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.
Type of article: Review
Published: 20 June 2016
Where: Nature Geoscience
Online coverage of the article (examples):
Our knowledge of mantle structure has changed significantly over the past few years, as the image below shows.
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.
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.
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.
The article does a great job in reviewing all the current thoughts on these mysterious blobs.
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’.