![]() ![]() Note: While Flood Basalts on the Earth are as large as Mylitta Fluctus, most Flood Basalts do not come from a single source. Each of the later lava flow sets could have formed in less than 2 to 80 days. On the basis of Earth lavas, it is thought that the shield formed in about 10 to 70 years. The later eruptions then produced the longer flows of the main flow field. This source is a large shield volcano that was formed by the first eruption event. The flows seem to have formed in 6 separate eruptions, and most come from a single center in the southeast (marked source). Many of these flows contain central lava channels like those seen on Hawaii. These vary in length from 400 to 1000 km, and form ~30 km to 100 km in width. This flow field contains many lava flows. Note the large crater which is partly flooded in the southeast (arrows). It lies on the southern edge of Lavinia Planitia, and drops some 2000 meters from south to north. Thus, it covers an area slightly larger than the state of Arizona (300,000 square km). It is about 1000 km long (600 miles) by 460 km wide. Mylitta Fluctus is one of the largest lava flow fields on Venus. If this is true, then these novas may turn into arachnoids or coronas in a few million years. Given their size and shape, they may mark an early stage of uplift over small mantle plumes. Since the higher plains on Venus are thought to lie over mantle plumes, this suggests that novas are linked to mantle melting in some way. They are seldom found alone or in the lowland plains. Although rare, novas tend to occur near large volcanoes or near groups of coronas and arachnoids. Most are between 150 and 200 km across, and thus are the same size as many of the arachnoids. About 50 Novas have been mapped, with sizes ranging from about 50 km to 300 km. Some of these faults seem to feed lava flows, but such flows are not common. Instead, they show a starburst-like pattern of faults and a broad, dome- like uplift. Novas show fewer signs of real volcanism than the coronas or the arachnoids. In this case, these dikes could drain magmas away from the plume and limit the eruption of lavas at the surface. Indeed, it has been suggested that the radial ridges may be large dikes. However, the lack of lava flows also suggests that there are more intrusions in arachnoids than in coronas. Since smaller plumes should have less magma and should cause less uplift, this model seems to fit the facts. Thus, they probably formed over smaller plumes. They are smaller than most coronas, however, and they tend to show fewer lavas. Thus, they are thought to form in much the same way as coronas. Instead, most lie just above the lowland plains (i.e., the green map areas).Īrachnoids look like coronas and form near coronas. Also, like the coronas, arachnoids are rarely found in the lowest plains. Over 250 arachnoids have been mapped, and they tend to cluster near both coronas and other arachnoids. The rings range from about 50 km to 200 km in size, with the outer ridges running out another 200 to 400 km. Like coronas, they have a round ring of faults or ridges, but these rings lie inside a set of radial ridges. With later cooling, the uplift then sinks to yield the down-dropped centers seen in the oldest coronas.Īrachnoids are smaller cousins of the coronas. Thus, the uplifted surface is not fully buried, and a complex mix of faults and lavas is formed. These plumes also feed local eruptions, but they are too small for a long string of eruptions. First, rising magmas and heat lift the surface. Coronas range in size from about 100 km to nearly 1000 km, but most are 200 to 250 km across.Ĭoronas are thought to form over small mantle plumes. Lava plains and small shields are found in both the centers and the moats, and pancake domes are very common as well. They often have a flat, raised or down-dropped center and an outer moat-like trough. These are large, round to oval shaped features with a distinct ring of faults or ridges. ![]() Some 200 to 300 coronas are known, of which 175 are mapped above. Due to differences in faulting, three types are found on Venus. Also, lavas make up only a small part of these structures. In contrast, the tectono-volcanic structures are thought to form by faulting over rising magmas. Second, most volcanoes are just large piles of lava. First, volcanoes often form on older rifts or faults, but they do not cause this faulting. These features mix both lavas and faulting thus, they are called TECTONO-VOLCANIC structures. Volcanism is also part of some very complex features on Venus. REFERENCE MAP (Tectonic/Volcanic Features) ![]()
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