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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One of my favorite things about geology and studying the Earth's surface is the interpretations we can make from geologic layers, volcanoes, and rock formations. Each layer is formed from a specifically curated environment to create geologic layers and then preserve them for millions of years. Studying geologic structures then give a crystal-ball look into Earth's evolution, changing environments, and processes that shaped the changing surface of the planet.
Most geologic layers give us insights into the evolution of Earth's outer crust: depositional environments, erosional environments, climate, and tectonic movement. Volcanoes, on the other hand, give a window into the chemical makeup and structures of the interior of the Earth. Studying geologic layering then not only gives a timelapse look into the outer layers of Earth, but the internal structures of the planet as well.
Magma forms deep within Earth where it can erupt and form volcanic structures. Most volcanoes and sources of magma are at mid-ocean ridges where tectonic plates are spreading apart, or the result of subduction zones where oceanic plates collide with another plate and are forced downward into the Earth's mantle. The addition of sediments, water, and the cold slab with the surrounding mantle forms magma which rises and erupts at volcanoes.
Though subduction zones or mid-ocean ridges at tectonic plate boundaries are the most common place to form volcanoes, those such as Yellowstone or Hawaii which sit in the middle of tectonic plates must form from a different source of magma.
You may be aware that Hawaii and Yellowstone are formed from what are called "hotspots". But what exactly is a hotspot and how does it differ from a "typical" volcano?
Magma Source
Volcanoes forming at subduction zones and mid-ocean ridges result from magmas generated in the shallower parts of the crust and uppermost mantle (<100 kilometers depth).
The most widely proposed model for hotspots is that magma form very deep in the Earth's interior near the Core1-Mantle-Boundary (CMB) and rises through the mantle with an initial voluminous amount of magma as a plume head, and then continually release magma through periodic pulses. The structure of a hotspot mantle plume is a mushroom-shaped head followed by a long, bulbous tail of a continuing magma stream.
Plumes are proposed to be caused by thermal instabilities within boundary layers in the Earth's interior. The primary thermal layers in the Earth's interior occur at the transition zone (410 km - 660 km deep) and at the CMB. The larger-scale whole-mantle convection hypotheses suggest that as the mantle exchanges heat and flows, instabilities can arise that lead to large plumes of magma forming that buoyantly rise to the Earth's surface.
The source of magma near the CMB is considered to be at a relatively fixed point, whereas the tectonic plates move continually across the surface of the Earth, giving the appearance of a moving magma source. Hawaii is perhaps the best example of tectonic plates moving across a fixed hotpot point, creating an extensive chain of volcanoes.
Iceland is another famously studied hotspot island, known for its highly active volcanoes and common earthquake swarms. Iceland is proposed to be a hotspot-sourced island that resides on a mid-ocean ridge. The island lies directly on the boundary between the North American Plate and the Eurasian Plate in the northern Atlantic Ocean. The island is slowly being pulled apart and constantly rebuilt by both mid-ocean ridge magmas and the upwelling of a deep mantle plume.
Geochemically, proposed hotspot-resulting magmas (i.e., Iceland Hawaiian islands) contain very different elements than subduction-related volcanoes or mid-ocean ridges. The exact ratios, elements, and isotope signatures are beyond the surface-level discussion of this article, so you can take my word on geochemical differences!
Beneath Iceland, seismologists have found zones of low seismic velocities extending to at least 400 km beneath the island, indicating regions of melted rocks to great depths. These zones of low seismic velocities support the model that the island is underlain by a columnar tube of hot upwelling magma.
Plume Types
There have been three primary types of mantle plumes proposed for hotspots originating from a deep-mantle source:
1) Primary plumes originating from the CMB which rise to the surface as a singular upwelling (e.g., Hawaii).
2) Secondary plumes rising from domes created by larger plume heads that spread out at transition zone depths (e.g., in the Pacific Ocean or near Africa).
3) Tertiary plumes created from superficial sources nearer to the Earth's surface, that may be created by lithospheric flexure or decompression melting along spreading centers.
Summary
Hotspots differ from "traditional" volcanism as they tend to be associated with volcanic centers off tectonic plate boundaries, and geochemically differ from mid-ocean ridge layers and subduction-related volcanoes. Hotspot centers also have been associated with columns of low seismic velocity layers which have been interpreted as large plumes floating to the surface.
Most plumes appear to have a deep-mantle source and be created from thermal instabilities at the Core-Mantle Boundary. The release of plumes may also be a way of cooling down the Earth's core: as is the natural course for large planetary bodies over time.
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1: Initial posting read “Crust-Mantle-Boundary” and should be “Core-Mantle-Boundary”. Edit has been made.
My three favorite posts from the last week:
References
Courtillot, V., Davaille, A., Besse, J., Stock, J., (2003). Three distinct types of hotspots in the Earth's mantle. Earth and Planetary Science Letters. 205, 295-308.
Frost,D. A., Avery,M. S., Buffett,B. A., Chidester,B. A., Deng,J., Dorfman,S. M., et al. (2022). Multidisciplinary constraints on the thermal-chemical boundary between Earth's core and mantle. Geochemistry, Geophysics, Geosystems, 23, e2021GC009764. https://doi.org/10.1029/2021GC009764
Sleep, N. H., “Hotspot Volcanism and Mantle Plumes”, Annual Review of Earth and Planetary Sciences, vol. 20, p. 19, 1992. doi:10.1146/annurev.ea.20.050192.000315.
Sleep, N. H. (2006). Mantle plumes from top to bottom. Earth-Science Reviews, 77(4), 231-271. https://doi.org/10.1016/j.earscirev.2006.03.007
Wolfe, C., Th. Bjarnason, I., VanDecar, J. et al. Seismic structure of the Iceland mantle plume. Nature 385, 245–247 (1997). https://doi.org/10.1038/385245a0
I was coincidentally up too late the night before last reading about hotspots and the idea that they're the source of certain rich ore deposits. It is difficult to build an intuitive understanding of what is thought to be going on down there from these jawbreaker style static illustrations. Someone really needs to put together some cool animations "realistically" depicting this stuff.