Contact metamorphic rocks form when rocks are heated to high temperatures, usually through contact with an igneous intrusion at a relatively low pressure.
The effects of contact metamorphism usually only extend for a short distance from the cooling igneous rock that is providing the heat.
Heat intense enough to cause contact metamorphism effectively cooks the rocks adjacent to the intrusion and may be hot enough to sometimes cause a small amont of localised melting.The zone around the intrusion that gets hot enough for the intruded rocks to metamorphose is called the metamorphic aureole.
Contact metamorphism does not involve directed pressure that deforms or squashes the rocks, so contact metamorphic rocks don't usually have foliated textures like textures found in regionally metamorphosed rocks. The non-foliated appearance of rocks like marble and quartzite are often described as granular or sugary.
Protolith rocks composed of just one mineral, for example pure limestone made of nothing but calcite, simply recystallise. In the process of doing so the calcite forms a denser rock consisting of roughly equigranular calcite crystals. This process also destroys any internal structures the limestone may have had, such as fossils, laminations etc. The resultant metamorphic rock is called Marble. The crystal size can vary but the most popular marble for building and sculpture have a relatively small crystal size.
∗ Marble - this rock was once a limestone
This 3D model shows a block of marble. It is a popular rock for sculpture and for use in buildings because it is easy to shape, cut and polish because the only mineral present - calcite - is relatively soft and easy to work. In this model, the extremely flat smooth surfaces are not natural but are cut surfaces that have been polished. If you zoom in and look closely you might just make out some tiny crystal faces.
∗ Quartzite - this rock was once a sandstone
Superficially marble and quartzite might appear similar; not all marbles are white and not all quartzites are grey. However, this rock was formed from a pure quartz sandstone. The quartz grains have recrystallised to form a denser rock consisting of roughly equigranular quartz crystals. This process also destroys any internal structures the sandstone may have had, such as crossbedding, laminations etc. The crystal size can vary but are rarely coarser than the original sand grains from which they have formed.
∗ Hornfels - what happens to some other rocks
When the protolith is composed of more than one mineral some complex rearrangements of the mineralogy can occur, forming a suite of new metamorphic minerals. The result is a denser rock of tightly interlocking equigranular cystals that make the rock very hard, especially when it is very fine grained (as most hornfels are).
The protolith that forms a hornfels can ba a sedimentary rock, an igneous rock or another metamorphic rock but because the hornfels are often fined grained it can be hard, even impossible, to discern what the grains are without the aid of a microscope.
The rock below is a hornfels - it a very fine grained rock with very few defining features other than it is very hard. Follow this link to see what it looks like under a microscope, showing that it is made up of a variety of tiny mineral grains.
The 3D model below is from Picanninny Point in northeast Tasmania. It shows a contact where granitic magma has intruded into sedimentary rocks and solidified.
The granite rock is the light-coloured material and the sedimentary rock is the dark material. The heat from the granite has "cooked" the surrounding rock and the original sedimentary layers are no longer visible. There are thin veins in this rock that are composed of material that was melted by the granite.
∗ Calc-silicate formation
During contact metamorphism, if the right minerals are present in the protolith, chemical reactions can happen to form new minerals. A good example of this is where a limestone rock, made mostly of CaCO3, contains some quartz grains (SiO2). Think of a coral reef near a coastal beach where river sand might wash into the limestone forming environment.
The heat of contact metamorphism drives the chemical reaction: CaCO3 + SiO2 => CaSiO3 + CO2
This forms a new mineral called Wolastonite with the formula CaSiO3.
In the 3D image below we see an outcrop where an impure limestone (containing quartz and other minerals in addition to calcite) has been intruded by a dolerite. The hot magma caused contact metamorphism of the limestone and then cooled to a dark coloured igneous rock.
The greenish colour of the rocks surrounding the dark dolerite is due to the minerals wollastonite and diopside which have grown during contact metamorphism.
This model is from Roches Beach and shows a Jurassic dolerite intrusion into Permian impure limestone.
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