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.

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.

Marble was once a limestone!

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.

Quartzite was once a sandstone!

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 to the left is a hornfels - it a very fine grained rock with very few defining features other than it is very hard.

Let's see what it looks like under a microscope!

To see what some rocks are made of geoscientists make thin-sections. Yes, they are really thin slices of rock!

A thin-section is about 0.33 of a millimetre thick. Most minerals become transparent or translucent when they are that thin so we can easily look at them under a light microscope an see what size they are how they are arranged inside the rock.

The images to the left are photographs of a thin section taken from the same hornfels shown on the previous page. The image is about 40mm wide so each mineral grain you can see in the thin-section is tiny, much too small to see without a microscope.

The first image is taken with plain light. You can clearly see a variety of mineral grains.

The image (right) is taken with a special polarising light microscope. This enables us to see the optical properties of each mineral - a bit like a finger print, each mineral has its own characteristics. From this we know that this hornfels contains:

• Quartz (50–60%)
• Feldspar (20–30%)
• Carbonate "spots" (5–10%)
• Muscovite (5–10%)
• Opaque minerals (<5%)

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.