The Center Of An Oversight

Planets are born from the chaos of countless collisions. The Earth was no exception, yet very little is known about the effects of collisions throughout Earth’s history. Scientists face great challenges in piecing together the deep past of our planet because it is highly active geologically, with its crust constantly churned and recycled by plate tectonics. The Earth’s rigid outermost shell, the lithosphere, made up of the crust and the uppermost part of the mantle, is composed of a mosaic of irregular plates floating atop the more malleable deeper mantle and being pulled around by underlying currents of slowly flowing rocks. The familiar arrangement of oceans and continents today and in the future too is slowly but steadily being shaped by these internal forces. As the new oceanic crust emerges, the older crust is removed, so the surface area of the Earth remains constant. This crust removal happens at the boundaries of colliding plates, the subduction zones. In these locations, one of the two colliding plates, generally an oceanic plate, which is denser, dives under the continental plate, sinking and melting to become recycled in the deeper mantle. In the process, the margins of continents are gradually being eroded. The oldest known rocks are about 4 billion years old and are found in Canada and Greenland. In contrast, the vast expanses of oceanic floor are much younger, less than 200 million years old. The overall process of crust generation and destruction is often compared to a conveyor belt, and this restless churn is ultimately responsible for Earth’s young surface.

Not Afraid

Rocks exposed at the surface also incur erosion due to weathering. Atmospheric processes such as rain and wind are responsible for the smoothing of the landscape over time that can erode and transport vast amounts of surface material. This material is typically washed back into the oceans, building up as sediment layers, and from there eventually to the mantle, where high temperature and pressure obliterate the rocks’ past. All the precious data regarding the nature of the earliest crust, its composition, and the environmental effects of cosmic bombardment was recorded in surface rocks, and the Earth keeps erasing this information. But all is not lost. What, then, is the surviving record of terrestrial cosmic catastrophes? Scientists have found about 170 craters on Earth, ranging in size from about 1 to 300 km. The existing crater population of the Earth indicates that sizable collisions were sporadic events, on average a crater larger than 50 km formed about every 150 million years, with no preserved craters from the first 2.5 billion years of Earth’s history. This observation is at the center of an oversight that has plagued studies of the evolution of the early Earth for decades. In essence, geologists typically were not trained to recognize the geological effects of cosmic collisions, while astronomers were lacking a theoretical and observational framework to assess the magnitude of the impact flux through time. So the role of collisions in shaping the geological history of the Earth has fallen into a crack, and was largely relegated to the interests of a few scientists. What can be done to bridge this gap? More importantly, the Moon is the nearest celestial object to the Earth and the two bodies share a common collisional history.

Cross My Heart

So a direct comparison of the lunar and terrestrial crater records might offer an interesting perspective on the latter. True, the Earth’s much denser atmosphere prevents most of the smaller meteors from reaching the surface, but that does not apply to collisions at these larger scales. Considering that the Moon has a surface area comparable to that of Africa, it is quite dramatic to imagine all those lunar craters crammed into that continent. Moreover, the Earth has a larger gravitational attraction than the Moon, so it should have received proportionally more collisions. Scientists estimate that the Earth, if subject to the same bombardment history of the Moon, should have at least 30,000 craters larger than 50 km. This means that for every 3,000 craters larger than 50 km that ever formed, we have discovered only one. Where have all the terrestrial craters gone? They are buried and covered by vegetation, water and soil, or have been eroded away. The larger the crater, the more likely it is to be preserved for a long time. In reality, terrestrial weathering is so ferocious that the three largest impact structures have remained unknown until recently. Chicxulub crater is mostly underwater in the Gulf of Mexico and buried under thick sediments. This area was subject to detailed geophysical investigations surveying for oil from the late 1940s. A gravity map, which shows subtle local variations in the Earth’s gravity field, revealed the presence of circular features, a possible sign of a hidden large impact structure, but the observation went mostly unnoticed.

Bite Your Lip

Sudbury is located in a remote area in subarctic Canada, and interest in this location converged in the 1880s when geological prospecting showed a high concentration of metals. The definitive signature of an impact origin was gathered in the 1960s–70s with the discovery of shocked quartz and shattered cones. These brief accounts of the recognition of three major terrestrial impact structures exemplify how complex the identification can be, regardless of the age of the structure. Chicxulub is only 66 million years old, while Vredefort is about 2 billion years old. The circular set of hills is about 70 km wide and is the remnant of a central dome due to the rebound of the Earth’s crust. The black sinuous line crossing the hills at the top of the image is the Vaal River. These connections, however, may not be preserved in older collisions, and scientists need to look for more subtle ways to overcome the unforgiving effects of time. South Africa preserves a record of landforms older than two billion years. Geologists have scoured these regions scrupulously due to their commercial value, resulting in flourishing mining activities to extract gold, diamonds, and a long list of valuable minerals. Most of the mining activity takes place in ancient basement rocks lying below recent sedimentary layers.