Earth's Paint Palette

The Rainbow Mountains of China also known as the Earth's Paint Palette. It is located within Zhangye Danxia Landform Geological Park, Gansu Province, North-West China.

The mountains are famous for their exhibition of an array of red, blue, green and yellow coloured rocks resulting from diagenetic and mineralogical process.

Diagenesis is the chemical and physical changes undergone by a sediment after its initial deposition as it turns into rock.

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The Rainbow Mountains are cretaceous sandstones and siltstones that were deposited in China before the Himalayan Mountains were formed.
These mountains are made of exposed sedimentary rocks that were before now, well hidden below the surface of the earth.

The bulk of the processes from formation of the rocks to the display of colours is attributed to weather conditions as in wind, rain and time.
This lead to sculpturing of extraordinary shapes, including towers, pillars, and ravines, with varying colours, patterns, and sizes.

• Weathering and erosion removed the overlying layers of continental siliciclastic rocks and exposed underlying formations with different mineralogy and chemistry. The sand and silt deposited alongside iron and trace minerals provide it with the key ingredients to form the colours we see today.

• Precipitated groundwater moves through the sandstone grains and deposits trace minerals in between the grains. This precipitate can build up to a point where there is no longer an pore space between the individual grains, cementing them in place.

Formation of varying colours
The primary colour of a sandstone is deep red and this red coloring is due to formation of iron-oxide (also known as hematite (Fe2O3) from the oxidation of iron. It is the coating and cementation of hematite between the sandstone grains that makes its colouration visible.

Most of the time, iron-oxides result in the display of a dark red pigment. However, there are instances where oxides form different colors. For example:

• oxidized limonite or goethite will produce brown or yellow staining of sandstones.
• magnetite can form black staining of sandstones.
• iron sulfide if present, will display a metallic yellow colour.
• green coloring is often due to chlorite or iron silicate clays.

These are just some examples of how sandstones can be altered in colouring during diagenesis.