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How did Earth get its continents? New theories emerge

Research published in the journal Science has provided new insights into the composition of the Earth’s crust and what makes it uniquely habitable. The study, conducted by a team of scientists at the Smithsonian Institution and Cornell University, tested and eliminated one popular hypothesis about why continental crust is lower in iron and more oxidized compared to oceanic crust.

Understanding the composition of the Earth’s crust is crucial to determining why it is habitable for life and how it developed over time.

“Continents are part of what makes Earth uniquely habitable for life among the planets of the solar system, yet surprisingly little is understood about what gave rise to these huge pieces of the planet’s crust and their special properties,” explained study lead author Elizabeth Cottrell.

The iron-poor composition of continental crust is a major reason why vast portions of the Earth’s surface stand above sea level as dry land, making terrestrial life possible today.

Where does the crust come from?

The building blocks of new continental crust come from the depths of the Earth at what are known as continental arc volcanoes, which are found at subduction zones where an oceanic plate dives beneath a continental plate. 

The crystallization of garnet in the magmas beneath these continental arc volcanoes removes non-oxidized iron from the terrestrial plates, simultaneously depleting the molten magma of iron and leaving it more oxidized. This is known as the garnet explanation for the iron-depleted and oxidized state of continental crust.

However, Cottrell and her colleagues set out to test whether the crystallization of garnet deep beneath these arc volcanoes is indeed essential to the process of creating continental crust as is understood. 

How the study was done

To accomplish this, the team had to find ways to replicate the intense heat and pressure of the Earth’s crust in the lab and then develop techniques sensitive enough to measure not just how much iron was present, but to differentiate whether that iron was oxidized.

To recreate the massive pressure and heat found beneath continental arc volcanoes, the team used what are called piston-cylinder presses in the museum’s High-Pressure Laboratory and at Cornell. A hydraulic piston-cylinder press is about the size of a mini fridge and is mostly made of incredibly thick and strong steel and tungsten carbide. 

Force applied by a large hydraulic ram results in very high pressures on tiny rock samples, about a cubic millimeter in size. The combination of the piston-cylinder press and heating assembly allows for experiments that can attain the very high pressures and temperatures found under volcanoes.

What the researchers learned

The study’s findings suggest that the garnet explanation for the iron depletion and oxidation in continental arc magmas is not entirely accurate. 

Cottrell said, “You need high pressures to make garnet stable, and you find this low-iron magma at places where crust isn’t that thick and so the pressure isn’t super high.” The team’s experiments showed that the iron-depleted, oxidized chemistry typical of Earth’s continental crust likely did not come from crystallization of the mineral garnet, as previously proposed.

One of the key consequences of Earth’s continental crust’s low iron content relative to oceanic crust is that it makes the continents less dense and more buoyant, causing the continental plates to sit higher atop the planet’s mantle than oceanic plates. 

This discrepancy in density and buoyancy is a major reason that the continents feature dry land while oceanic crusts are underwater, as well as why continental plates always come out on top when they meet oceanic plates at subduction zones.

13 total experiments garner results

The researchers conducted 13 experiments in which they grew garnet samples from molten rock under extreme temperatures and pressures, similar to those found deep inside the Earth’s crust.

The team then collected garnet samples from the Smithsonian’s National Rock Collection and from other researchers around the world. The garnets had already been analyzed, providing the researchers with a way to compare the concentrations of oxidized and unoxidized iron between the samples from the experiments and the previously analyzed samples. 

The researchers then used X-ray absorption spectroscopy at the Advanced Photon Source at the US Department of Energy’s Argonne National Laboratory in Illinois to investigate the structure and composition of the materials based on how they absorbed X-rays.

The experts found that the garnets they grew in their experiments had not incorporated enough unoxidized iron from rock samples to account for the levels of iron depletion and oxidation found in magmas that form continental crust. 

“These results make the garnet crystallization model an extremely unlikely explanation for why magmas from continental arc volcanoes are oxidized and iron depleted,” Cottrell said.

