Ultra-low velocity zone

Last updated

Ultra low velocity zones (ULVZs) are patches on the core-mantle boundary that have extremely low seismic velocities. The zones are mapped to be hundreds of kilometers in diameter and tens of kilometers thick. Their shear wave velocities can be up to 30% lower than surrounding material. The composition and origin of the zones remain uncertain. The zones appear to correlate with edges of the African and Pacific large low-shear-velocity provinces (LLSVPs) [1] as well as the location of hotspots. [2] [3]

Contents

Discovery and constraints

ULVZs are discovered by the delay and scattering of body waves that reflect and diffract on or are refracted by the core-mantle boundary. Different body waves types give different constraints on the dimensions or velocity contrasts of the ULVZ. Even though ULVZs are discovered in places, it remains difficult to map out their extent and constrain their density and velocity. Usually trade-offs between various parameters exist. In general though, ULVZs appear to be a hundred to a thousand kilometers across and tens of kilometers thick (although existing thinner or smaller ULVZs might fall below the resolution of seismology). Their shear wave velocity reduction is on the order of −10 to −30% and the compressional wave velocity reduction tends to be weaker. [3] [4] [5]

Composition and origin

ULVZs are hypothesized to be enriched in iron, be partially molten [6] or a combination of both, or result from the presence of carbon. Different scenarios have been proposed for the iron enrichment: iron could be leaking from the core, [7] have accumulated over past subduction, [8] or be remnants of a basal magma ocean. [9] Both silicate perovskite [10] and periclase [11] (which are thought to be present in the lowermost mantle) show reduced velocities with increasing iron at these pressures and temperatures.

Experiments with iron and water under the same conditions form an iron peroxide FeO2Hx that will contribute to ULVZ. [12]

Distribution and dynamics

ULVZs have higher density than their surroundings to remain stable on the core-mantle boundary. In a general mantle convection setting, the density contrast as well as the amount of material available would control the morphology/shape of the ULVZ. [13] So far a range of sizes for ULVZs has been found. [14] The location and shape of the ULVZs can also be controlled by the presence of thermo-chemical piles (or LLSVPs). The denser ULVZ material heaps up at the edges of these piles. [1]

Hawaiian ULVZ

The Hawaiian ULVZ appears to be the largest ULVZ mapped to date. [4] It sits on the core-mantle boundary slightly to the west of the Hawaiian hotspot at the northern boundary of the Pacific large low-shear-velocity province. It is mapped out to be roughly 1000 km across and 20 km high. Its large aspect ratio dynamically suggests it is very dense. [13] Its shear wave velocity reduction is roughly 20% compared to surrounding material. It remains speculative if there is a correlation between this large ULVZ and the presence of the strongest hotspot flux at the surface; potentially the ULVZ could be an anchor to a whole-mantle plume.

Cartoon of ultra low velocity zones (red structures) and the Pacific large low shear velocity province (red transparent) on the core of the Earth (blue) Pacific ULVZs.jpeg
Cartoon of ultra low velocity zones (red structures) and the Pacific large low shear velocity province (red transparent) on the core of the Earth (blue)

Samoan ULVZ

The Samoan is another mega-ultra-low velocity zone which lies directly beneath the Samoa hotspot. [5] [15] This zone is roughly 800 by 250 km (roughly the size of Florida) and is 10–15 km high. Its material appears 45% slower in shear wave velocity, 15% slower in compressional wave velocity and 10% denser. Additionally, the ULVZ appears to lie in a gap of the Pacific LLSVP [16] (not represented in the illustration here), leading to the hypothesis that this slow material is pushed to the center by surrounding large piles.

Related Research Articles

<span class="mw-page-title-main">Asthenosphere</span> Highly viscous, ductile, and mechanically weak region of Earths mantle

The asthenosphere is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between c. 80 and 200 km below the surface, and extends as deep as 700 km (430 mi). However, the lower boundary of the asthenosphere is not well defined.

<span class="mw-page-title-main">Earth's outer core</span> Fluid layer composed of mostly iron and nickel between Earths solid [[inner core]] and its mantle

Earth's outer core is a fluid layer about 2,260 km (1,400 mi) thick, composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. The outer core begins approximately 2,889 km (1,795 mi) beneath Earth's surface at the core-mantle boundary and ends 5,150 km (3,200 mi) beneath Earth's surface at the inner core boundary.

<span class="mw-page-title-main">Mantle plume</span> Upwelling of abnormally hot rock within Earths mantle

A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hotspots, such as Hawaii or Iceland, and large igneous provinces such as the Deccan and Siberian Traps. Some such volcanic regions lie far from tectonic plate boundaries, while others represent unusually large-volume volcanism near plate boundaries.

