Why Do Some Rocks on Earth Turn Red? Scientists Explain

A Rutgers-led study sheds new light on the phenomenon and addresses questions regarding the Late Triassic climate roughly 200 million years ago.

Why do Rocks Turn Red?

Christopher Lepre, lead author and assistant professor in the Department of Earth and Planetary Sciences at Rutgers University says, "All the red color we have seen in New Jersey rocks and in the American Southwest is due to the presence of hematite, a mineral."

He adds, "As far as we know, there are only a few locations where this red hematite phenomenon is widespread: one bing the geologic 'red beds' on Earth, and another the surface of Mars."

The study published in the Geological Society of America takes a step forward towards understanding how long it takes for redness to form, chemical reactions involved, and the role of hematite.

The research challenges conventional thinking that hematite only offers limited use in interpreting the ancient past mostly because it is a product of natural chemical changes that occur after the beds were initially deposited.

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What is Hematite?

Hematite is one of the most abundant minerals on the planet's surface and in the shallow crust. It is an iron oxide (Fe2O3) chemical that is commonly responsible for rock-forming in metamorphic, sedimentary, and igneous rocks in various locations around the globe.

Hematite is also the most valuable ore of iron. Although it was previously mined in different locations in the world, today, almost all of the production comes from a few dozen large deposits.

Most of these ores are now produced in Australia, Brazil, India, China, Russia, the United States, and many more.

Hematite Turns Rocks Red

Lepre and his team demonstrated that hematite concentrations track 14.5 million years of Late Triassic monsoonal rainfall over the vast Colorado Plateau, Arizona back when it was part of the supercontinent Pangea.

Analyzing the data, researchers assessed the relationships between environmental disturbances, the evolution of vertebrates on land, and climate.

Scientists examined parts of a 1,700-foot-long core from the Chinle Foundation in the Petrified Forest National Park, Arizona. The same core was previously studied that led to findings that gravitational tugs from both Venus and Jupiter influenced Earth's orbit roughly every 405,000 years that would influence the planet's climate for at least 2015 million years.

Lepre and his colleagues measured the visible light spectrum to determine the concentrations of hematite within the rocks. This is the first time that the technique is used in studying rocks that date back to over 200 million years ago during the Late Triassic epoch.

Many believe that the redness is caused by recent iron in rocks reacting with air similar to rusts.

Lepre says, "The hematite is indeed old and probably resulted from the interactions between the ancients soils and climate change. This climate information allows us to sort out some causes and effects whether they were due to climate change or an asteroid impact in Canada."

The team, in collaboration with Navajo Nation members, submitted a multi-million dollar grant proposal in an effort to retrieve more cores from the Colorado Plateau.

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