The Somali Plate is a minor tectonic plate, which straddles the equator in the eastern hemisphere. It is centered on the island of Madagascar and includes about half of the East coast of Africa, from the Gulf of Aden in the North through the East African Rift Valley; the southern boundary with the Nubian-African plate is a diffuse plate boundary consisting of the Lwandle plate. The Arabian plate diverges to the north forming the Gulf of Aden; the Indian Plate, Australian Plate, Antarctic Plate all diverge from the Somali plate forming the eastern Indian Ocean. The Somali-Indian boundary spreading ridge is known as the Carlsberg Ridge; the Somali-Australian boundary spreading ridge is known as the Central Indian Ridge. The Somali-Antarctic boundary spreading ridge is known as the Southwest Indian Ridge; the western boundary with the African Plate is diverging to form the East African Rift, which stretches south from the triple junction in the Afar depression. The southern boundary with the Nubian-African plate is a diffuse plate boundary with of the Lwandle plate.
The Seychelles and the Mascarene Plateau are located northeast of the Madagascar. From 1.4 to 1.2 Ga the Kibaran orogeny fused the Congo cratons. From 1000 to 600 Ma the super-continent Gondwana was formed and the Pan-African orogeny sutured the Tanzanian and Kalahari cratons; the rifting of Gondwana occurred from 190 Ma to 47 Ma separating Madagascar from the eastern coast of Africa and placing the Seychelles/Mascarene Plateau northeast of Madagascar. The rifting of the Red Sea started around 30 million years ago and the first rifting occurred in the northern East African Rift System around 20 million years ago. Emerick, C. M. 1985, Age progressive volcanism in the Comores Archipelago and Northern Madagascar, Oregon State Univ. Corvallis, PhD Thesis, Abstract
South American Plate
The South American Plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African Plate, with which it forms the southern part of the Mid-Atlantic Ridge. The easterly edge is a divergent boundary with the African Plate. At the Chile Triple Junction, near the west coast of the Taitao–Tres Montes Peninsula, an oceanic ridge known as the Chile Rise is subducting under the South American Plate. Geological research suggests that the South American Plate is moving westward away from the Mid-Atlantic Ridge: "Parts of the plate boundaries consisting of alternations of short transform fault and spreading ridge segments are represented by a boundary following the general trend." As a result, the eastward-moving and more dense Nazca Plate is subducting under the western edge of the South American Plate, along the continent's Pacific coast, at a rate of 77 mm per year. The collision of these two plates is responsible for lifting the massive Andes Mountains and for creating the numerous volcanoes which are strewn throughout them.
Fifteen-Twenty Fracture Zone
Aegean Sea Plate
The Aegean Sea Plate is a small tectonic plate located in the eastern Mediterranean Sea under southern Greece and far western Turkey. Its southern edge is a subduction zone south of Crete, where the African Plate is being swept under the Aegean Sea Plate. To the north is the Eurasian Plate, a divergent boundary responsible for the formation of the Gulf of Corinth. Bird, P.. "An updated digital model of plate boundaries". Geochemistry, Geosystems. 4: 1027. Doi:10.1029/2001GC000252
The Gorda Ridge, a tectonic spreading center, is located 200 kilometres off the northern coast of California and southern Oregon. Running NE – SW it is 300 kilometres in length; the ridge is broken into three segments. The Gorda Ridge runs in a north-easterly direction, bounded at both ends by transform faults. At the southern end, the ridge meets the Mendocino transform fault, while the northern end butts against the Blanco transform fault. To its east is the Gorda Plate, which together with the Juan de Fuca Plate to its north, is what remains of the once-vast Farallon Plate; these two oceanic plates are moving east, subducting underneath the North American Plate in what is known as the Cascadia Subduction Zone. To the west and south of the ridge is the Pacific Plate, moving west diverging from the Gorda Plate; the divergence of the Pacific Plate and the Gorda Plate has led to the creation of the Gorda Ridge. On the Pacific Plate 50 km west of the northern portion of the ridge, sit eight seamounts, known as the President Jackson Seamounts.
30 Mya the Farallon Plate subducted beneath the North American Plate, segmenting the Pacific Farallon Ridge. This subduction created new microplates and new ridges, including the Juan de Fuca Plate and Juan de Fuca Ridge; as the Juan de Fuca Plate continued to subduct underneath the North American Plate it segmented, creating the Gorda Plate and Gorda Ridge. The Pacific Plate is moving in a northwest direction, creating a divergence with the Gorda Plate at a speed of 5 cm per year; the Juan de Fuca Plate is moving east-northeast, subducting under the North America Plate at a much slower rate of 2.5–3 cm per year. Due to the ridge being segmented into three distinct parts, each section has its own spreading rate, caused by the slab-pull and ridge-push of the surrounding tectonic plates; the northern segment is the narrowest, with portions as narrow as 3 km across, has the fastest spreading rate of 2.9 cm per year. The central segment is 10 km wide with a spreading rate of 2.4 cm per year. The southern segment has sections as wide as 18 km, has the slowest spreading rate of 1.2 cm per year.
