At the posted coordinates you will see the Christ the King statue, erected in 1935 as a devotion. The statue itself is made from marble, a type of metamorphic rock.

Metamorphic Rocks

Metamorphic rocks are one of the three main categories of rocks: the other two being sedimentary rocks and igneous rocks. These rocks are formed when existing rocks are modified by heat and pressure whilst underneath the ground. This process alters the chemical composition, texture and mineralogy of the original rocks.

There are two types of metamorphic rocks: foliated and non-foliated. We will discuss more of this later on.

Marble

Most marble forms at convergent plate boundaries where large areas of Earth's crust are exposed to regional metamorphism. Some marble also forms by contact metamorphism when a hot magma body heats adjacent limestone or dolostone.

Before metamorphism, the calcite in the limestone is often in the form of lithified fossil material and biological debris. During metamorphism, this calcite recrystallizes and the texture of the rock changes. In the early stages of the limestone-to-marble transformation, the calcite crystals in the rock are very small. In a freshly-broken hand specimen, they might only be recognized as a sugary sparkle of light reflecting from their tiny cleavage faces when the rock is played in the light.

As metamorphism progresses, the crystals grow larger and become easily recognizable as interlocking crystals of calcite. Recrystallization obscures the original fossils and sedimentary structures of the limestone. It also occurs without forming foliation, which normally is found in rocks that are altered by the directed pressure of a convergent plate boundary.

Recrystallization is what marks the separation between limestone and marble. Marble that has been exposed to low levels of metamorphism will have very small calcite crystals. The crystals become larger as the level of metamorphism progresses. Clay minerals within the marble will alter to micas and more complex silicate structures as the level of metamorphism increases.

However, the chemical composition does not change during metamorphism, as limestone and marble are both calcium carbonates (CaCO3).

Foliation in Marble?

Foliation, in layman's terms are just layered minerals that are noticable.

Phyllite, showing foliation. Notice the distinct streaks. (Source: geology.com)

The reason foliation can occur is due to the fact that the metamorphic rock is made out of different minerals. If the minerals are allowed to form layers during metamorphism, you can notice the foliation.

On the other hand, marble is almost 100% calcite. Even if the calcite formed layers and streaks of crystals it will be hugely unnoticed. There however, exceptions to this, such as if the marble has high levels of impurities in it, although this is very rare.

 The marble you see here has some veins and swirls to it. These are typically the result of numerous mineral-based impurities like various mineral impurities such as clay, sand, silt, chert or iron oxides; actually present as layers or grains in the limestone.

Questions

Send the answers to the following via my profile. Log once answers are sent. I will get back to you if there are any unsatisfactory answers.

1. Observe the statue. Give a percentage of the purity of the marble.
2. Notice the veins and swirls, what colour are they?
3. Are these veins and swirls equivalent to foliation in metamorphic rocks? Justify
4. Limestone, made of calcium carbonate is easily weathered. Knowing that marble has the same chemical composition, can you see the statue start to weather over time? Compare it to limestone;
5. Fun Fact: Microbial factors can cause marble to degrade. Name one species of bacteria that can cause marble to degrade.