The findings raise new questions about how Earth’s continental crust forms. 

“What is doing the oxidizing or iron depleting?” Cottrell asked. “If it’s not garnet crystallization in the crust and it’s something about how the magmas arrive from the mantle, then what is happening in the mantle? How did their compositions get modified?” 

Conclusion of the research

The study’s findings suggest that conditions in Earth’s mantle below the continental crust may be setting these oxidized conditions.

Cottrell acknowledged that these questions are difficult to answer, but the leading theory is that oxidized sulfur could be the cause of the oxidation, which a current Peter Buck Fellow is investigating under her mentorship at the museum.

The study is part of the Smithsonian National Museum of Natural History’s new Our Unique Planet initiative, a public-private partnership that supports research into some of the most significant questions about what makes Earth special. Other research in the initiative will investigate the source of Earth’s liquid oceans and how minerals may have served as templates for life.

In summary, the study’s findings provide new insights into the composition of the Earth’s crust and what makes it uniquely habitable. The team’s experiments have shown that the garnet explanation for the iron depletion and oxidation in continental arc magmas is not entirely accurate, opening up new avenues for research.

The research was supported by funding from the Smithsonian, the National Science Foundation, the Department of Energy and the Lyda Hill Foundation.

More about Earth’s continents

The history of Earth’s continents is a complex and fascinating subject that has been the focus of intense scientific research for many decades. 

According to current scientific understanding, Earth’s continents have been in a constant state of change over billions of years, as a result of a range of geological processes including tectonic plate movement, erosion, sedimentation, and volcanic activity.

The earliest evidence for the presence of continents on Earth comes from rocks that are around 4 billion years old. These rocks are known as ancient cratons and are found in many locations around the world, including parts of Canada, Australia, and Africa. 

These cratons are thought to represent the original building blocks of the continents and are composed of some of the oldest rocks on the planet.

Over time, these cratons have been subject to a range of geological processes that have shaped them into the continents that we know today. One of the most significant of these processes is tectonic plate movement, which is the gradual shifting of large pieces of the Earth’s crust. 

Continental shift and drift

As plates move around, they can collide with each other, creating mountain ranges and other geological features. This process is still ongoing today, with the Earth’s continents continuing to shift and change.

Another important factor in the evolution of Earth’s continents is erosion. As water and wind wear away at rocks and soil, they can reshape the landscape and create new landforms. Sedimentation, the process by which sediment is deposited in layers, can also contribute to the growth and evolution of continents over time.

Volcanic activity is also a key player in the history of Earth’s continents. As magma rises to the surface, it can create new land through the process of volcanic eruption. This is how many islands are formed, and it has also contributed to the growth and evolution of continents throughout history.

Scientists have studied the history of Earth’s continents through a range of techniques, including studying the composition and age of rocks, analyzing seismic data, and using computer models to simulate the movement of tectonic plates. 

Through these methods, they have been able to gain a better understanding of how the continents have changed over time and how they may continue to change in the future.

Overall, the history of Earth’s continents is a fascinating subject that sheds light on the dynamic and ever-changing nature of our planet.

Supercontinents of Earth’s past

Geologists have identified several supercontinents that existed in the past based on evidence from rock formations, fossils, and other geological data. Here are some of the major supercontinents and their estimated time periods:

  1. Vaalbara – about 3.6 to 2.8 billion years ago
  2. Ur – about 3 billion years ago
  3. Kenorland – about 2.7 to 2.1 billion years ago
  4. Columbia (also called Nuna) – about 1.8 to 1.5 billion years ago
  5. Rodinia – about 1.1 billion to 750 million years ago
  6. Pannotia – about 600 million years ago
  7. Pangaea – about 335 to 175 million years ago

It’s worth noting that these supercontinents were not the only landmasses on Earth during their respective time periods, and there were also smaller continents and islands that formed and broke up over time.

Additionally, the exact time periods and configurations of these supercontinents are still the subject of ongoing research and debate in the scientific community.


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