Due to the Gorda Ridge's proximity to the Mendocino Triple Junction, the area experiences a significant amount of seismic activity. The majority of activity is seen on the Gorda Plate, however some occurs on the ridge itself. Most events are generated by the divergence of the Gorda Plate. Since 1983 there have been 80 magnitude 3 earthquakes happening at this location every year. On 28 February 1996 the northern segment of the Gorda Ridge experienced a burst of seismic activity, which lasted three weeks. Concurrent with seismicity were a series of slow volcanic eruptions, forming thick flows of pillow basalt; these pillow basalts are thickest to the north, indicating this region's activity lasted longer than the other portions of the ridge. The estimated volume of erupted magma during this event is 18x106 m3, forming a blanket of new oceanic crust, averaging 75 m thick. Unlike other intermediate spreading centers, the Gorda Ridge has a large rift valley, seen in areas of slow spreading centers.
This is caused by the oceanic crust beneath the ridge being thinner and mantle temperatures being cooler than most intermediate spreading centers. The Gorda Ridge has an average depth of 3000 m, with a few locations reaching depths of 3500 m; the walls of this valley are steep, in most cases giving a vertical relief of over 1000 m. The floor of the southern ridge valley has been filled in with 1000 m of sediment from the continental margin delivered by turbidity currents; the central ridge valley contains exposed basalt, the northern ridge valley has a light sediment covering. Divergent Boundary Explorer Ridge Juan de Fuca Ridge
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, gases to escape from a magma chamber below the surface. Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle. Therefore, on Earth, volcanoes are found where tectonic plates are diverging or converging, most are found underwater. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. Volcanoes can form where there is stretching and thinning of the crust's plates, e.g. in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "plate hypothesis" volcanism. Volcanism away from plate boundaries has been explained as mantle plumes; these so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth.
Volcanoes are not created where two tectonic plates slide past one another. Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere. Volcanic winters have caused catastrophic famines; the word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology. The study of volcanoes is sometimes spelled vulcanology. At the mid-oceanic ridges, two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock; because the crust is thin at these ridges due to the pull of the tectonic plates, the release of pressure leads to adiabatic expansion and the partial melting of the mantle, causing volcanism and creating new oceanic crust.
Most divergent plate boundaries are at the bottom of the oceans. Black smokers are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed. Subduction zones are places where two plates an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges, under the continental plate, forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, thus creating magma; this magma tends to be viscous because of its high silica content, so it does not attain the surface but cools and solidifies at depth. When it does reach the surface, however, a volcano is formed. Typical examples are the volcanoes in the Pacific Ring of Fire. Hotspots are volcanic areas believed to be formed by mantle plumes, which are hypothesized to be columns of hot material rising from the core-mantle boundary in a fixed space that causes large-volume melting.
Because tectonic plates move across them, each volcano becomes dormant and is re-formed as the plate advances over the postulated plume. The Hawaiian Islands are said to have been formed in such a manner; this theory, has been doubted. The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; the features of volcanoes are much more complicated and their structure and behavior depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus. Vents that issue volcanic material and gases can develop anywhere on the landform and may give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Hawaii's Kīlauea. Other types of volcano include cryovolcanoes on some moons of Jupiter and Neptune. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when the mud volcano is a vent of an igneous volcano.
Volcanic fissure vents are linear fractures through which lava emerges. Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent, they do not explode catastrophically. Since low-viscosity magma is low in silica, shield volcanoes are more common in oceanic than continental settings; the Hawaiian volcanic chain is a series of shield cones, they are common in Iceland, as well. Lava domes are built by slow eruptions of viscous lava, they are sometimes formed within the crater of a previous volcanic eruption, as in the case of Mount Saint Helen
The Capricorn Plate is a proposed minor tectonic plate lying beneath the Indian Ocean basin in the southern and eastern hemispheres. The original theory of plate tectonics as accepted by the scientific community in the 1960s assumed rigid plates and narrow, distinct plate boundaries. However, research in the late 20th and early 21st centuries suggests that certain plate junctions are diffuse across several dozen or hundred kilometres; the Capricorn Plate is a rigid piece of oceanic crust along the far western edge of the former Indo-Australian Plate. The Capricorn Plate was once joined with the Indian Plate and the Australian Plate to form the Indo-Australian Plate, but recent studies suggest that the Capricorn Plate began separating from the Indian and Australian Plates between 18 million years ago and 8 million years ago along a wide, diffuse boundary
Mount Shasta is a active volcano at the southern end of the Cascade Range in Siskiyou County, California. At an elevation of 14,179 feet, it is the second-highest peak in the Cascades and the fifth-highest in the state. Mount Shasta has an estimated volume of 85 cubic miles, which makes it the most voluminous stratovolcano in the Cascade Volcanic Arc; the mountain and surrounding area are part of the Shasta–Trinity National Forest. Mount Shasta is connected to its satellite cone of Shastina, together they dominate the landscape. Shasta rises abruptly to tower nearly 10,000 feet above its surroundings. On a clear winter day, the mountain can be seen from the floor of the Central Valley 140 miles to the south; the mountain has attracted the attention of poets and presidents. It is dormant; the mountain consists of four overlapping volcanic cones that have built a complex shape, including the main summit and the prominent satellite cone of 12,330 ft Shastina, which has a visibly conical form. If Shastina were a separate mountain, it would rank as the fourth-highest peak of the Cascade Range.
