Spatial Relationships between Plate Boundaries, Volcanoes, and Earthquakes
The geographic distributions of volcanism and earthquake activity are highly similar and tend to be concentrated in linear patterns along plate boundaries. Although there are exceptions, the locations of volcanoes and earthquakes match fairly well, yet their nature and severity differ from place to place. In general, the frequency and severity of volcanic eruptions or earthquakes vary according to their proximity to a certain type of plate boundary or by their central location on a tectonic plate.
Plate divergence, whether it breaks continents or seafloor, creates fractures that provide avenues for molten rock to reach the surface. The divergent midocean ridges are volcanic and tend to have rather mild volcanic eruptions and small to moderate earthquakes that originate at a shallow depth. These tectonic activities affect ocean islands associated with midocean ridges, such as the Azores and Iceland.
Volcanoes also tend to occur where continental crust is breaking and diverging. In these regions, earthquakes tend to be small to moderate, but continental crust mixed with mafic magma produces a wider variety of volcano types, some of which are potentially quite violent. Examples in the East African rift valleys include Mount Kilimanjaro and Mount Kenya.
The potential severity of earthquakes and eruptions is much greater where plates are converging. Along the ocean trenches where crustal material is subducted, volcanoes typically develop along the edge of the overriding plate. The largest region where this occurs is the “Pacific Ring of Fire,” a volcanically active and earthquake-prone rim around the Pacific Ocean. When oceanic crust subducts beneath continental crust along an ocean trench, it melts into magma that moves upward under the continental borders. Subduction along the Pacific is associated with volcanoes in the Andes, the Cascades, and the Aleutians; the Kuril Islands and the Kamchatka Peninsula in Russia; and Japan, the Philippines, New Guinea, Tonga, and New Zealand. Many of these volcanoes erupt andesite and can be dangerously explosive. Earthquakes are common events along the Pacific Rim, mostly small to moderate, but the largest earthquakes ever recorded have been related to subduction in this region. The depth of earthquakes becomes deeper under the overriding plates, tracking the subducting plates downward to where they are recycled into the mantle.
Another volcanic and seismic belt marks the collision zone between northward-moving Southern Hemisphere plates and the Eurasian plate. The volcanoes of the Mediterranean region, Turkey, Iran, and Indonesia are located along this collision zone, seismic activity is common, and occasionally major earthquakes occur.
Transform plate boundaries, where lateral sliding occurs, also mark the locations of many earthquakes. The potential for major earthquakes mainly exists in places such as along the San Andreas Fault zone in California where thick continental crust is resistant to sliding easily. Volcanic activity along transform boundaries is moderate on the seafloor to modest in continental locations.
Certain active earthquake and volcanic areas do not occur near active plate boundaries, rather they exist within the central parts of plates. The Hawaiian Islands, the Galapagos Islands, and the Yellowstone National Park area are examples of intraplate “hot spots.” Some may be associated with a plume of magma rising from the mantle. Oceanic areas that are over hot spots tend to have strong volcanic activity and modest earthquake activity. In midcontinental areas where large earthquakes have occurred, these regions are suture zones where continents are pushing together, such as in the Himalayas, or where broken edges of ancient landmasses shift, today located in the midcontinent but deeply buried by more recent rocks.
Volcanic and earthquake activities that are not located along active plate margins are intriguing and show that we still have much to learn about Earth’s internal processes and their impact on the surface. Yet, plate tectonics has greatly influenced our understanding of the variations in volcanism, earthquake activity, and the landforms associated with these processes.
References: Essentials of Physical Geography {Eighth Edition}: Robert E. Gabler | James F. Petersen | L. Michael Trapasso
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