Background information

A metamorphic aureole is a belt of rocks around an intrusion which has been altered by heat from the intrusion. This type of metamorphism is called thermal or contact metamorphism. In thermal metamorphism the rocks have been altered by heat, no directed pressure is involved. For the country rocks to be metamorphosed they must heated up and held a high temperature for long time because the chemical reactions which cause the formation of new minerals are very slow. For a metamorphic aureole to form heat must be transferred from the intrusion, causing it to cool and the country rock to warm up.

Heat transfer

Heat can be transferred in three different ways: conduction, convection and radiation. Conduction is the transfer of heat energy within a solid or between solids. When you hold a hot cup of coffee the heat you feel has been transferred by conduction though the cup. Within dry solid rocks heat can only be transferred by conduction. Convection is the transfer of heat by the movement of a fluid. Convection occurs because hot fluid is less dense than cold fluid so the hot fluid rises and the cold sinks. The coffee in your cup cools at the top and the coffee there then sinks and is replaced by warmer coffee from lower down. Convection currents occur in basic magmas which are relatively runny but not in acid magmas because they are too viscous. In permeable rocks convection currents in the ground water transfer heat upwards. Hot springs and some mineral deposits are the result of this convection. Radiation is the third way in which heat is transferred. It is the heat you feel in front of a fire. Because it doesn’t need contact between objects (as in conduction) or movement (as in convection), radiated heat passes through gases and through a vacuum. The Earth is heated by radiated energy from the Sun during the day, and loses heat from its surface by radiation at night. The diagram below shows the apparatus used in the real-life version of this experiment. Either make a sketch of the diagram or print a copy, then use labelled arrows to show where heat energy is transferred by each of the three methods. The second diagram below shows a section through an intrusion surrounded by country rock (like that shown in the map in the introduction). Again, either make a sketch copy or print the diagram. Mark on the section labelled arrows to show where each type of heat energy is transferred.

Heat conduction and heat capacity

In this experiment we will only measure the transfer of heat by conduction. Thermal conductivity is a measure of the ability of a material to transfer heat. Steel has a high thermal conductivity, it transmits heat easily. On the other hand china has a low conductivity, so that it is a good thermal insulator. Dry rocks have a low conductivity but the conductivity will be increased if the rock is porous and the pores are filled with water. The thermal gradient between two points is the difference in temperature between the points divided by the distance. If the material has a high conductivity then there will little difference in temperature between the points and therefore a low thermal gradient. If however the material has a low conductivity the difference in temperature will be greater and thus the gradient will be higher. Specific heat capacity is the amount of heat energy required to heat a substance by one degree centigrade. Some examples are given below: The amount of heat energy in an object depends on the specific heat of the material as well as its temperature and its mass. So for an intrusion the heat energy it contains is: specific heat capacity x temperature x mass. Which will cause the most metamorphism a granite batholith at 800° or a dolerite dyke intruded at 1100°? Which will require the most energy to heat up dry sand or wet sand? What factors do you think will affect the speed at which the intrusion cools?