Mount Shasta's surface is free of deep glacial erosion except, for its south side where Sargents Ridge runs parallel to the U-shaped Avalanche Gulch. This is the largest glacial valley on the volcano. There are seven named glaciers on Mount Shasta, with the four largest radiating down from high on the main summit cone to below 10,000 ft on the north and east sides; the Whitney Glacier is the longest, the Hotlum is the most voluminous glacier in the state of California. Three of the smaller named glaciers occupy cirques near and above 11,000 ft on the south and southeast sides, including the Watkins and Mud Creek glaciers; the oldest-known human settlement in the area dates to about 7,000 years ago. At the time of Euro-American contact in the 1820s, the Native American tribes who lived within view of Mount Shasta included the Shasta, Modoc, Atsugewi, Klamath and Yana tribes; the historic eruption of Mount Shasta in 1786 may have been observed by Lapérouse, but this is disputed. Although first seen by Spanish explorers, the first reliably reported land sighting of Mount Shasta by a European or American was by Peter Skene Ogden in 1826.
In 1827, the name "Sasty" or "Sastise" was given to nearby Mount McLoughlin by Ogden. An 1839 map by David Burr lists the mountain as Rogers Peak; this name was dropped, the name Shasta was transferred to present-day Mount Shasta in 1841 as a result of work by the United States Exploring Expedition. Beginning in the 1820s, Mount Shasta was a prominent landmark along what became known as the Siskiyou Trail, which runs at Mount Shasta's base; the Siskiyou Trail was on the track of an ancient trade and travel route of Native American footpaths between California's Central Valley and the Pacific Northwest. The California Gold Rush brought the first Euro-American settlements into the area in the early 1850s, including at Yreka and Upper Soda Springs; the first recorded ascent of Mount Shasta occurred after several earlier failed attempts. In 1856, the first women reached the summit. By the 1860s and 1870s, Mount Shasta was the subject of literary interest. In 1854 John Rollin Ridge titled a poem "Mount Shasta."
A book by California pioneer and entrepreneur James Hutchings, titled Scenes of Wonder and Curiosity in California, contained an account of an early summit trip in 1855. The summit was achieved by John Muir, Josiah Whitney, Clarence King, John Wesley Powell. In 1877, Muir wrote a dramatic popular article about his surviving an overnight blizzard on Mount Shasta by lying in the hot sulfur springs near the summit; this experience was inspiration to Kim Stanley Robinson's short story "Muir on Shasta". The 1887 completion of the Central Pacific Railroad, built along the line of the Siskiyou Trail between California and Oregon, brought a substantial increase in tourism and population into the area around Mount Shasta. Early resorts and hotels, such as Shasta Springs and Upper Soda Springs, grew up along the Siskiyou Trail around Mount Shasta, catering to these early adventuresome tourists and mountaineers. In the early 20th century, the Pacific Highway followed the track of the Siskiyou Trail to the base of Mount Shasta, leading to still more access to the mountain.
Today's version of the Siskiyou Trail, Interstate 5, brings thousands of people each year to Mount Shasta. From February 13–19, 1959, the Mount Shasta Ski Bowl obtained the record for the most snowfall during one storm in the U. S. with a total of 15.75 feet. Mount Shasta was declared a National Natural Landmark in December 1976; the lore of some of the Klamath Tribes in the area held that Mount Shasta is inhabited by the Spirit of the Above-World, who descended from heaven to the mountain's summit at the request of a Klamath chief. Skell fought with Spirit of the Below-World, who resided at Mount Mazama by throwing hot rocks and lava representing the volcanic eruptions at both mountains. Italian settlers arrived in the early 1900s to work in the mills as stonemasons and established a strong Catholic presence in the area. Many other faiths have been attracted to Mount Shasta over the years—more than any other Cascade volcano. Mount Shasta City and Dunsmuir, small towns near Shas