Welcome to Let’s Talk Outcrop, a weekly newsletter delivered every Tuesday where I explain Earth Science topics such as interesting geologic formations, Earth’s structure, physical Earth processes, or natural disasters (earthquakes or volcanoes). Other topics include famous geologic maps, minerals, or interplanetary science.
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We are all, in some way or another, familiar with earthquakes. Either through personal experience having felt shaking from such an event, through close friends’ or family’s stories who have been through an earthquake, or seeing the aftermath of their occurrence and destruction on news channels.
However your personal experience works with earthquakes, you are aware that these events happen every day, and major events happen multiple times a year. Fortunately, many earthquakes are of low magnitude and minimally destructive or occur in unpopulated places such as the Alaskan islands or at the bottom of the oceans.
What you may be less familiar with, are the structures that result post-earthquakes.
Earthquakes happen along what geoscientists call faults. This is also probably not news to you.
However, if it is, here is a quick description.
Earthquakes are the result of geologic formations breaking. Tectonic forces add strain to solid rocks in Earth's crust and builds over time. Once the strain in the rock overcomes the rock's strength, the rock formation breaks and the energy released propagates through the earth as an earthquake. The resulting rock fracture is a fault.
Similarly to breaking a stick in your hand, when you bend a stick it won't break right away. However, the more you bend and stress the stick, eventually it breaks.
Faults then record locations, and sometimes timing, of earthquakes. One of the most seismically active locations on Earth - the places where the most earthquakes happen - is at mid-ocean ridges where tectonic plates are moving apart and forming extensive underwater mountain ridges of new oceanic crust.
Recent oceanic research cruises have discovered long, linear features parallel to mid-ocean ridges that slope both towards and away from the ridge, and are now known to be found on oceanic crust across the world.
Further analyses have revealed that these features are expansive normal faults that can be 6-12 miles long (10-20 kilometers), 1-3 miles wide (2-5 kilometers), and 165-1000 feet high (50-300).
These normal faults have been termed "abyssal hills" or "abyssal-hill faults" and cover nearly all of the ocean floor, except where they are covered by thick sediments. Over 30% of the seafloor is characterized as abyssal hills. Considering that 70% of the Earth's surface is covered by oceans and underlain by oceanic crust, abyssal-hill faults are the most ubiquitous geomorphic feature on the planet.
Abyssal-hill faults by nature are formed at tectonic spreading centers where oceanic plates are separating from one another. To that end, the tectonic plates are in a state of extensional stresses. Extensional regimes result in normal faults where rocks pull apart and one block moves down relative to another.
Abyssal-hill faults parallel the mid-ocean ridge but do not appear to be formed on the ridge itself. Faults are observed several miles (3-5 kilometers) off the ridge and are then formed by a separate process from crustal formation and ridge deformation.
The accepted model today is that normal faults form on the flanks of the mid-ocean ridge due to extension and crustal bending and unending. At the ridge, new crust material arrives as molten rock and is buoyant causing it to rise to a higher elevation and giving ocean ridges their mountainous appearance.
As the plates continue to move apart and the newer crust cools, it sinks lower into the mantle and forms a bend in the crust. The extensional forces driving separation at the axis, pull on the plate in combination with the plate bending. Both the extensional stress and bending (or unbending) strains the rock to the point of fracturing, causing earthquakes and resulting abyssal-hill faults.
Faults remain in the crust indefinitely and are carried as permanent scars in the oceanic crust that are subjects of many scientific studies.
Abyssal-hill faults and their structures record information about crust formation that are valuable pieces of evidence for learning more about the Earth's evolution.
Abyssal-hill fault formation is sensitive to several aspects of seafloor formation: tectonic spreading rates, crustal thermal structure, and plate geometry.
Spreading centers with fast spreading rates create smaller earthquakes and smaller resulting faults in the resulting oceanic crust. Tectonic spreading centers with slower spreading rates create larger offsets and longer faults. The size of abyssal-hill faults can then be used to back-project the spreading rates of tectonic plates.
Abyssal hills also parallel mid-ocean ridge axes which gives a direct estimation of the mid-ocean ridge orientation at the time of fault formation. Tectonic plates are prone to reorganizations and plate boundaries do shift and move over millions of years. Using abyssal hill orientations, researchers can gauge the spreading directions of tectonic plates easily.
Abyssal hills are the most common geomorphic feature on the planet, though only recently studied due to the technological requirements for studying the seafloor. Though these features cover much of our planet, they are covered by several miles of seawater. The seafloor may look simple at first glance, but close analysis offers many insights into the continuously changing surface of our planet.
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References
Buck, W., Poliakov, A. Abyssal hills formed by stretching oceanic lithosphere. Nature 392, 272–275 (1998). https://doi.org/10.1038/32636
Goff, J. Finding chaos in abyssal hills. Nature 392, 225–227 (1998). https://doi.org/10.1038/32524
Macdonald, K., Fox, P., Alexander, R. et al. Volcanic growth faults and the origin of Pacific abyssal hills. Nature 380, 125–129 (1996). https://doi.org/10.1038/380125a0
Olive, J. A., Behn, M. D., Ito, G., Buck, W. R., Escartín, J., & Howell, S. (2015). Sensitivity of seafloor bathymetry to climate-driven fluctuations in mid-ocean ridge magma supply. Science, 350(6258), 310-313
Second question! Are there any physical features on the Earth's land surface for whose formation we have no generally accepted explanation? Are there "mystery" features still unexplained? I would doubt it given the sophistication of our science, but it would be fascinating if there were and you devoted an issue to one such!
Mars was once host to extensive oceans and also had tectonic processes. Has there ever been evidence on its current surface of any remnants of something like our abyssal hills?