of new mineral growth and/or a strong alignment of mineral grains within the rock.
To understand this, let's start with the definition of each type of rock:
1. Metamorphic rocks: These are formed from pre-existing rocks, such as igneous or sedimentary rocks, through the process of metamorphism. During metamorphism, rocks typically undergo changes in temperature, pressure, and chemical composition, leading to the development of new minerals, reorientation of mineral grains, and the formation of banding or foliation.
2. Clastic sedimentary rocks: These rocks are formed from the accumulation, compaction, and cementation of fragments (clasts) derived from pre-existing rocks. Layers or strata in clastic sedimentary rocks are formed as sediments settle and are subsequently compacted and lithified.
Now, to identify the distinguishable characteristic present in metamorphic rocks but absent in clastic sedimentary rocks:
Banding in metamorphic rocks is a result of the unique conditions experienced during metamorphism. The intense heat, high pressure, and tectonic forces acting on the rock can cause minerals to realign or grow, giving rise to distinct bands or layers with different mineral compositions, textures, or colors. These bands may consist of different minerals, such as quartz, feldspar, or mica, aligned parallel to the direction of stress or pressure.
In contrast, the layering in clastic sedimentary rocks is primarily a result of the sequential deposition of sediments, with each layer reflecting a specific period of geological time. The layers are typically composed of similar-sized clasts or grains of different minerals, with little to no alignment or growth of new minerals.
To summarize, the presence of new mineral growth and strong alignment of mineral grains resulting in banding is a characteristic observed in metamorphic rocks but not in the layering of clastic sedimentary rocks. Understanding the geological processes behind the formation of these rocks helps explain this difference.