Africa's confusing jumble of rocks could represent oldest known earthquakes: ScienceAlert

Our world may seem fragile, but Earth has been around for a very long time. If we venture back into the past, will we arrive at a time when things look radically different?

The answer lies in some of the oldest ruins on Earth, found in a remote corner of South Africa's Highveld region – an area known to geologists as the Barberton Greenstone Belt.

Deciphering the geological formations in this area has proven difficult, despite numerous attempts. But our new research has shown that the key to solving this code lies in geologically young rocks found on the seafloor in the Pacific Ocean off the coast of New Zealand.

This opened a new perspective on what our planet looked like when it was still young.

Our work began with a new detailed geological map (by Cornel de Rond) of part of the Barberton greenstone belt. This revealed part of the ancient deep sea floor, which was formed about 3.3 billion years ago.

However, there was something very strange about this seafloor, and it took our study of rocks in New Zealand, at the other end of Earth's long history, to understand it.

We argue that the widely held view that the early Earth was a hotter, earthquake-free place, with a surface so weak that it was unable to form solid plates, is wrong.

Instead, the young Earth was constantly shaken by large earthquakes, caused by one tectonic plate sliding under another in a subduction zone as part of plate tectonics – just as is the case in New Zealand today.

Mixed rocks

Geologists have long found it difficult to interpret the ancient rocks of the Barberton greenstone belt.

Layers that formed on land or in shallow water — for example, beautiful barite crystals that crystallized as evaporites, or the remains of bubbling mud pools — are found sitting on top of rocks that have accumulated on the deep sea floor. Lumps of volcanic rock, flint, sandstone and conglomerates are turned upside down and mixed.

We realized that this map looked remarkably similar to a geological map (drawn by Simon Lamb) that had been prepared following submarine-triggered landslides. These large earthquakes were triggered along New Zealand's largest fault, the Hikurangi Subduction Zone megathrust.

The rock consists of a mixture of sedimentary rocks, which were originally laid on the sea floor off the coast of New Zealand about 20 million years ago. This area is located on the edges of a deep oceanic trench, where Pacific tectonic plates slide down into the subduction zone, causing frequent large earthquakes.

This diagram of a subduction zone in New Zealand shows how bedrock in a shallow shelf area slides into deeper water, where huge blocks pile up on top of each other. (Simon Lamb, CC BY-SA)

Rocks in New Zealand are the key to reading the geological record in the Barberton Greenstone Belt.

What was previously thought to be untranslatable, turned out to be the remains of a massive landslide containing sediments deposited on land or in very shallow water, mixed with those that had accumulated on the deep sea floor.

This detail of a new map by Cornel de Ronde of the Barberton Greenstone Belt shows rocks mixed with the remains of underwater landslides composed of huge sliding blocks. We believe this is an inevitable consequence of one tectonic plate sliding beneath another in the subduction zone, which is periodically shaken by major earthquakes. (Cornel de Ronde, CC BY-SA)

The importance of this lies in the fact that New Zealand's geological record was uniquely created by the deep impacts of large subduction zone earthquakes. This is still happening today, most recently in November 2016, when the 7.8-magnitude Kaikoura earthquake triggered massive undersea landslides and debris avalanches that flowed into deeper waters.

We have found the oldest record of these earthquakes, hidden in the Highveld region of South Africa.

The key to other secrets

Our work may have uncovered other mysteries as well, because subduction zones are also associated with explosive volcanic eruptions.

In January 2022, the Tonga-Hunga Haapai volcano in Tonga erupted with the force of a 60-megaton atomic bomb, sending a huge cloud of ash into space. Over the next 11 hours, more than 200,000 lightning strikes flashed through this cloud.

In the same volcanic region, underwater volcanoes erupt an extremely rare type of lava called boninite. This is the earliest modern example of lava that was common on the early Earth.

The massive amounts of volcanic ash found in the Barberton greenstone belt may serve as an ancient record of similar volcanic violence. The associated lightning strikes may have created the crucible of life in which basic organic molecules were formed.

Hidden deep in the southwest Pacific Ocean, there are echoes of our planet shortly after its formation. They provide unexpected clues about the origins of the world we know today, and perhaps life itself. The key to this turns out to be the subduction of tectonic plates.

Simon Lamb, Associate Professor of Geophysics, Te Heeringa Waka – Victoria University of Wellington and Cornel de Ronde, Principal Scientist, GNS Science

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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