Scientists reveal reason behind ‘blood rain’ that turned Iran’s shoreline crimson

The “blood rain” that turned an Iranian island’s coastline crimson last week was due to the local soil containing high concentrations of oxidised iron, experts said.

Social media footage showed rainwater streaming down a cliff to the beach on Hormuz Island, located in the Strait of Hormuz at the junction of the Gulf of Oman and the Persian Gulf.

One of the videos of the rare sight that garnered over a million likes on Instagram showed tourists watching blood-red rainwater flow down the island, dubbed “rainbow island” due to its colourful soil and mineral deposits.

"The island is a salt dome, a teardrop-shaped mound of rock salt, gypsum, anhydrite, and other evaporites that has risen upward through overlying layers of rock,” Nasa’s Earth Observatory explained.

“Rock salt or halite is weak and buoyant, so it loses its brittleness and flows more like a liquid when under high pressure."

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The phenomenon of “blood rain” usually occurs when high concentrations of red-coloured dust or particles are mixed into rain, giving it a crimson appearance, the UK Met Office said.

In the case of the Iranian island, however, the crimson colour of the shoreline was a result of the region’s iron-rich red soil interacting with rainwater, Simon Frasers University earth sciences professor Brent Ward told CBC News.

A previous study found that iron was the most abundant of all analysed metals in the soil. “The rising mass is not purely made of salt,” the observatory said in a statement. “Embedded within it are layers of clay, carbonates, shale, and iron-rich volcanic rocks, some of which have taken on vivid shades of red, yellow and orange as they moved upward and interacted with water and minerals from other rock layers.”

The island’s soil is so rich in minerals it is used as a local spice, playing an important role in the region’s food culture. The red soil is used in the preparation of a special local bread in Hormuz called “tomshi”.

Scientists warn against using the soil in cuisines due to its heavy metal content.

“The native people of this region of Iran call this soil 'Gelak' and use it as a spice. They turn the red soil into a sauce and use it in a variety of foods," a 2023 study published in the journal Chemosphere noted.