AboutDr Thomas Bell Expertise I can answer questions regarding surface earth processes and the chemical transformations that sediments and rocks undergo with burial. I can also answer questions regarding deep time, the evolution of the elements, and the last 4.5 billion years of earth history. I specialize in metallic ore forming processes, the major geologic time periods when they were produced and what they tell us about the evolution of our planet. Learn more about my professional interests at Stratamodel.com.
Experience I am a professional consulting geologist with a background in the petroleum, mining, environmental, and geotechnical industries with over 25 years of experience.
Education/Credentials Ph.D., Geology, University of California at Berkeley, 1984
M.A., Geology, University of California at Berkeley, 1980
B.S., Geology, San Jose State University, 1978
Question Hi,
I have been browsing the internet for quite a bit in search of the heat sources that drive metamorphism. I came accross either too general articles, or articles that are too complex for me to understand.
As far as I know, there are mantle heat flow, crustal heat production (radioactive decay), and magmatism (advective and transient).
Is the mantle heat flow due to convection or does it cause convection?
Could you please help me grasp the magmatism (advective and transient) and mantle heat flow concepts.
What are the relative importance of heat sources in metamorphism and the scales they operate at.
Thank you heaps..
Mariah.
Answer Mariah,
Lets start by separating the source of heat from the mechanisms of heat transport. Radioactive decay is the ongoing source of heat in Earth's interior. In general, heat 'moves' from its source by three mechanisms, conduction, advection, and radiation.
Suppose you heat water in a black kettle. While the flame is on, water heated at the bottom of the kettle rises to the top carrying heat with it. This is the process of convection, heated matter moving from a hot region to a cool region. The word we use for the movement of heat in this process is advection. If you touch the outside of the kettle, it is hot. Heat from the interior has warmed the outer side of the kettle. This is conduction, no matter has moved, only the heat has. If you hold your hand near the kettle, you can still feel the heat. Much of this heat is radiant heat reaching your hand in the form of infrared waves.
Each of these processes is at work on Earth. Heat generated in the core, mantle, and crust escapes by the same three processes. The mantle is solid but so soft it can flow. Heated mantle material flows from areas in the interior that are relatively hot to those that are cooler near the surface. This is the same process at work inside the kettle as hot water rises toward the surface. At the same time, heat is conducted from the top of the mantle through the crust like heat through the metal skin of the kettle. This is an example of heat flow. It gets a little messy at surface because of groundwater flow and atmospheric circulation so I don't have much to say about radiant heat loss to space from Earth's surface.
Though the mantle is soft, it is not in a liquid state due to the immense pressure. Where mantle material comes into contact with the lower crust, heat is transfered by conduction which can heat the crust to the melting point. Molten rock can then move toward surface carrying heat with it. This magmatism is a major process for transferring heat from the mantle to the surface.
Metamorphism is divided into two main classes, contact metamorphism and regional metamorphism. Advective and conductive heat transport plays a role in both types of metamorphism.
Where molten rock is intruded into the crust, it looses heat to the surrounding rocks driving chemical reactions that transform the original minerals into a new assemblage of minerals. Heat arrives at the site of metamorphism by advective transport in the flowing molten rock. At the smaller scale, heat conducted through solid rock drives the metamorphic reactions. The scale of contact metamorphism is local, sometimes only a few centimeters thick around a thin sheet of magma and sometimes several hundred meters thick forming a shell like zone around a larger intrusion.
Regional metamorphism occurs when rocks formed at moderate depths are buried to great depths. The scale is much larger as you might guess from the term 'regional'. Young areas of regional metamorphism can be hundreds of kilometers in dimension. Older metamorphic terrains make up the cores of the continents and can be thousands of kilometers in dimension. As temperatures rise, the original minerals in the rocks undergo chemical reactions to form a new suite of minerals that are in or near equilibrium with the new temperature and pressure conditions.
Regional metamorphism has been thought of as a process that only occurs in mountain belts where heat flow (conductive heat transfer) and even magmatic activity (advective heat transfer) are high. This is a somewhat narrow view of the environments where regional metamorphism occurs. In fact deep sedimentary basins that mark regions of the crust that are sagging into the mantle are also sites of regional metamorphism though deformation of the sedimentary rocks may not be so extreme as that in mountain belts.
Though it is hard to generalize, heat flow is probably a more important source of heat for regional metamorphism. If the metamorphic rocks are heated enough to cause melting, advective heat transfer can begin to play a role as well.
