Graphics by Regina Milkereit. As the rift matures, the magma-ponding zone near the base of the crust becomes progressively shallower due to crustal thinning and to sills piling one above the other and functioning as new, shallower reservoirs. This drives the system towards the condition of case a, and to transition to in-rift volcanism. Earlier models considered flexure of the lithosphere and interaction of magma with large boundary faults to explain rift-flank volcanism [5].
Boundary faults certainly influence magma ascent in the upper crust. However, our models suggest that if unloading stresses are ignored, dykes from a ponding zone beneath the rift, ascending in a stress field associated with normal faults, always reach the surface within the graben. The flexural behaviour of the crust may add local variations in the stress pattern, but these are not expected to alter the overall distribution of stresses [5]. Rift-induced gravitational unloading: A mechanism for the formation of off-rift volcanoes in continental rift zones.
If dykes start above the stress barrier or if the stress barrier is below the crust , within-rift volcanism occurs. Dykes ascend subvertically, or propagate laterally within the rift parallel to its axis Figure 2a , and dyke-fed volcanism is distributed within the graben. If dykes start within the stress barrier or the stress barrier extends across the crust and up to the brittle-ductile transition , our model predicts off-rift volcanism with sill formation.
The injected magma at the base of the crust forms sub-horizontal magmatic sheets that, depending on their initial distance from the rift axis, are either trapped as stacked sills above the ponding zone or escape to the sides of the stress barrier. From there they turn into subvertical dykes and eventually reach the surface Figure 2b. For a given set of parameters, dykes emerge very tightly spaced at a distance from the rift axis equal to about 1 - 2 and up to 3 graben half-widths.
If a dyke starts beneath the stress barrier, off-rift volcanism occurs without sill formation. Vertical dykes are deflected towards the rift sides and produce a more scattered arrival distribution at the surface Figure 2c. This occurs with deep nucleation below shallow and narrow grabens.
This is the least frequent scenario for a reasonable range of values for graben depths and widths and crustal thicknesses.
McKenzie, D. Some remarks on the development of sedimentary basins. Earth Planet. Lubimova, E. Heat flow patterns from Baikal and other rift zones. Tectonophysics, 8 , Bown, J. Effect of finite extension rate on melt generation at rifted continental margins. San Andreas Fault: We see part of a very active region in California where one crustal plate is sliding sideways with respect to the other.
The fault is marked by the valley running up the right side of the photo. Major slippages along this fault can produce extremely destructive earthquakes. Along much of their length, the crustal plates slide parallel to each other. These plate boundaries are marked by cracks or faults. Along active fault zones, the motion of one plate with respect to the other is several centimeters per year, about the same as the spreading rates along rifts. One of the most famous faults is the San Andreas Fault in California, which lies at the boundary between the Pacific plate and the North American plate Figure 5.
The Pacific plate, to the west, is moving northward, carrying Los Angeles, San Diego, and parts of the southern California coast with it. In several million years, Los Angeles may be an island off the coast of San Francisco.
Unfortunately for us, the motion along fault zones does not take place smoothly. The creeping motion of the plates against each other builds up stresses in the crust that are released in sudden, violent slippages that generate earthquakes. Because the average motion of the plates is constant, the longer the interval between earthquakes, the greater the stress and the more energy released when the surface finally moves.
For example, the part of the San Andreas Fault near the central California town of Parkfield has slipped every 25 years or so during the past century, moving an average of about 1 meter each time. In contrast, the average interval between major earthquakes in the Los Angeles region is about years, and the average motion is about 7 meters. The last time the San Andreas fault slipped in this area was in ; tension has been building ever since, and sometime soon it is bound to be released.
Sensitive instruments placed within the Los Angeles basin show that the basin is distorting and contracting in size as these tremendous pressures build up beneath the surface.