Seismic tomography or seismotomography is a technique for imaging the subsurface of the Earth using seismic waves. The properties of seismic waves are modified by the material through which they travel. By comparing the differences in seismic waves recorded at different locations, it is possible to create a model of the subsurface structure. Most commonly, these seismic waves are generated by earthquakes or man-made sources such as explosions. Different types of waves, including P-, S-, Rayleigh, and Love waves can be used for tomographic images, though each comes with their own benefits and downsides and are used depending on the geologic setting, seismometer coverage, distance from nearby earthquakes, and required resolution. The model created by tomographic imaging is almost always a seismic velocity model, and features within this model may be interpreted as structural, thermal, or compositional variations. Geoscientists apply seismic tomography to a wide variety of settings in which the subsurface structure is of interest, ranging in scale from whole-Earth structure to the upper few meters below the surface.

<span class="mw-page-title-main">Earth's mantle</span> A layer of silicate rock between Earths crust and its outer core

Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01×1024 kg (8.84×1024 lb) and makes up 67% of the mass of Earth. It has a thickness of 2,900 kilometers (1,800 mi) making up about 46% of Earth's radius and 84% of Earth's volume. It is predominantly solid but, on geologic time scales, it behaves as a viscous fluid, sometimes described as having the consistency of caramel. Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust.

<span class="mw-page-title-main">Core–mantle boundary</span> Discontinuity where the bottom of the planets mantle meets the outer layer of the core

The core–mantle boundary (CMB) of Earth lies between the planet's silicate mantle and its liquid iron–nickel outer core, at a depth of 2,891 km (1,796 mi) below Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth due to the differences between the acoustic impedances of the solid mantle and the molten outer core. P-wave velocities are much slower in the outer core than in the deep mantle while S-waves do not exist at all in the liquid portion of the core. Recent evidence suggests a distinct boundary layer directly above the CMB possibly made of a novel phase of the basic perovskite mineralogy of the deep mantle named post-perovskite. Seismic tomography studies have shown significant irregularities within the boundary zone and appear to be dominated by the African and Pacific Large low-shear-velocity provinces (LLSVP).

<span class="mw-page-title-main">Large igneous province</span> Huge regional accumulation of igneous rocks

A large igneous province (LIP) is an extremely large accumulation of igneous rocks, including intrusive and extrusive, arising when magma travels through the crust towards the surface. The formation of LIPs is variously attributed to mantle plumes or to processes associated with divergent plate tectonics. The formation of some of the LIPs in the past 500 million years coincide in time with mass extinctions and rapid climatic changes, which has led to numerous hypotheses about causal relationships. LIPs are fundamentally different from any other currently active volcanoes or volcanic systems.

<span class="mw-page-title-main">Iceland hotspot</span> Hotspot partly responsible for volcanic activity forming the Iceland Plateau and island

The Iceland hotspot is a hotspot which is partly responsible for the high volcanic activity which has formed the Iceland Plateau and the island of Iceland. It contributes to understanding the geological deformation of Iceland.

Pyrolite is a term used to characterize a model composition of the Earth's mantle. This model is based on that a pyrolite source can produce mid-ocean ridge basalts (MORB) by partial melting. It was first proposed by Ted Ringwood (1962) as being 1 part basalt and 4 parts harzburgite, but later was revised to being 1 part tholeiitic basalt and 3 parts dunite. The term is derived from the mineral names PYR-oxene and OL-ivine. However, whether pyrolite is entirely representative of the Earth's mantle remains debated.

<span class="mw-page-title-main">Society hotspot</span> Pacific volcanic hotspot

The Society hotspot is a volcanic hotspot in the south Pacific Ocean which is responsible for the formation of the Society Islands, an archipelago of fourteen volcanic islands and atolls spanning around 720 kilometres (450 mi) of the ocean which formed between 4.5 and <1 Ma.

<span class="mw-page-title-main">Low-velocity zone</span>

The low-velocity zone (LVZ) occurs close to the boundary between the lithosphere and the asthenosphere in the upper mantle. It is characterized by unusually low seismic shear wave velocity compared to the surrounding depth intervals. This range of depths also corresponds to anomalously high electrical conductivity. It is present between about 80 and 300 km depth. This appears to be universally present for S waves, but may be absent in certain regions for P waves. A second low-velocity zone has been detected in a thin ≈50 km layer at the core-mantle boundary. These LVZs may have important implications for plate tectonics and the origin of the Earth's crust.

<span class="mw-page-title-main">Geology of Cape Verde</span>

Cape Verde is a volcanic archipelago situated above an oceanic rise that puts the base of the islands 2 kilometers (1.2 mi) above the rest of the seafloor. Cape Verde has been identified as a hotspot and the majority of geoscientists have argued that the archipelago is underlain by a mantle plume and that this plume is responsible for the volcanic activity and associated geothermal anomalies.

