Gneiss is a common type of metamorphic rock found extensively within Earth’s crust. It forms when existing rocks undergo significant transformation due to intense heat and pressure deep underground. This process, known as metamorphism, changes the rock’s mineral composition, texture, and structure without melting it. This article will explain what gneiss is and detail the various types of rocks that can be altered into this distinctive banded rock.
What is Gneiss?
Gneiss is a high-grade metamorphic rock recognized by its characteristic banded appearance, or gneissic banding. This banding results from the segregation of light-colored minerals, like quartz and feldspar, into distinct layers separated by darker bands of minerals, such as mica and amphibole. The rock forms under significant heat, typically ranging from 600 to 800 degrees Celsius, and immense pressure. Its common mineral composition includes quartz, feldspar, mica, and amphibole, with variations depending on the original rock.
The Forces That Create Gneiss
Gneiss forms due to geological forces that cause profound changes in precursor rocks. High temperatures, often found at depths of 10 to 30 kilometers within the Earth’s crust, cause minerals within the rock to recrystallize. This recrystallization occurs in a solid state; its mineral grains rearrange and grow larger. The intense heat also facilitates chemical reactions, leading to the formation of new, more stable minerals.
Immense pressure from overlying rock layers and tectonic forces also plays a crucial role. This pressure compacts the rock, reducing pore space and increasing its density. Directed stress, or differential stress, is particularly significant in creating the characteristic banding of gneiss. During events like mountain building, uneven pressures cause mineral grains to align perpendicular to the direction of maximum stress. This alignment, along with the segregation of minerals into distinct layers, defines the foliated texture.
Common Precursor Rocks
Many different rock types can transform into gneiss, depending on their initial mineral composition and the specific metamorphic conditions they experience. The term “orthogneiss” refers to gneiss formed from igneous rocks, while “paragneiss” originates from sedimentary rocks. The mineralogy of the original rock dictates the resulting gneiss composition.
Granite is a common igneous precursor that transforms into orthogneiss. Granite’s primary minerals, quartz and feldspar, are stable under the high temperatures and pressures. During metamorphism, these minerals recrystallize and align, developing the distinct banding characteristic of orthogneiss. Other igneous rocks, such as diorite or gabbro, can also become orthogneiss, with their different starting compositions leading to variations in the final gneiss mineralogy.
Sedimentary rocks like shale or mudstone are frequent sources for paragneiss. These fine-grained rocks are rich in clay minerals, which recrystallize into new minerals such as mica, feldspar, and garnet under metamorphic conditions. Arkose, a type of sandstone rich in feldspar, is another sedimentary rock that can transform into paragneiss. Volcanic rocks, including felsic examples like rhyolite or mafic ones like basalt, can also undergo metamorphism to form gneiss. Their specific mineral content and the grade of metamorphism determine the resulting gneiss type.