According to a new study, the oceanic crust produced by Earth today is thinner than crust made 170 million years ago during the time of the supercontinent Pangea. The thinning is related to the cooling of Earth’s interior prompted by the splitting of the ancient supercontinent.
The study, led by University of Texas Institute for Geophysics researcher Dr. Harm Van Avendonk, suggests that since the breakup of Pangea, the cooling rate of the mantle has increased from 6-11 degrees Celsius per 100 million years to 15-20 degrees per 100 million years.
Since cooler mantle temperatures produce less magma, it’s a trend that’s making modern day ocean crust thinner.
“Earth’s mantle has cooled by 6-11 degrees Celsius every 100 million years since the Archaean, 2.5 billion years ago,” Dr. Van Avendonk and co-authors said.
“In more recent times, the surface heat loss that led to this temperature drop may have been enhanced by plate-tectonic processes, such as continental breakup, the continuous creation of oceanic lithosphere at mid-ocean ridges and subduction at deep-sea trenches.”
“We find that oceanic crust formed in the mid-Jurassic, about 170 million years ago, is 1 mile (1.7 km) thicker on average than crust produced along the present-day mid-ocean ridge system.”
“If a higher mantle temperature is the cause of thicker Jurassic ocean crust, the upper mantle may have cooled by 15-20 degrees Celsius per 100 million years over this time period.”
The research that led to the connection between the splitting of Pangea and crust thickness started when the authors noticed an unexpected trend when studying existing data from young and old seafloor.
They analyzed 234 measurements of crustal thickness from around the world and found that, on a global scale, the oldest ocean crust examined — 170 million year old rock created in the Jurassic — is about one mile thicker than the crust that’s being produced today.
The link between crust thickness and age prompted two possible explanations — both related to the fact that hotter mantle tends to make more magma: mantle hot spots — highly volcanic regions, such as the Hawaiian Islands and Iceland — could have thickened the old crust by covering it in layers of lava at a later time. Or, the mantle was hotter in the Jurassic than it is now.
The analysis ruled out the hot spot theory — thick layers of old crust formed just as easily at distances greater than 600 miles (965 km) from hotspots, a distance that the researchers judged was outside the influence of the hotspots.
In contrast, the analysis supported the hypothesis of mantle heating during the age of Pangea, and mantle cooling after the breakup of the supercontinent.
“The finding that splitting up Pangea cooled the mantle is important because it gives a more nuanced view of the mantle temperature that influences tectonics on Earth,” Dr. Van Avendonk said.