Editing Metamorphic rock (section)

=== Contact aureoles ===
[[Image:Eozoon01.jpg|thumb|A contact metamorphic rock made of interlayered [[calcite]] and [[Serpentine group|serpentine]] from the [[Precambrian]] of Canada. Once thought to be a [[pseudofossil]] called ''[[Eozoön]] canadense''.  Scale in mm.]]
[[File:Rock contact metamorphism eng big text.jpg|thumbnail]]
Contact metamorphism takes place when magma is injected into the surrounding solid rock ([[country rock (geology)|country rock]]).{{sfn|Yardley|1989|p=12}} The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it. Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a ''[[contact aureole]]''. Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed (unchanged) country rock some distance away. The formation of important [[ore]] minerals may occur by the process of [[metasomatism]] at or near the contact zone.<ref>{{cite book |last1=Marshak |first1=Stephen |year=2009 |title=Essentials of Geology |publisher=W. W. Norton & Company |edition=3rd |isbn=978-0393196566}}</ref> Contact aureoles around large [[pluton]]s may be as much as several kilometers wide.<ref>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |page=427}}</ref>

The term ''[[hornfels]]'' is often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism.<ref name=EB1911>{{EB1911|wstitle=Petrology |volume=21 |page=332–33 |inline=1 |first=John Smith |last=Flett}}</ref> The contact aureole typically shows little deformation, and so hornfels is usually devoid of schistosity and forms a tough, equigranular rock. If the rock was originally banded or [[Foliation (geology)|foliated]] (as, for example, a laminated sandstone or a foliated calc-[[schist]]) this character may not be obliterated, and a banded hornfels is the product.<ref name=EB1911/> Contact metamorphism close to the surface produces distinctive low-pressure metamorphic minerals,{{sfn|Yardley|1989|p=12}} such as [[spinel]], andalusite, [[vesuvianite]], or [[wollastonite]].{{sfn|Klein|Hurlbut|1993|pp=385, 456, 466, 485}}

Similar changes may be induced in shales by the burning of [[coal]] seams.<ref name=EB1911/> This produces a rock type named ''clinker''.<ref>{{cite journal |last1=Milligan |first1=Mark |title=Geosights: Colorful coal "clinker" close to Castle Gate, Carbon County |journal=Survey Notes |date=September 2007 |volume=39 |issue=3 |url=https://geology.utah.gov/map-pub/survey-notes/geosights/coal-clinker-castle-gate/ |access-date=28 February 2021}}</ref>

There is also a tendency for [[metasomatism]] between the igneous magma and sedimentary country rock, whereby the chemicals in each are exchanged or introduced into the other.  In that case, hybrid rocks called [[skarn]] arise.<ref name=EB1911/>{{sfn|Yardley|1989|p=126}}