http://www.geology.sdsu.edu/how_volcanoes_work/intraplvolc_page.html Part 1: Here is how volcanic CO2 degassing works for Intraplate volcanism, also known as hotspot. Although most volcanic rocks are generated at plate boundaries, there are a few exceptionally active sites of volcanism within the plate interiors. These intraplate regions of voluminous volcanism are called hotspots. Twenty-four selected hotspots are shown on the adjacent map. Most hotspots are thought to be underlain by a large plume of anomalously hot mantle. These mantle plumes appear to be generated in the lower mantle and rise slowly through the mantle by convection. Experimental data suggests that they rise as a plastically deforming mass that has a bulbous plume head fed by a long, narrow plume tail. As the head impinges on the base of the lithosphere, it spreads outward into a mushroom shape. Such plume heads are thought to have diameters between ~500 to ~1000 km. Many scientists believe that mantle plumes may be derived from near the core-mantle boundary, as demonstrated in this computer simulation from the Minnesota supercomputing lab. Note the bulbous plume heads, the narrowplume tails, and the flattened plume heads as they impinge on the outer sphere representing the base of the lithosphere. Decompressional melting of this hot mantle source can generate huge volumes of basalt magma. It is thought that the massive flood basalt provinces on earth are produced above mantle hotspots. Although most geologists accept the hotspot concept, the number of hotspots worldwide is still a matter of controversy. HOTSPOT TRACKSThe Pacific plate contains several linear belts of extinct submarine volcanoes, called seamounts, an example of which is the Foundation seamount chain shown here. The formation of at least some of these intraplate seamount chains can be attributed to volcanism above a mantle hotspot to form a linear, age-progressive hotspot track. Mantle plumes appear to be largely unaffected by plate motions. As lithospheric plates move across stationary hotspots, volcanism will generate volcanic islands that are active above the mantle plume, but become inactive and progressively older as they move away from the mantle plume in the direction of plate movement. Thus, a linear belt of inactive volcanic islands and seamounts will be produced. A classic example of this mechanism is demonstrated by the Hawaiian and Emperor seamount chains.
The "Big Island" of Hawaii lies above the mantle plume. It is the only island that is currently volcanically active. The seven Hawaiian Islands become progressively older to the northwest. The main phase of volcanism on Oahu ceased about 3 million years ago, and on Kauai about 5 million years ago. This trend continues beyond the Hawaiian Islands, as demonstrated by a string of seamounts (the Hawaiian chain) that becomes progressively older toward Midway Island. Midway is composed of lavas that are ~27 million years old. Northwest of Midway, the volcanic belt bends to the north-northwest to form the Emperor seamount chain. Here, the seamounts become progressively older until they terminate against the Aleutian trench. The oldest of these seamounts near the trench is ~70 million years old. This implies that the mantle plume currently generating basaltic lavas on the Big Island has been in existence for at least 70 million years! The Hawaiians were very good at recognizing the difference in the older, eroded volcanic islands and newer islands to the southeast, where volcanic features are more pristine. Legend has it that Pele, the Hawaiian goddess of fire, was forced from island to island as she was chased by various gods. Her journey is marked by volcanic eruptions, as she progressed from the island of Kaua'i to her current home on the Big Island. The legend corresponds well with the modern scientific notion of the age progression of these volcanic islands. The reason that I'm interested in these volcanoes is the CO2 released from them. Current estimates of CO2 release rate from hotspot volcanoes range from 0.5 to 3 x10^18 mol/my. This is 0.006 to 0.036 Pg C/yr (Gt C/yr), only a small fraction of the current fossil fuel burning rate 10 Gt C/yr today!
The other form of volcanic CO2 release is through release from mantle at mid-ocean ridge, that is 1 to 3 x10^18 mol/my, similar to that of hotspot CO2 release. There is also release in arc volcanoes from subducted CaCO3 (turns out to be the largest 2 to 3 x10^18 mol/my), and release in arc volcanoes from the mantle (very small, 0.3 to 0.5 x10^18 mol/my). All these add up to a total global degassing of 4 to 10 x10^18 mol/my. Unless my calculation is wrong, this equals to 0.048 to 0.12 Gt C/yr CO2 release rate. This is a simple mass balance calculation, and without our anthropogenic CO2 emission, this is perhaps the rate of CO2 is released in natural system, but this CO2 is balanced by silicate weathering occurring on land. So the net CO2 release from volcanoes is essentially 0!!!
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Ying CuiI'm a geoscientist doing research on extreme environmental change and its impact to biodiversity in the geologic past. ArchivesCategories |