The theory of plate tectonics is an
important scientific theory that was proposed about 50 years ago. Like all scientific
theories, it is constantly being tested and refined. This blog post provides
some updates to our understanding based on recent research.
Evidence for plate tectonics
The evidence Alfred Wegener used for his hypothesis of continental drift included the jigsaw fit of continents, fossil locations, mountain belts and glacial tracks. Although this evidence suggested that continents had once been connected, Wegener did not propose a reasonable mechanism for the breakup of continents and his hypothesis was rejected.
Fossils of terrestrial organisms helped Wegener to put together the puzzle of Pangea (USGS, public domain)
Seafloor mapping, convection currents in the mantle and magnetic reversals (measurable in seafloor
rocks) provided evidence for forces strong enough to move continents. This
evidence of forces and continental movement were combined into the modern
theory of plate tectonics. The theory explains how new ocean crust is produced
at mid-ocean ridges, while being subducted at deep ocean trenches. The less
dense continental crust is moved about by expansion and contraction of ocean
basins, as well as being torn apart by rift zones.
Magnetic reversals provide evidence for seafloor spreading at mid-ocean ridges (WJ Kious & RI Tilling 1996, This Dynamic Earth: The Story of Plate Tectonics, public domain)
Mid-ocean ridge research
Most mid-ocean ridge studies
indicate that new crust is produced equally on both sides for both fast- and
slow-spreading ridges. However, some ridges spread very slowly and behave
differently. At ultraslow-spreading ridges, mantle rock is being brought to the
surface on the faster side of the fault which is termed a detachment fault by some researchers. Seawater sinks into cracks around the ridge and
reacts with mantle olivine to form serpentine. Serpentine allows faults to move
more quickly, increasing slippage and olivine uplift in a positive feedback
loop. The seismic appearance and rock profiles around ultra-slow ridges are
changing ideas about the crust-mantle boundary, as well as mantle composition.
Models of mantle convection
The exact pattern of convection in
the mantle is still debated. Simple diagrams like that below provide a basic
summary of patterns. However, this is an area of much research. Isotopic data (Barry
et al. 2017) suggests that the whole-mantle model
may be the most accurate, with convection cycles taking 100 million years or
more. However, recent computer models (Cameri 2018) show greater activity in
the upper mantle overlying larger mantle upwelling.
Convection may occur in the
upper mantle (top), whole mantle (bottom) or separately in both the upper and lower
mantle.
Computer modelling shows the complexities of mantle movement and how this relates to mantle temperature. (F Crameri (2018). Geocynamic diagnostics, scientific visualisation and StagLab 3.0. Geosci Model Dev 11:2541-2562, Creative Commons 4.0)
Subducting continental crust
Our original understanding of plate tectonics suggests that continental crust does not subduct because of its low density. Instead, colliding continents are folded and pushed up into mountain ranges that eventually weather and erode. This idea has been challenged by several lines of evidence (Ducea 2016).
- Ultrahigh-pressure metamorphic rocks of continental origin provide evidence that continental crust can be dragged 90-100 km into the mantle. This is too far for normal mountain-building processes and consistent with subduction.
- Modelling of the Indo-Asian collision shows that more than 1000 km of Indian lithosphere was subducted under Asia during formation of the Himalayas.
- Seismic imaging shows that Indian lithosphere is still being subducted in the Pamir-Hindu Kush collision.
It has long been recognised
that oceanic plates subduct (A), but recent research has provided many lines of
evidence for continental subduction (B). Each type of subduction leaves
distinctive evidence in the overlying crust (C). (LQ Dai, ZF Zhao, YF Zheng (2015).
Tectonic development from oceanic subduction to continental collision:
Geochemical evidence from postcollisional mafic rocks in the Hong’an-Dabie
orogens. Gondwana Research 27(3): 1236-1254, https://doi.org/10.1016/j.gr.2013.12.005)
Explore
- Experiment with evidence for plate tectonics as shown in the AusEarthEd videos Continental Drift and What Lies Below?
- Investigate convection currents using one or more methods shown in this video.
- Snow and Edmonds (2007) review the structure of mid-ocean ridges and how ultraslow-spreading ridges have changed our ideas of plate tectonic mechanics.
- See modern visualisations of the mantle as Professor Hans-Peter Bunge explains mantle convection.