BBC - GCSE Bitesize: Plate tectonics - Higher tier
Learn and revise about plate tectonics focusing on the Earth's structure, plate movement and boundaries with BBC Bitesize KS3 Geography. We're so excited to continue to grow and support the parents and teachers championing children's Most volcanoes occur at plate boundaries, where two tectonic plates meet. Normally volcanoes appear between 2 divergent plates. Mantle A flood basalt appears when a mantle plume melts an area of basalt magma. The seeds of an earthquake lie in the tectonic plates that make up the of the meeting point of those same two plates and the Arabian plate.
Massive forces would have been needed to break this single lithosphere into multiple plates and to initiate plates descending into the mantle. The dense, downgoing slabs pull on the parts of the plates still at the surface, driving plate tectonics. Some subducting slabs stall at the transition zone, while others descend toward the core-mantle boundary.
The forces involved are incredible. The momentum of the massive sinking slabs overcomes the friction generated by the upper mantle adjacent to the slabs as they descend. If plate tectonics is primarily driven by the forces generated by downgoing slabs, how could tectonics have gotten started before there were subducting slabs? It may have to do with heat.
Today, temperatures in the mantle hover around 1, degrees Celsius. But numerical models by Jun Korenagaof Yale University, and colleagues indicate that about 3 billion years ago, the mantle was hotter by about to degrees.
These extreme temperatures — as hot as 1, degrees Celsius — had a profound effect on the early crust: Computer models from Gerya published in Nature in suggest that the hotter temperatures of early Earth may have made for weaker, more easily broken plates.
That heat would have also created a very different mantle environment. But while this hotter and weaker scenario could have helped initiate the process, strength is required to sustain it, van Hunen says.
BBC - Travel - Swim between two tectonic plates
The Farallon Plate began subducting under the North American Plate during the Jurassic, and is thought to have been completely overrun by about 50 million years ago. The fate of its remnant slabs as they descended into the mantle may explain several features of the overlying continent, including the rise of the Rockies and the activity of the New Madrid Seismic Zone. The colors show anomalies in rigidity, which correlate with temperature anomalies.
Green and blue represent relatively cooler regions, and orange and red represent hotter regions. Empirical data are also needed to calibrate models, and to answer questions about what happens to slabs once they start subducting: Where do they go, and how has this process changed over time? Deep seismic tomography, which uses seismic waves to image the interior structure of Earth, provides the best look at slab shapes and what happens to them as they descend. Some slabs seem to stall out along the boundary between the upper and lower mantle, located at a depth of about kilometers.
But in other cases, such as the Mariana Trench in the western Pacific, slabs appear to keep going down through this boundary into the lower mantle. Recently, scientists have been working on developing sophisticated models that can rewind the tape and show how slabs move and are assimilated into the mantle over geologic timescales. But how accurate these reconstructions are as they retrace slab movements back in time is debated.
Royden and colleagues are currently working on a suite of combined numerical and analytical models to address these questions. Computers are just now able to handle very large, complex models, Royden says. Discrimination between valid and invalid models can only be made on the basis of data. Geochemists use two types of isotopes to study the history of rocks and minerals: Stable isotopes occur in different ratios in different geological settings such as in the mantle versus the crustso these isotopes, such as oxygen and oxygen, can be used to determine where different magmas originated.
Radiogenic, or parent-daughter isotope pairs, such as uranium and lead, rubidium and strontium, samarium and neodymium, and lutetium and hafnium, decay at specific rates and can be used to evaluate the timing of geological processes, such as when the continents formed. These isotopes also fractionate differently in different magmatic settings so they can also be used to decipher the processes involved in magma generation and answer questions about how rocks — and continents — formed.
Isotope geochemistry may address one of the biggest mysteries surrounding the onset of plate tectonics: For example, in a study in Nature Geoscience, researchers used isotopes of uranium and lead preserved in zircon crystals found in ancient rocks in Western Australia to confirm the age of the oldest known continental crust to 4.
The bulk of continental crust is thought to have formed prior to plate tectonics starting on Earth. Some researchers suggest that the continental crust could have formed by mantle plume-like volcanism. Evidence for this early crustal formation comes from isotopic signatures preserved when certain elements diffused into the liquid magma when melting occured in the mantle.2013-09 ICELAND: Diving between American and Eurasian tectonic plates @ Silfra Cracks
And in a study in Science, Dhuime and his group looked at ratios of these different isotopes collected in a worldwide database of more than 7, zircons to model the volume of continental crust through time. That suggests, Dhuime says, that plate tectonics has operated more or less continuously since it began, without any interruptions that might have thrown off the equilibrium between the creation of new crust and the destruction of old crust.
As to when it began, further isotopic research has shown that something big began to happen about 3 billion years ago — perhaps the onset of plate tectonics.
Rubidium decays to strontium with a long half-life of nearly 49 billion years, making it an ideal tool for studying conditions on early Earth. When mantle material melts to form new crust, rubidium preferentially migrates into granitic melt more so than strontium, so the more felsic granitic the crust, the higher the rubidium-strontium ratio will be in that crust. By tabulating rubidium-strontium ratios for those 13, samples, Dhuime and colleagues showed that these particular isotopes can be used as proxies for the silica content, which is a known marker for the thickness and volume of early continental crust.
The combination of oxygen, hafnium and uranium-lead isotopes in zircon indicates a change in the volume of crust about 3 billion years ago, which Dhuime and his colleagues say may be related to increased recycling associated with the onset of plate tectonics. In summary, the isotopic clues suggest that continental crust started forming 4.
How is the strength of an earthquake measured? Earthquakes are measured using a network of seismometers, instruments that record the motion of the Earth as it vibrates or shakes.
Many countries have hundreds or thousands of seismometers, allowing them to accurately predict the size and location of an earthquake using a process of triangulation. The maximum motion of the earthquake measured by these seismometers is then used in calculating the earthquake's magnitude, which signifies the amount of energy released by the quake.
There's no theoretical upper limit to the magnitude scale, though limitations in the amount of energy that can build up in plate compression mean it's unlikely there'd ever be a quake above magnitude 10 on Earth.
Every time the magnitude increases by one — like from magnitude 5 to magnitude 6 — there's a fold increase in the amount of energy released by the quake. But magnitude will not always predict how destructive an earthquake will be, because it also depends on where the earthquake occurs and at what depth its hypocentre is. The hypocentre of an earthquake is the point in the Earth at which a rupture actually starts, which could be kilometres below the surface, while the epicentre is the point on the surface vertically above the hypocentre.
The more shallow the hypocentre of the earthquake, the greater the shaking at the Earth's surface. Seismic waves that are generated deeper below the Earth's surface have further to travel and thus lose their power along the way.
How frequent are the biggest earthquakes?
- The science of earthquakes explained
- Major Plates of the Lithosphere: Earth's Tectonic Plates
- Mantle Plume
The largest earthquake ever recorded was a magnitude 9. From toan earthquake above magnitude 8 occurred once a year on average.
No single year had more than two earthquakes above magnitude 8 except forwith quakes in the Solomon Islands, Peru, Indonesia, and near the Kuril Islands.