International Rock Day

 

International Rock Day observed on July 13 is a celebration of all things rock. Rocks make up the planet we call home, so their importance cannot be overstated for this simple fact alone. We build our houses out of them; they literally provide a roof over our heads. Civilizations and industries have mined and quarried them to extract their precious treasures. One could say they are a cornerstone of modern life, as so many household products are made from rocks. We have even named a music genre after them, because…well rocks, rock!

Get To Know The Three Rock Types

What better way to celebrate our love of all things lithic (that means rocky!) than to learn more about the three types of rocks. This trio of stone-faced brothers are diverse and fascinating, so let us jump right in and get to know them a little better.

1 – Sedimentary

Sedimentary rocks types of rocks formed by accumulation or deposition of minerals or organic matter. This mineral and organic matter are collectively called “sediment“. Mineral sediments are typically eroded and weathered material sourced from older rocks. Organic sediments come from biological sources, such as sea creature shells or plant material (to name but two!).

There are two types of sedimentary rocks, clastic and carbonate. In clastic rocks, sediment is transported by rivers and other waterways but may also be carried by wind, ice or mass movement (such as landslides). Eventually, it will become deposited somewhere like a lake or in the sea where it settles to the bottom. In carbonate rocks, organic sediment is formed (without transportation) as a calcium-rich ooze produced by shelly sea or freshwater creatures.

As the sediment piles up over time, the sediment towards the bottom of the pile is compacted. Water is squeezed out from between the sediment, and various chemical reactions begin to occur. The rock begins to stick together, in processes called cementation and diagenesis. And so, our sedimentary rock is born!

Examples of sedimentary rocks are;

• shale
• sandstone
• chalk
• limestone

2 – Metamorphic

A metamorphic rock forms from other rocks, that have been changed by a process called metamorphism. Over time a rock becomes buried, as new rocks are laid down above. Over time the rocks heat up, as they are buried deeper and deeper. The pressure also increases, and the chemical and physical changes occur and the rock re-crystalised to form a newer and harder rock. As the Earth’s tectonic plates move, some regions undergo immense compression. The rocks are compressed and often pushed up, to form mountains. In this way, these deep metamorphic rocks can be brought up to the surface in these rising mountain ranges.

Examples of metamorphic rocks are;

• slate
• gneiss
• marble

3 – Igneous

Igneous rocks are formed from molten rock. The molten rocks come from within the Earth, many kilometres deep where it is very hot. This liquid rock is called magma. In some places, magma erupts at the earth surface from volcanoes. Magma that has been erupted is known as lava. Igneous rocks can form when rocks cool within the crust (intrusive igneous rocks), or at the Earth’s surface (extrusive igneous rocks). Generally, the size of the crystals related to the speed at which the rock has cooled. For example, a slow cooler igneous rock formed within the crust will have time to grow large crystals (forming rocks such as granite). Conversely, lava erupted into water will have minute crystals and can even form volcanic glass such as obsidian.

Examples of igneous rocks are;

• granite
• basalt
• obsidian 

Interesting Rock Facts

• Rocks have been fundamental to the evolution of humans. They were used as the earliest hunting tools and weapons, as well as being used to start fires and keep us warm (striking flint produces sparks, which can be used to ignite wood and grass). You have heard of the Stone Age; it was named this with good reason!
• Some of the earliest artists paints were made from powdered rocks and minerals. For example, some of the oldest cave paintings made by the Neanderthals were made with haematite (red, iron oxide).
• The most famous rock in the world is (probably) the Rosetta Stone. This stone, discovered in 1799, enabled early linguists to decipher a language that had baffled scholars for years; Egyptian hieroglyphics.
• The biggest pure gold nugget ever found, was discovered in Australia in 1869. It weighs as much as an adult human (156lbs).
• Coal is, in fact, a sedimentary rock. It is formed from compressed layers of plant matter.