<span class="mw-page-title-main">Mantle wedge</span> Triangular section of mantle between a subducting and overriding tectonic plate

A mantle wedge is a triangular shaped piece of mantle that lies above a subducting tectonic plate and below the overriding plate. This piece of mantle can be identified using seismic velocity imaging as well as earthquake maps. Subducting oceanic slabs carry large amounts of water; this water lowers the melting temperature of the above mantle wedge. Melting of the mantle wedge can also be contributed to depressurization due to the flow in the wedge. This melt gives rise to associated volcanism on the Earth's surface. This volcanism can be seen around the world in places such as Japan and Indonesia.

<span class="mw-page-title-main">Large low-shear-velocity provinces</span> Structures of the Earths mantle

Large low-shear-velocity provinces (LLSVPs), also called large low-velocity provinces (LLVPs) or superplumes, are characteristic structures of parts of the lowermost mantle, the region surrounding the outer core deep inside the Earth. These provinces are characterized by slow shear wave velocities and were discovered by seismic tomography of deep Earth. There are two main provinces: the African LLSVP and the Pacific LLSVP, both extending laterally for thousands of kilometers and possibly up to 1,000 kilometres vertically from the core–mantle boundary. These have been named Tuzo and Jason respectively, after Tuzo Wilson and W. Jason Morgan, two geologists acclaimed in the field of plate tectonics. The Pacific LLSVP is 3,000 kilometers across and underlies four hotspots on Earth's crust that suggest multiple mantle plumes underneath. These zones represent around 8% of the volume of the mantle, or 6% of the entire Earth.

The Skagerrak-Centered Large Igneous Province (SCLIP), also known as the European-Northwest African Large Igneous Province (EUNWA), and Jutland LIP, is a 300 million year old (Ma) large igneous province (LIP) centered on what is today the Skagerrak strait in north-western Europe. It was named by Torsvik et al. 2008.

<span class="mw-page-title-main">Deep water cycle</span> Movement of water in the deep Earth

The deep water cycle, or geologic water cycle, involves exchange of water with the mantle, with water carried down by subducting oceanic plates and returning through volcanic activity, distinct from the water cycle process that occurs above and on the surface of Earth. Some of the water makes it all the way to the lower mantle and may even reach the outer core. Mineral physics experiments show that hydrous minerals can carry water deep into the mantle in colder slabs and even "nominally anhydrous minerals" can store several oceans' worth of water.

<span class="mw-page-title-main">Deep carbon cycle</span> Movement of carbon through Earths mantle and core

The deep carbon cycle is geochemical cycle (movement) of carbon through the Earth's mantle and core. It forms part of the carbon cycle and is intimately connected to the movement of carbon in the Earth's surface and atmosphere. By returning carbon to the deep Earth, it plays a critical role in maintaining the terrestrial conditions necessary for life to exist. Without it, carbon would accumulate in the atmosphere, reaching extremely high concentrations over long periods of time.

<span class="mw-page-title-main">Lower mantle</span> The region from 660 to 2900 km below Earths surface

The lower mantle, historically also known as the mesosphere, represents approximately 56% of Earth's total volume, and is the region from 660 to 2900 km below Earth's surface; between the transition zone and the outer core. The preliminary reference Earth model (PREM) separates the lower mantle into three sections, the uppermost (660–770 km), mid-lower mantle (770–2700 km), and the D layer (2700–2900 km). Pressure and temperature in the lower mantle range from 24–127 GPa and 1900–2600 K. It has been proposed that the composition of the lower mantle is pyrolitic, containing three major phases of bridgmanite, ferropericlase, and calcium-silicate perovskite. The high pressure in the lower mantle has been shown to induce a spin transition of iron-bearing bridgmanite and ferropericlase, which may affect both mantle plume dynamics and lower mantle chemistry.

<span class="mw-page-title-main">Kevin C. A. Burke</span> British geologist (1929–2018)

Kevin C. A. Burke was a geologist known for his contributions in the theory of plate tectonics. In the course of his life, Burke held multiple professorships, most recent of which (1983-2018) was the position of professor of geology and tectonics at the Department of Earth and Atmospheric Science, University of Houston. His studies on plate tectonics, deep mantle processes, sedimentology, erosion, soil formation and other topics extended over several decades and influenced multiple generations of geologists and geophysicists around the world.

Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.

