Mohorovičić discontinuity

Mohorovičić discontinuity
Name	Mohorovičić discontinuity
Other names	Moho
Type	Seismic discontinuity
Depth	5–90 km (varies)
Discovered	1909
Discoverer	Andrija Mohorovičić

Mohorovičić discontinuity is the seismic boundary separating Earth's crust from the mantle, identified by a marked change in seismic wave velocities and global variations in depth, composition, and seismic behavior. It was recognised through analysis of seismic arrivals following earthquakes and has become central to studies by institutions such as US Geological Survey, Imperial College London, Scripps Institution of Oceanography, ETH Zurich, and Lamont–Doherty Earth Observatory. The feature influences interpretations by organizations and projects including International Seismological Centre, Incorporated Research Institutions for Seismology, IRIS, Global Seismographic Network, and continental surveys like British Geological Survey and Geological Survey of Canada.

Discovery and Naming

The discontinuity was discovered in 1909 by Andrija Mohorovičić while analysing seismic records from earthquakes in the Adriatic Sea and along the Dinaric Alps, using instruments and comparisons common to observatories such as the Vienna Observatory and the Royal Observatory of Belgium. Contemporary scientists like Beno Gutenberg, Inge Lehmann, and Charles Richter later refined seismic interpretation methods that validated Mohorovičić's interpretation. The name derives from the discoverer and was popularized through citations in publications associated with institutions including Princeton University, University of Zagreb, University of Göttingen, Columbia University, and journals such as Nature (journal), Science (journal), and Geophysical Journal International. Subsequent field programs by United States Navy expeditions and expeditions organized by Alfred Wegener Institute extended observations to oceanic areas explored by ships like RV Vema and RV Knorr.

Physical Characteristics

The boundary is characterized by an abrupt increase in P-wave and S-wave velocities recorded by arrays from Global Seismographic Network and regional networks run by Japan Meteorological Agency, Geological Survey of Japan, Instituto Geográfico Nacional (Spain), and Geological Survey of India. Typical continental depths are between about 30 and 50 km beneath shields such as the Canadian Shield, Fennoscandian Shield, and the Kaapvaal Craton, while oceanic crust underlies an interface shallower near 5–10 km beneath features mapped by Challenger Deep surveys and research by Woods Hole Oceanographic Institution. Depth variations correlate with tectonic provinces studied by teams from California Institute of Technology, Massachusetts Institute of Technology, University of California, Berkeley, and University of Cambridge. Sharpness and impedance contrast produce conversions and reflections utilized in seismic profiling by agencies such as National Oceanic and Atmospheric Administration, USGS, and European counterparts including Geological Survey of Finland.

Geophysical Detection and Measurement

Detection relies on seismograms from instruments developed and maintained by entities like Seismological Society of America, Japanese University Network, European-Mediterranean Seismological Centre, and networks such as Global Centroid Moment Tensor catalog. Techniques include analysis of P-wave refracted arrivals (Pn), converted phases (Ps, Sp), receiver function methods pioneered in work by researchers at Stanford University and University of Arizona, as well as seismic reflection and refraction profiles executed by oceanographic groups including National Oceanography Centre (UK), Scripps Institution of Oceanography, and IFREMER. Complementary measurements from gravity missions such as GRACE and magnetotelluric surveys by labs at ETH Zurich and GFZ German Research Centre for Geosciences assist constraints; borehole data from projects like Kola Superdeep Borehole and drilling efforts by International Ocean Discovery Program provide ground truth. Tomographic inversions developed at Princeton University, University College London, and Seismological Laboratory, Caltech reveal three-dimensional variations exploited in models by USGS and European Space Agency-linked initiatives.

Composition and Mineralogy

The discontinuity marks a transition from crustal lithologies—felsic rocks studied in regions like the Appalachian Mountains, Himalayas, and Alps—to ultramafic mantle peridotite assemblages similar to xenoliths sampled near Kimberley (South Africa), San Carlos (Arizona), and volcanic provinces such as the Basin and Range Province, Iceland, and Hawaiian Islands. Experimental petrology from laboratories at Carnegie Institution for Science and Geological Survey of Japan indicates phase changes involving minerals like olivine, orthopyroxene, clinopyroxene, and garnet, with polymorphic transformations under pressure related to studies by Frank Press and Don L. Anderson. High-pressure experiments using diamond anvil cells at facilities including Lawrence Livermore National Laboratory and Max Planck Institute for Chemistry constrain elastic properties and equations of state informing velocity contrasts measured by seismologists such as Adam Dziewonski and Don Helmberger.

Role in Plate Tectonics and Mantle Dynamics

The interface influences lithospheric mechanical strength and affects processes addressed by tectonic researchers at University of Oxford, University of Tokyo, Australian National University, and University of Cape Town. Variations in Moho depth relate to crustal thickening at convergent margins like the Andes, Himalaya collisions, and rift systems including the East African Rift and Mid-Atlantic Ridge spreading centers overseen by programs like IODP and InterRidge. Mantle upwelling, delamination, and slab rollback models developed by researchers affiliated with Scripps Institution of Oceanography and ETH Zurich incorporate Moho behavior into mantle convection frameworks used by NASA-funded Earth system models and studies from NOAA and European Space Agency. The boundary modulates magmatism in hotspots such as Yellowstone National Park, Galápagos Islands, and Iceland, and affects seismic hazard assessments performed by United States Geological Survey, Japan Meteorological Agency, and regional seismic centers.

Variations and Global Distribution

Global surveys combining data from Global Seismographic Network, IRIS, USArray, European Seismic Network, and regional observatories reveal systematic variations: thick crust over orogenic belts like Tibet and the Hindu Kush, thin crust at oceanic spreading ridges including the East Pacific Rise and Mid-Atlantic Ridge, and anomalous structures beneath large igneous provinces such as Deccan Traps and Siberian Traps. Cratonic regions including the Pilbara Craton and the Zimbabwe Craton show deep, reflective discontinuities linked to ancient tectonic events catalogued by International Continental Scientific Drilling Program and regional geological surveys. Mapping efforts by teams from Lamont–Doherty Earth Observatory and University of Leeds produce global compilations used by organizations like World Data Center for Seismology and UNESCO-sponsored geoscience programs to inform studies of continental evolution, resource exploration, and seismic risk.

Category:Geology