Pottery shards found in China demonstrate that humans have known for over 20,000 years that heat turns mud into something else. More recently, petrologists have conducted a great many experiments that document and detail changes caused by heating minerals and rocks. So looking for sources of heat in the earth is a good place to start looking for possible metamorphism. One obvious mechanism for heating is to intrude a hot magma into a cold rock. Heat will pass from the hot magma to the cold host rock, cooling the magma and metamorphosing the host. This metamorphism is called contact metamorphism.
Figure 2.01. Contact Metamorphism by the Purcell Sill White marble bands border an olivine diabase sill in the Helena Formation (Siyeh Limestone) in Glacier National Park, Montana, USA. Click on the image to see a larger version with more information.
The photo in Figure 2.01 shows a beautifully exposed example of contact metamorphism. In the photo, an approximately 20 meter thick, dark diabase sill is visible where it has intruded limestone beds of the Proterozoic Helena Formation in Glacier National Park, Montana, USA. Heat from the sill has changed (metamorphosed) the limestone, recrystallizing calcite and quartz, whitening the color, but not producing new minerals. The bleached zone extends for 2-3 meters on both sides of the sill, confirming that the diabase layer is a sill and not a lava flow. The margins of the diabase are fine-grained and locally glassy, evidence that the host rock was cold and caused rapid crystallization ("chilling") of the intruding magma. Grain size increases from the margin into the sill for approximately 120 cm (Mejstrick, 1975).
Figure 2.02. Chilled margin on diabase dike. White plagioclase phenocrysts are absent from the margin of this diabase dike from Hull, MA, USA. Click on the image to see a larger version with more information.
Figure 2.02 is a photograph of a chilled margin on a smaller (<2m thick) Mesozoic diabase dike in Hull, MA, USA, also evidence for intrusion into a cold host rock. There is no apparent contact metamorphism of the host rock, a Proterozoic granodiorite. Possible explanations include: (1) the MA contact zone did not get as hot as the MT contact zone, (2) the MA contact zone was not hot as long as the MT zone, (3) the contact zone temperature was not high enough to change the equilibrium mineral assemblage of the granodiorite. How hot should the host rocks get when a diabase magma is intruded? What factors control the temperature? Can larger intrusions metamorphose a mountain range by contact metamorphism?
