This Earthcache will reveal to you the secrets of metamorphism, the process in which ordinary rock is transformed by intense heat and pressure into beautiful rocks that are prized in sculpture and architecture. At the published coordinates, you will find a large sample of serpentinite, which will be the subject of our study for this EC. However, before we embark on your learning journey, we will first need to learn about the formation of metamorphic rocks.
Part 1: What is Metamorphism?
Overview
Metamorphic rocks are one of the three main groups of rocks, the others being igneous and sedimentary, and they are created from the other two through a process known as metamorphism. During this process, rocks and minerals are transformed due to changes in temperature, pressure, and/or chemical environment. This process occurs deep within the Earth's crust, where rocks are subjected to intense heat and pressure or exposed to fluids that alter their composition.
During metamorphism, the mineral composition, texture, and physical properties of rocks can change significantly. For example, a sedimentary rock may be transformed into a metamorphic rock through the process of recrystallization, where the original mineral grains are replaced by new ones that are larger and more tightly packed. This can lead to the formation of new minerals, such as mica, garnet, or quartz.
Metamorphism can also cause rocks to become more dense, harder, and more resistant to erosion, making them more suitable for building and construction purposes. Some of the most common examples of metamorphic rocks include marble (shown below), slate, and gneiss, which are formed from limestone, shale, and granite, respectively.
Let us now examine two ways metamorphism can occur: contact and regional metamorphism.
Contact Metamorphism
Contact metamorphism is a type of metamorphism that occurs when rocks are altered by contact with a heat source, such as an intrusion of magma. When magma moves into a rock formation, it heats up the surrounding rocks, causing them to undergo chemical and physical changes.
The intensity of contact metamorphism depends on several factors, including the temperature and duration of the heat exposure, the composition of the original rocks, and the type and size of the intruding magma body. In general, the closer the rocks are to the magma source, the greater the degree of metamorphism.
During contact metamorphism, the heat causes the minerals in the original rocks to recrystallize or form new minerals. For example, limestone can be transformed into marble, a harder and more dense rock, by the introduction of heat from a nearby magma body. Shale can be transformed into hornfels, a fine-grained metamorphic rock, under similar conditions.
One of the most distinctive features of contact metamorphism is the presence of a "contact zone" around the magma intrusion, known as an areole, where the original rocks have been altered by the heat. This zone can range in size from a few centimeters to several kilometers, depending on the size of the magma body and the surrounding rocks.
Regional Metamorphism
Regional metamorphism is a type of metamorphism that occurs over large areas, typically at convergent plate boundaries or in mountain-building processes. It is caused by the combination of heat, pressure, and chemical activity over a long period of time, often millions of years.
In regional metamorphism, rocks are subjected to high temperatures and pressures, usually from tectonic forces, as they are buried deep within the Earth's crust. This process can cause the minerals in the rocks to change, creating new minerals and altering the texture and composition of the original rock.
One of the most common types of rock to undergo regional metamorphism is shale, which can be transformed into slate, phyllite, schist, or gneiss depending on the degree of metamorphism. The metamorphic grade of a rock is determined by the temperature and pressure conditions it has experienced, with higher grades indicating more intense metamorphism.
Part 2: Classifying Metamorphic Rocks
Metamorphic rocks are classified into broad groups called “facies”. Rock belonging to the same metamorphic facies undergo similar metamorphic changes due to similar temperature and pressure conditions. Think of it like different neighborhoods in a city, each with a different character and vibe. In this case, each neighborhood represents different metamorphic facies, with its own unique set of rocks and minerals.
Metamorphic facies are classified based on the mineral assemblages that are formed under specific pressure and temperature conditions. These assemblages are determined by the original mineralogy of the rock, and the degree and duration of heat and pressure that the rock has been exposed to. A diagram of the different facies is shown below:
As you can see, there are several different metamorphic facies, ranging from low-grade to high-grade, depending on the intensity of metamorphism. For example, the zeolite facies represents the lowest grade of metamorphism, characterized by the formation of minerals such as zeolites, while the granulite facies represents the highest grade, characterized by the formation of minerals such as garnet and sillimanite.
And… finally, let’s learn about the rock you’ll see at the GZ: serpentinite.
Part 3: On Serpentinite
Serpentinite is a type of metamorphic rock that is composed mainly of the mineral serpentine, which is formed through serpentinization by the alteration of minerals such as olivine or pyroxene in rocks, such as peridotite, in the presence of water and/or carbon dioxide. It is typically green in color and has a characteristic waxy or greasy appearance.
Serpentinite is commonly found in areas of the Earth's crust where there has been a significant amount of tectonic activity, such as subduction zones or fault zones. It is formed when water infiltrates cracks and crevices in rocks that are being subjected to high pressure and low temperature conditions, causing them to break down and transform into serpentinite.
Its beautiful green hue and patterns make serpentinite sought after as cladding material and for sculptures, such as the one you see here. Metamorphic rocks are a rare sight in Singapore, where most of the island is either underlain by granite or sedimentary deposits, so this rare “gem” provided a useful lesson. And so... on to your tasks!
Part 4: Earthcache Tasks
At the published coordinates, you will find a water feature and sculpture that is a gift to Singapore from the city of Vienna, Austria. This sculpture is made entirely from serpentinite, and exhibits many of the features of the rock that has been described in Part 3. The rock that is used for this sculpture is known as Tauerngrün from the foot of the Großvenediger, a mountain in the Austrian Alps.
The serpentinite from this area exhibits veins due to dissolved minerals percipitating into cracks in the serpentinite during its formation. The veins in Tauerngrün are usually deposits of talc and calcite.
Based on the information provided and your observations on site, answer the following questions:
- Describe the colour and surface patterns of the serpentinite you see at the published coordinates.
- What is the range of the width of the veins in the serpentinite used for the water feature? Describe their shape, colour and pattern. Is it consistent with the colour and texture of talc and/or calcite (which are usually white/light coloured)?
- Is serpentinite formed through contact or regional metamorphism? Explain your answer based on your observations and the information provided in the listing,
- Serpentinite is a rock classified under the greenschist facies. Under what range of temperature and pressure conditions does it form?
- Take a photo of yourself or a personal object with the sign containing the name of the sculpture and the sculptor, and attach it to your log.
As with all Earthcaches, please send your responses to me via email or the Message Centre. There is no need to get all the answers perfectly right as long as a conscious and honest effort is made.
Logs that do not meet the task requirements may be deleted.
I hope you enjoyed your geology field trip here, and happy caching!