Mantle Currents and Ice Sheets Caused the Great Unconformity, Study Says

Missing rock layers spanning hundreds of millions of years vanished because deep mantle currents pushed continents high above sea level where massive ice sheets scoured them clean, according to new research published in Nature on Thursday.

The Tech TL;DR:

• Mantle convection slowly lifted ancient continental blocks between 920 million and 420 million years ago.
• Enormous ice sheets during the Snowball Earth period ground away the exposed rock layers.
• Subsequent subsidence allowed younger sedimentary layers to form and preserve the geological gap known as the Great Unconformity.

Geologists studying rock records across North America, northern Europe, and northern China have long puzzled over the Great Unconformity, where flat sedimentary rock layers rest directly atop crystalline basement rock up to two billion years old. This geological gap, visible in locations such as the Grand Canyon, erases up to a quarter of Earth’s history.

Previous hypotheses attributed the missing rock to Snowball Earth glaciation, tectonic reshuffling, or sea-level change, but testing these ideas remained difficult because the primary evidence consists of missing material.

“It is a thrill to be able to propose a solution to this geological puzzle,” said Santosh Dhungana, PhD candidate and lead author from the University of Wollongong.

Dhungana and co-author Nicolas Flament used the Australian supercomputer Gadi to model mantle convection over the past billion years. Their simulations showed that hot mantle currents lifted ancient continental blocks including Laurentia, Baltica, and North China between 920 million and 420 million years ago. During this period of high elevation, glaciers and ice sheets slid downhill across the land, acting more efficiently than rivers to erode the exposed underlayers.

“Published in Nature, the study argues the gentle rise and fall of whole continents over hundreds of millions of years, caused by hot currents deep below the Earth’s crust, led to the confounding disappearance of huge swathes of Earth’s geological record,” stated a University of Wollongong report.

After the prolonged period of uplift and erosion, the mantle convection models indicate that these continental blocks began sinking during the Phanerozoic eon over the last 540 million years. As the worn rock surfaces subsided into the ocean, newer layers of sediment accumulated on top. This sequence matches the sudden appearance of younger sedimentary layers right around the Cambrian explosion, which brought a vast diversification of animal life on Earth.

In contrast to Laurentia, Baltica, and North China, models for Australia and South America showed only two cycles of uplift and subsidence from 900 million years ago, which explains why those regions display little evidence of the Great Unconformity.