A new study conducted at the University of Haifa and published in the journal Communications Earth & Environment found that the Earth’s crust off the east coast of the United States cooled up to 1.6 times faster than predicted. The researchers believe that water flowing through volcanic rocks removed heat from deep within the crust, cooled it, and accelerated its subsidence.
A new study conducted at the University of Haifa and published in the journal Communications Earth & Environment found that the Earth’s crust in the transition zone between continents and oceans cooled at a rate up to 1.6 times faster than previously assumed by conventional geological models. Accelerated cooling made the rocks denser, the region subsided more rapidly, creating space for thick layers of sediment to accumulate. “This finding changes the way we understand the evolution of continental margins. If the crust cools faster than we previously thought, then the subsidence of the seafloor and the accumulation of sedimentary layers above it also occur more rapidly. Processes that we have long attributed solely to passive cooling now require an additional explanation,” said Dr. Guy Lang of the Leon H. Charney School of Marine Sciences, one of the study’s authors.
When continents rift and move apart, the Earth’s crust stretches and becomes thinner in response. As the process progresses, hot magmatic material rises from deep within the Earth and cools to form new oceanic crust. This process leads to the formation of a new ocean. At the edges of the continents, extensive regions known as passive continental margins remain. Over time, these regions cool due to heat conduction, subside, and become covered by thick layers of sediment. The thickness of these sedimentary layers is determined by the rate at which the crust cools: the faster the cooling, the thicker the sedimentary layers and the faster they accumulate. In many of these regions, enormous volume of molten rock rose from deep within the Earth during continental breakup. Some of this magma intruded into the crust, while some erupted and covered it with thick layers of basalt. These regions are known as magma-rich continental margins. For decades, geological models have struggled to explain why these margins subsided faster and why much thicker sedimentary layers accumulated above them than predicted by models based on passive heat conduction. Dr. Lang, Prof. Yizhaq Makovsky, and Prof. Uri ten Brink, from the Dr. Moses Strauss Department of Marine Geosciences at the University of Haifa and the U.S. Geological Survey, set out to explain the rapid subsidence of an extensive region off the Atlantic coast of the United States. This margin, which formed roughly 190 million years ago when North America and Africa split apart to open the Central Atlantic Ocean, is a classic example of a magma-rich continental margin.
The researchers developed a new mathematical model that reconstruct how the continental margin cooled and subsided following continental breakup. The model incorporated three main processes: stretching of the crust and the layers beneath it, addition of volcanic rocks to the crust, and changes in how fast heat moves through rock. The researchers compared the model’s predictions with subsurface data from the study area, using seismic measurements to estimate the thickness of the crust and the thickness of the sedimentary layers that accumulated above it during the first 26 million years following continental breakup. To test the robustness of the results, the researchers performed 30,000 repeated calibrations of the model using different samples from the dataset. They then compared its predictions with the subsidence history reconstructed from a deep test well in the region.
The study found that during that period, sediments up to approximately eight kilometers thick accumulated in the study area, whereas conventional geological models predicted an accumulation up to 2.5 times lower. Even after the researchers accounted for the non-uniform stretching of the crust and the volcanic rocks added to it, the models still failed to explain the full extent of the subsidence. Only when the model incorporated a faster cooling rate did it agree with the measured data. The researchers suggest that this accelerated cooling was made possible by water flowing through porous basaltic rocks, heating at depth and carrying the heat upward. The more rapidly the heat was removed, the denser the crust became and the faster the region subsided. Similar processes are seen today in Iceland and East Africa. “The ability to reconstruct more accurately the cooling and subsidence rates of continental margins is important far beyond understanding the evolution of the region examined in this study. It could change how we interpret the thickness of sedimentary layers, reconstruct ancient sea-level changes, and assess the thermal history of sedimentary basins in which oil and gas systems developed,” the researchers concluded.