The relative motion of the plates is horizontal. They can occur underwater or on land, and crust is neither destroyed nor created. Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it exceeds the threshold of the rocks, the energy is released — causing earthquakes.
Privacy Statement Disclaimer and Copyright. Staff Search. Earthquakes Earthquakes at a Plate Boundary. Tectonic Plates and Plate Boundaries. Plate Collision in NZ. Subduction occurs when there is a difference in the density of the plates. Oceanic crust is typically denser than continental crust and is forced downwards into the hot mantle when it collides with continental crust. The less dense conitinental crust is forced upward.
At the start the Iapetus oceanic crust was subducted beneath continental crust, but eventually the oceanic crust was gone and continents collided. The Caledonian mountains formed at this time and it is thought they were as high as the modern Himalayas when they formed.
The Himalayas are an example of the collision of two continental plates where the Indian plate is crashing into the Eurasian plate and is being forced upwards. They are continually growing at an average rate of 1cm per year, this will be 10km in 1 million years. See earthquakes , tsunami , and volcanoes for more on the effects of convergent plate boundaries redirect to Topics section. Passive plate bo u ndarie s. This is when two plates slide past each other.
When the plates move, the jagged edges of the plate boundaries snag and catch each other and can get jammed. This causes a build-up of pressure. When the plates eventually pass each other, the pressure is released in the form of an earthquake.
The closest passive plate boundary to Ireland is the boundary between the African and Eurasian plate south of Portugal. Source: Wikimedia Commons The movement of the two plates can be in opposite directions or in the same direction but at different speeds for example the San Andreas Fault in California. Not all plate boundaries are as simple as the main types discussed above.
In some regions, the boundaries are not well defined because the plate-movement deformation occurring there extends over a broad belt called a plate-boundary zone. One of these zones marks the Mediterranean-Alpine region between the Eurasian and African Plates, within which several smaller fragments of plates microplates have been recognized.
Because plate-boundary zones involve at least two large plates and one or more microplates caught up between them, they tend to have complicated geological structures and earthquake patterns. We can measure how fast tectonic plates are moving today, but how do scientists know what the rates of plate movement have been over geologic time?
The oceans hold one of the key pieces to the puzzle. Because the ocean-floor magnetic striping records the flip-flops in the Earth's magnetic field, scientists, knowing the approximate duration of the reversal, can calculate the average rate of plate movement during a given time span. These average rates of plate separations can range widely. The Arctic Ridge has the slowest rate less than 2. Evidence of past rates of plate movement also can be obtained from geologic mapping studies.
If a rock formation of known age -- with distinctive composition, structure, or fossils -- mapped on one side of a plate boundary can be matched with the same formation on the other side of the boundary, then measuring the distance that the formation has been offset can give an estimate of the average rate of plate motion.
This simple but effective technique has been used to determine the rates of plate motion at divergent boundaries, for example the Mid-Atlantic Ridge, and transform boundaries, such as the San Andreas Fault. Current plate movement can be tracked directly by means of ground-based or space-based geodetic measurements; geodesy is the science of the size and shape of the Earth. Ground-based measurements are taken with conventional but very precise ground-surveying techniques, using laser-electronic instruments.
However, because plate motions are global in scale, they are best measured by satellite-based methods. The late s witnessed the rapid growth of space geodesy, a term applied to space-based techniques for taking precise, repeated measurements of carefully chosen points on the Earth's surface separated by hundreds to thousands of kilometers. The three most commonly used space-geodetic techniques -- very long baseline interferometry VLBI , satellite laser ranging SLR , and the Global Positioning System GPS -- are based on technologies developed for military and aerospace research, notably radio astronomy and satellite tracking.
Among the three techniques, to date the GPS has been the most useful for studying the Earth's crustal movements. Twenty-one satellites are currently in orbit 20, km above the Earth as part of the NavStar system of the U. Department of Defense. These satellites continuously transmit radio signals back to Earth.
To determine its precise position on Earth longitude, latitude, elevation , each GPS ground site must simultaneously receive signals from at least four satellites, recording the exact time and location of each satellite when its signal was received. By repeatedly measuring distances between specific points, geologists can determine if there has been active movement along faults or between plates. The separations between GPS sites are already being measured regularly around the Pacific basin.
By monitoring the interaction between the Pacific Plate and the surrounding, largely continental plates, scientists hope to learn more about the events building up to earthquakes and volcanic eruptions in the circum-Pacific Ring of Fire.
Early in the rift-forming process, streams and rivers will flow into the sinking rift valley to form a long linear lake.
As the rift grows deeper it might drop below sea level, allowing ocean waters to flow in. This will produce a narrow, shallow sea within the rift. This rift can then grow deeper and wider.
If rifting continues, a new ocean basin could be produced. The East Africa Rift Valley is a classic example of this type of plate boundary. The East Africa Rift is in a very early stage of development. The plate has not been completely rifted, and the rift valley is still above sea level but occupied by lakes at several locations.
The Red Sea is an example of a more completely developed rift. There the plates have fully separated, and the central rift valley has dropped below sea level. Visit the Interactive Plate Boundary Map to explore satellite images of divergent boundaries between continental plates. Two locations are marked within the rift valley of East Africa, and another location is marked within the Red Sea. Effects that are found at this type of plate boundary include: a rift valley sometimes occupied by long linear lakes or a shallow arm of the ocean; numerous normal faults bounding a central rift valley; shallow earthquake activity along the normal faults.
Volcanic activity sometimes occurs within the rift. Contributor: Hobart King Publisher, Geology.
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