References

  1. 1 2 McNamara, Allen K.; Garnero, Edward J.; Rost, Sebastian (2010). "Tracking deep mantle reservoirs with ultra-low velocity zones". Earth and Planetary Science Letters . 299 (1–2): 1–9. Bibcode:2010E&PSL.299....1M. doi:10.1016/j.epsl.2010.07.042.
  2. ↑ Czechowski L.(1993). The Origin of Hotspots and The D” Layer. In: Montag H., Reigber C. (eds) Geodesy and Physics of the Earth. International Association of Geodesy Symposia, vol 112.
  3. 1 2 Williams, Q. (1998). "A Correlation Between Ultra-Low Basal Velocities in the Mantle and Hot Spots". Science . 281 (5376): 546–549. Bibcode:1998Sci...281..546W. doi:10.1126/science.281.5376.546.
  4. 1 2 Cottaar, Sanne; Romanowicz, Barbara (2012). "An unusually large ULVZ at the base of the mantle near Hawaii". Earth and Planetary Science Letters . 355–356: 213–222. Bibcode:2012E&PSL.355..213C. doi:10.1016/j.epsl.2012.09.005.
  5. 1 2 Thorne, Michael S.; Garnero, Edward J.; Jahnke, Gunnar; Igel, Heiner; McNamara, Allen K. (2013). "Mega ultra low velocity zone and mantle flow". Earth and Planetary Science Letters . 364: 59–67. Bibcode:2013E&PSL.364...59T. doi:10.1016/j.epsl.2012.12.034.
  6. ↑ Williams, Q.; Garnero, E. J. (1996). "Seismic Evidence for Partial Melt at the Base of Earth's Mantle". Science . 273 (5281): 1528–1530. Bibcode:1996Sci...273.1528W. doi:10.1126/science.273.5281.1528.
  7. ↑ Otsuka, K.; Karato, S. (2012). "Deep penetration of molten iron into the mantle caused by a morphological instability". Nature . 492 (7428): 243–246. Bibcode:2012Natur.492..243O. doi:10.1038/nature11663. PMID   23235879.
  8. ↑ Dobson, D. P.; Brodholt, J. P. (2005). "Subducted banded iron formations as a source of ultralow-velocity zones at the core–mantle boundary". Nature . 434 (7031): 371–374. Bibcode:2005Natur.434..371D. doi:10.1038/nature03430. PMID   15772658.
  9. ↑ Labrosse, S.; Hernlund, J. W.; Coltice, N. (2007). "A crystallizing dense magma ocean at the base of the Earth's mantle". Nature . 450 (7171): 866–869. Bibcode:2007Natur.450..866L. doi:10.1038/nature06355. PMID   18064010.
  10. ↑ Mao, W. L.; Mao, H. K.; Sturhahn, W.; Zhao, J.; Prakapenka, V. B.; Meng, Y.; Shu, J.; Fei, Y.; Hemley, R. J. (2006). "Iron-Rich Post-Perovskite and the Origin of Ultralow-Velocity Zones". Science . 312 (5773): 564–565. Bibcode:2006Sci...312..564M. doi:10.1126/science.1123442. PMID   16645091.
  11. ↑ Wicks, J. (2013). Sound velocities and equation of state of iron-rich (Mg,Fe)O (Thesis). California Institute of Technology. doi:10.7907/Z94B2Z98.
  12. ↑ Liu, Jin; Hu, Qingyang; Young Kim, Duck; Wu, Zhongqing; Wang, Wenzhong; Xiao, Yuming; Chow, Paul; Meng, Yue; Prakapenka, Vitali B.; Mao, Ho-Kwang; Mao, Wendy L. (2017). "Hydrogen-bearing iron peroxide and the origin of ultralow-velocity zones". Nature . 551 (7681): 494–497. Bibcode:2017Natur.551..494L. doi:10.1038/nature24461. PMID   29168804.
  13. 1 2 Bower, Dan J.; Wicks, June K.; Gurnis, Michael; Jackson, Jennifer M. (2011). "A geodynamic and mineral physics model of a solid-state ultralow-velocity zone". Earth and Planetary Science Letters . 303 (3–4): 193–202. Bibcode:2011E&PSL.303..193B. doi:10.1016/j.epsl.2010.12.035.
  14. ↑ Torsvik, Trond H.; Smethurst, Mark A.; Burke, Kevin; Steinberger, Bernhard (2006). "Large igneous provinces generated from the margins of the large low-velocity provinces in the deep mantle". Geophysical Journal International. 167 (3): 1447–1460. Bibcode:2006GeoJI.167.1447T. doi:10.1111/j.1365-
  15. ↑ "Is The Earth Cooking Up Another Super Volcano?". npr.org. National Public Radio.
  16. ↑ He, Yumei; Wen, Lianxing (2009). "Structural features and shear-velocity structure of the "Pacific Anomaly"". Journal of Geophysical Research: Solid Earth . 114 (B2): B02309. Bibcode:2009JGRB..114.2309H. doi: 10.1029/2008JB005814 .
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.