A new computer model shows blobs of cooler rocks (in blue) in Earth's mantle where none should be.Sebastian Noe/ETH ZurichA team of geophysicists created a detailed model of Earth's lower mantle using earthquake waves. This indicates the presence of zones of rocks that are colder, or have a different composition, than our current understanding of plate tectonics allows.When an earthquake happens, seismic waves spread from the hypocenter in all directions, traveling through Earth. The speed at which the waves travel depends on the type of wave, but also on the density and elasticity of the material through which the waves pass. Seismographic stations record these different waves, and on the basis of these recordings, geophysicists can draw conclusions about the structure and composition of the Earth and examine the processes that take place inside it.The rocks making up Earth's tectonic plates are cooler and have a higher density than the surrounding mantle and can be visualized as slabs sinking into Earth's interior. Scientists always found them where they expected them to be: in an area known as subduction zones, where two plates meet and one subducts beneath the other. This has helped scientists investigate the plate tectonic cycle, i.e., the emergence and destruction of plates at Earth's surface, through our planet's history.Now, however, a team of geophysicists from ETH Zurich and the California Institute of Technology has made a surprising discovery: using a new high-resolution model, they have discovered blobs of cold rocks under large oceans or in the interior of continentsfar away from plate boundaries.Unlike previous research, using only seismic waves generate by very large and powerful earthquakes, the new study used a supercomputer to handle a large dataset generated from earthquakes of varying magnitude.Read More: Mystery Of Giant Volcanic Eruption That Has Puzzled Scientists For Almost 200 Years Has Finally Been SolvedOne of the newly discovered zones is under the western Pacifica very large tectonic plate. However, according to current plate tectonic theories and knowledge, there should be no material from subducted plates there, because it is impossible that there were subduction zones nearby in the recent geological history.Seismic cross section through the Pacific showing a cold anomaly (in blue) beneath much of the plate ... [+] forming the ocean.Schouten et al.2025/scientifc reports"Apparently, such zones in the Earth's mantle are much more widespread than previously thought," explains Thomas Schouten, first author of the new study and doctoral student at the Geological Institute of ETH Zurich.The researchers do not know for certain what material is involved instead, and what that would mean for Earth's internal dynamics."That's our dilemma. With the new high-resolution model, we can see such anomalies everywhere in the Earth's mantle. But we don't know exactly what they are or what material is creating the patterns we have uncovered."The researchers believe it is possible that they are not just cold plate material that has subducted in the last 200 million years, as previously assumed, but material formed by various geological processesmaybe even no longer active on Earth.Over billions of years, convective currents driving plate tectonics should remix and homogenize the entire mantle. However, these findings suggest that convection is limited to the upper part of Earth's mantle. Another explanation sees the anomalies as old rocks that remain stuck in zones between the currents."We think that the anomalies in the lower mantle have a variety of origins," concludes Schouten. "It could be either ancient, silica-rich material that has been there since the formation of the mantle about 4 billion years ago and has survived despite the convective movements in the mantle, or zones where iron-rich rocks accumulate as a consequence of these mantle movements over billions of years."The study, "Full-waveform inversion reveals diverse origins of lower mantle positive wave speed anomalies," was published in scientific reports and can be found online here.Additional material and interviews provided by ETH Zurich.