EGAATA-1: ‘blobs’ of material at the core-mantle boundary

Here is the first ‘Excellent Geophysics Articles As Teaching Aids’ (EGAATA) post.  The point of these posts is to highlight excellent articles that could be a basis for teaching a section on the chosen topic. 

Executive summary: Modern geophysical imaging also shows two chemical “blobs” at the boundary of Earth’s core and mantle, about 2900 km from the surface. Some consensus is forming on their nature.

Title: Continent-sized anomalous zones with low seismic velocity at the base of Earth’s mantle.

Type of article: Review

Published: 20 June 2016

Where: Nature Geoscience

Authors: Edward J. Garnero, Allen K. McNamara and Sang-Heon Shim.

Online coverage of the article (examples):

Daily Mail: Massive ‘blobs’ of molten rock 100 times higher than Everest found in Earth’s mantle
Science Daily: Giant blobs of rock, deep inside the earth, hold important clues about our planet

Overview: 

Our knowledge of mantle structure has changed significantly over the past few years, as the image below shows.

Arizona State seismology expert Ed Garnero’s summary of how far we have come in over 100 years of studying the interior of the Earth. Ed GarneroCC BY

Two blobs (a.k.a. piles) found beneath the Pacific and under Africa are particularly important. Seismology expert (and author) Prof Ed Garnero (Arizona State University) once described the piles as “the largest structure within our planet of unknown origin and nature.” They are potentially very important to every aspect of our planet’s history – from our early planet formation to the impact on plate tectonics.

Details: 

Seismic imaging of the Earth’s interior has revealed two regions of anomalously warm and dense material at the base of the mantle, above the core. Recent data from geochemistry, mineral physics, and seismology indicates that these deep heterogeneities are chemically distinct from the surrounding mantle, leading to their description of ‘thermochemical piles’ (technically described as LLSVPs – Large Low Shear Velocity Provinces). It is understood that these continent-sized dense piles are dynamically stable and long-lived,  but not rigid and/or fixed structures. Based on the present-day geometry of the thermochemical piles, mantle flow from oceanic subduction processes at the surface is believed to mould LLSVPs, with deep-origin mantle plumes forming on their margins.

ngeo2733-f2
a Surface features (upper panel) and seismically determined lower-mantle phenomena (lower panel). See article for details. b–e, Idealized possibilities proposed to explain LLSVPs. In all cases, subducted material (possibly including post-perovskite, pPv) surrounds the structure of interest that maps as the LLSVP. b, Plume cluster. c, Thermochemical superplume. d, Stable thermochemical pile. e, Metastable thermochemical pile. LIPs, large igneous provinces; CMB, core–mantle boundary; ULVZs, ultralow velocity zones. Source (with permission)

Blob size and structure: 

It is difficult to properly confirm the size and extent of these blobs at present, but a rough estimate would be that they cover 30% of the core-mantle boundary, and in some places extend up to 1200 km in height.

Origin and future: 

Their origin is uncertain. Two end-member theories are that the piles were formed (a) during the formation of the planet (primordial) or (b) have grown over time through mantle processes accumulating material at its base. The future of these blobs is also not known – how these piles look at present is only a snapshot of how they behave over time. Both end-member theories of how the piles formed would produce very different possibilities of what they will evolve into. As a result, their future is very much tied to their past.

ngeo2733-f3
Two end-member evolutionary pathways for a present-day thermochemical pile are illustrated. a, Primordial layer: in this scenario, early Earth processes establish a global layer, which develops into separate piles over time. b, Growth of the thermochemical layer over time: here, material with elevated density collects at the core–mantle boundary over time, which grows into a thicker layer and eventually into distinct thermochemical piles. Source (with permission)

Summary:

The article does a great job in reviewing all the current thoughts on these mysterious blobs.

Title: Continent-sized anomalous zones with low seismic velocity at the base of Earth’s mantle.

Feel free to ask questions in the comments. I can provide further information (and materials) for any high school educators that would be interested in teaching these important ‘blobs’.  

One thought on “EGAATA-1: ‘blobs’ of material at the core-mantle boundary

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