A new view of the thermal structure of continental mountain ranges: the importance of igneous heat transport

When continental tectonic plates collide and build mountain ranges, the rocks in the interior of these ranges get hot enough to melt. This molten rock rises through the crust, and crystallises at shallower depths or is erupted at volcanoes. However, we don't know how much heat these molten rocks transport with them as they move through the crust. This uncertainly is important, because it is one of the main things we don't understand about what controls the distribution of temperature within the Earth. The reason we care about temperature is that it controls the strength of rocks, and the distribution of minerals within the Earth. These topics have wide-ranging importance, from the role they play in controlling the sizes, shapes, and evolution through time of mountain ranges, to the distribution of economic minerals.

Our proposed research will resolve the current uncertainty by testing how much heat is moved through the interiors of mountain ranges by molten rocks. This aim will be achieved by combining three research themes:

1. When rocks melt, they do so gradually over a range of temperatures, and the composition of the resulting magma depends on how hot the rocks became. Looking at the types of minerals in the rocks produced when the magma cools therefore tells us the temperature in the melting zone. As molten rocks are emplaced in the upper part of the crust, they heat up the surroundings, causing new minerals to grow, which we can use to estimate how hot these surrounding rocks became. We will apply this approach in four regions where molten rocks have been emplaced, so that we can work out how much variability there is between different locations.

2. We can use computer models to turn the observations in theme (1) into estimates of how long it took to emplace the bodies of molten rock, and how hot they were when this happened. New calculations described in this proposal demonstrate that the distribution of new minerals grown around intrusions is sufficient to make well-constrained estimates of these quantities. This objective is made possible by new measurements of how the properties of the crust vary as a function of temperature, and by the power of modern computers that means we are able to run all of the necessary calculations.

3. By combining the results of themes (1) and (2) we can make new computer models of the temperature within mountain ranges. These models will include the effects of the melting of rocks, and those rocks transporting with them the amount of heat that we have been able to estimate in objective (2). We will therefore be able to create a new and more accurate understanding of the temperature structure of mountain belts. By combining this information with our knowledge of the temperature-dependent strength of rocks, we will be able to address a long-running controversy over what happens in the deep interiors of mountain belts, and therefore how they evolve through time (for example whether the rocks are able to flow in 'channels' through the crust, much like toothpaste being squeezed from a tube).

The final part of our proposed research will be to examine the implications of our results for other areas of Earth Science. It is currently debated whether intrusions of molten rock occur as a single large body, or as a series of smaller bodies arriving at different times. Our results for how long the intrusions were appearing and heating the surrounding rocks will allow us to answer this question in the regions we study. Finally, we will be able to use our results to update our knowledge of which rocks we would expect to be produced in which locations within the Earth, as this depends on the temperature. The wider importance of this topic is that it allows us to use the distribution of rocks at the present day to infer what happened in the geological past, and also aids our efforts to understand the distribution of economically important minerals.