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The width of the CSZ varies along its length, depending on the temperature of the subducted oceanic plate, which heats up as it is pushed deeper beneath the continent. As it becomes hotter and more molten, it eventually loses the ability to store mechanical stress and generate earthquakes. On the Hyndman and Wang diagram the “locked” zone is storing up energy for an earthquake, and the “transition” zone, although somewhat plastic, could probably rupture. Earthquakes and volcanism Great subduction zone earthquakes are the most powerful earthquakes known to occur, and can exceed magnitude 9.0. They occur when enough energy (stress) has accumulated in the “locked” zone of the fault to cause a rupture known as a megathrust earthquake. The magnitude of a megathrust earthquake is proportional to length of the rupture along the fault. Because of the great length of the fault, the CSZ is capable of producing very large earthquakes if rupture occurs along its entire length.
This volcanism has included such notable eruptions as Mount Mazama (Crater Lake) about 7,500 years ago, Mount Meager about 2, 350 years ago, and Mount St. Helens in 1980. Major cities affected by a disturbance in this subduction zone would include Vancouver and Victoria, British Columbia; Seattle, Washington; and Portland, OregonMajor spreading ridges – especially the mid-Atlantic ridge and the East Pacific riseMid-Atlantic Ridge The North American and Eurasian Plates are moving away from each other along the line of the Mid Atlantic Ridge. The ridge extends into the South Atlantic Ocean between the South American and African Plates. The ocean ridge rises to between 2 to 3 km above the ocean floor, and has a rift valley at its crest marking the location at which the plates are moving apart. The Mid Atlantic Ridge, like other ocean ridge systems, has developed as a consequence of the divergent motion between the Eurasian and North American, and African and South American plates. As the mantle rises towards the surface below the ridge the pressure is lowered (decompression) and the hot rock starts to partially melt.
This produces basaltic volcanoes when an eruption occurs above the surface (Eyjafallajokull in Iceland) and characteristic basalt “pillow lava” in underwater eruptions. In this way, as the plates move further apart new ocean lithosphere is formed at the ridge and the ocean basin gets wider. This process is known as “sea floor spreading” and results in a symmetrical alignment of the rocks of the ocean floor, which get older with distance from the ridge crest. Evidence for this process comes from the magnetic properties of the erupted basalt. The Earth’s magnetic field has been shown to “flip” occasionally so that the North and South magnetic poles reverse with time. Basalt contains minute magnetic minerals that take on the direction of the Earth’s magnetic field at the time of eruption These polarity reversals are therefore recorded in the rocks forming at the Mid Atlantic Ridge