What Are the Rainbow Mountains? Who Colored Them the Way They Are?
The Rainbow Mountains are cretaceous sandstones and siltstones that were deposited in China before the Himalayan Mountains were formed. The sand and silt was deposited with iron and trace minerals that provided it with the key ingredient to form the colors we see today.
As a stretch of multicolored hills that look like painted rainbows, in Zhangye Danxia National Geological Park located in the Gansu province in China’s northwest covering 200 square miles. The site was named a UNESCO World Heritage Site in 2009 and is the destination for many Chinese and international tourists. Just like a tiered cake, the landscape of Zhangye Danxia was formed by layers called strata.
Wind and water eroded these layers over time and sediment that settled hardened over the years. Changes in climate affected the rate of erosion and the appearance of the sediment. Now, when the layers eroded, a new color of sediment came out forming another sheet of rock over the hill. This process happened many times, producing different sheets of different hues.
We know that mountains are formed when Earth’s tectonic plates push against each other. Around 50 million years ago, the plate that carried India pushed against the one that carried the rest of Asia. The collision was very powerful and as a result, a large mass of colorful hills popped out of the surface of where Zhangye Danxia is today.
So now that you have an idea of how the Rainbow Mountains formed, we will discuss a bit about how they got the color 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. This process is what imparts the trace minerals mentioned below and allows for the otherworldly coloring of sandstones around the world.
The primary color is a deep red sandstone, not unlike the Fountain Formation that outcrops in the Flatirons, Red Rocks Park, and the Garden of the Gods all in Colorado. The red coloring is due to an iron oxide coating and cementation, also known as hematite (Fe2O3), between the sandstone grains. This is the exact same process that takes place when a piece of metal is left out in the rain and forms a red layer of rust around the outside.
Weathering, mixed with water and oxygen oxidizes elemental iron into iron oxide, which is notable for its dark red coloring. The Rainbow Mountains are largely characterized by this iron oxide staining of its sandstone Danxia formation.
Most of the time iron oxides impart 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. If there is iron sulfide present, you will get a metallic yellow color imparted by the sulfur. Meanwhile, green coloring is often due to chlorite or iron silicate clays. These are just some examples of how sandstones can be altered in coloring during diagenesis.