Hornfels is a fine-textured metamorphic rock formed by contact metamorphism . Contact metamorphism occurs when a mass of hot magma intrudes into preexisting rock , whether by injecting itself into a crack or by ascending in a large body (e.g., pluton ). Rock in close proximity to the magma is temporarily softened or melted and recrystallizes with an altered texture, producing a hornfels. The term hornfels is often restricted to rocks produced by contact metamorphism of shale, slate , or mudstone.
In contrast to schists and gneisses, hornfelses show little or no foliation or layering. They form under conditions of approximately anisotropic (directionless) stress, so there is no tendency for the crystals to align in any particular direction. Traces of bedding present in the parent rock may remain in a hornfels but are not caused by metamorphosis.
Because they form by contact metamorphosis, hornfelses occur in shells or layers around bodies of intrusive magmatic rock. When seen in cross-section, as at Earth's surface, these shells or layers appear as rings or bands surrounding areas of magmatic rock. These rings are termed contact aureoles. A contact aureole may be only a few centimeters thick or several kilometers thick, depending on the size of the magmatic intrusion. An aureole of less-metamorphosed rocks, often spotted slates and semihornfels, frequently surrounds the hornfels aureole and blends smoothly with it. As is generally the rule with metamorphic rocks, coarser texture in a hornfels indicates more thorough melting and slower recrystallization.
Hornfels may be chemically altered by the magma that metamorphoses them, but generally reflect the chemical composition of their parent rocks; thus, quartz , feldspar , biotite, muscovite, pyroxenes, garnet, and calcite are common ingredients of hornfelses. However, as hornfelses are defined by process of origin (contact metamorphism), not by composition, one must establish that a rock has originated in a contact aureole to classify it as a hornfels.
See also Country rock; Intrusive cooling; Metamorphic rock
"Hornfels." World of Earth Science. . Encyclopedia.com. (September 15, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/hornfels
"Hornfels." World of Earth Science. . Retrieved September 15, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/hornfels
Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).
Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.
Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:
Modern Language Association
The Chicago Manual of Style
American Psychological Association
- Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
- In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.
"hornfels." A Dictionary of Earth Sciences. . Encyclopedia.com. (September 15, 2018). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/hornfels
"hornfels." A Dictionary of Earth Sciences. . Retrieved September 15, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/hornfels