How much slippage is required for this to occur? If the next major southern California earthquake occurs in and only relieves one-half of the accumulated strain, how much slippage will occur? Figure 6. We owe the beauty of our young, steep mountains to the erosion by ice and water.
When two continental masses are moving on a collision course, they push against each other under great pressure. Earth buckles and folds, dragging some rock deep below the surface and raising other folds to heights of many kilometers. This is the way many, but not all, of the mountain ranges on Earth were formed. The Alps, for example, are a result of the African plate bumping into the Eurasian plate. As we will see, however, quite different processes produced the mountains on other planets.
Once a mountain range is formed by upthrusting of the crust, its rocks are subject to erosion by water and ice. The sharp peaks and serrated edges have little to do with the forces that make the mountains initially.
Instead, they result from the processes that tear down mountains. Ice is an especially effective sculptor of rock Figure 6. In a world without moving ice or running water such as the Moon or Mercury , mountains remain smooth and dull. Volcanoes mark locations where lava rises to the surface. A second major kind of volcanic activity is associated with subduction zones, and volcanoes sometimes also appear in regions where continental plates are colliding. In each case, the volcanic activity gives us a way to sample some of the material from deeper within our planet.
One of the best-known hot spot is under the island of Hawaii, where it currently supplies the heat to maintain three active volcanoes, two on land and one under the ocean. The Hawaii hot spot has been active for at least million years. The tallest Hawaiian volcanoes are among the largest individual mountains on Earth, more than kilometers in diameter and rising 9 kilometers above the ocean floor.
Not all volcanic eruptions produce mountains. If lava flows rapidly from long cracks, it can spread out to form lava plains. The largest known terrestrial eruptions, such as those that produced the Snake River basalts in the northwestern United States or the Deccan plains in India, are of this type.
Similar lava plains are found on the Moon and the other terrestrial planets. Terrestrial rocks can be classified as igneous, sedimentary, or metamorphic. A fourth type, primitive rock, is not found on Earth.
The surface expression of plate tectonics includes continental drift, recycling of the ocean floor, mountain building, rift zones, subduction zones, faults, earthquakes, and volcanic eruptions of lava from the interior. One of the wells in Kenya produces enough power for 5, homes! If successful, this program would provide a sustainable energy source for millions of people, many of whom do not have access to electricity today.
Also called an extensional boundary. Also called the Somali Peninsula. Sea level is determined by measurements taken over a year cycle. Also called lithospheric plate. Also called a conservative plate boundary. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.
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Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. Search through these resources to discover more about unique landforms and landscapes around the world. Landforms are natural and distinctive features. Explore how they show up in various landscapes. These resources can be used to teach middle schoolers more about the natural world, its distinctive features, and landscapes.
These tectonic plates rest upon the convecting mantle, which causes them to move. The movements of these plates can account for noticeable geologic events such as earthquakes, volcanic eruptions, and more subtle yet sublime events, like the building of mountains.
Teach your students about plate tectonics using these classroom resources. Weathering is the process of the weakening and breakdown of rocks, metals, and manmade objects. There are two main types of weathering: chemical and physical. An example of chemical weathering is acid rain. Caused mostly by the burning of fossil fuels, acid rain is a form of precipitation with high levels of sulfuric acid, which can cause erosion in the materials in which it comes in contact. An example of physical weathering is wind blowing across the desert playas.
This process causes rocks to form a specific pyramid-like shape and they are called ventifacts. Select from these resources to teach about the process of weathering in your classroom. Continental drift describes one of the earliest ways geologists thought continents moved over time. Today, the theory of continental drift has been replaced by the science of plate tectonics. In , after decades of tediously collecting and mapping ocean sonar data, scientists began to see a fairly accurate picture of the seafloor emerge.
The Tharp-Heezen map illustrated the geological features that characterize the seafloor and became a crucial factor in the acceptance of the theories of plate tectonics and continental drift. Today, these theories serve as the foundation upon which we understand the geologic processes that shape the Earth. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.
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