Venus has more volcanoes than any other planet in the solar system. Over 1600 major volcanoes or volcanic features are known (see map), and there are many, many more smaller volcanoes. (No one has yet counted them all, but the total number may be over 100,000 or even over 1,000,000). These volcanoes come in a variety of forms. Most are either Large Shields or Smaller Shield volcanoes, but there are also many Complex Features, several Unusual Constructs, and a few Large Flow Features. None is known to be active at present, but our data is very limited. Thus, while most of these volcanoes are probably long dead, a few may still be active.
Venus is like the Earth in many ways. It is nearly the same size and it has a similar bulk composition. Of all the planets, its orbit around the sun is the closest to EarthUs orbit. It has both clouds and a thick atmosphere. Like the Earth, it even has a fairly young surface age (~500 million years). Despite these basic similarities, however, Venus differs greatly from the Earth in detail.
First, since the atmosphere is mostly CO2, Venus has an extreme Greenhouse Effect. In fact, the surface temperature on Venus is about 470!C (about 880!F). Further, the surface air pressure on Venus is about 90 times greater than that at sealevel on Earth. This is roughly equivalent to the WATER pressure on Earth one kilometer beneath the oceanUs surface. These surface conditions have two effects. (1) There is no water on the surface of Venus. Indeed, there is almost no water in the air, either. The clouds are mostly made of sulfuric acid and they are much, much higher than most clouds on the Earth. (2) Due to the high atmospheric pressure, the winds on Venus are also relatively slow. Thus, neither wind nor rain can really affect the surface on Venus. As a result, volcanic features will look freshly formed for a long time.
Second, Venus shows no evidence for plate tectonics. There are no long, linear volcano chains. There are no clear subduction zones. Although rifts are common, none look like the mid-ocean ridges on Earth. Also, continent-like regions are rare, and show none of the jigsaw fits seen on Earth. Thus, where volcanism on Earth mostly marks plate boundaries and plate movements, volcanism on Venus is much more regional and much less organized.
Third, volcanism on Venus shows fewer eruptive styles than on the Earth. Almost all volcanism on Venus seems to involve fluid lava flows. There is no sign of explosive, ash-forming eruptions on Venus, and little evidence for the eruption of sludgy, viscous lavas. This may reflect a combination of several effects. First, due to the high air pressure, venusian lavas need much higher gas contents than Earth lavas to erupt explosively. Second, the main gas driving lava explosions on Earth is water, which is in very short supply on Venus. Lastly, many viscous lavas and explosive eruptions on Earth occur near plate subduction zones. Thus, the lack of subduction zones should also reduce the likelihood of such eruptions on Venus.
Venus has over 150 large shield volcanoes. These shields are mostly between 100 and 600 km across, with heights between about 0.3 and 5.0 km. The largest shields, however, are over 700 km in diameter and up to 5.5 km in height. For reference, Mauna Loa is ~120 km across at its base, and it has a total height of ~8 km (from the sea floor). Thus, the large Venusian shields are broader, but much flatter then the largest shield volcanoes on Earth. Indeed, the largest shield volcanoes on Venus cover nearly the same area as Olympus Mons, which has a basal diameter of ~800 km. (Note, Olympus Mons is still much bigger than the Venusian shields due to its immense height.)
These large shields all look much like shield volcanoes on Earth. They are mostly covered by long, radial lava flows. They all have very gentle slopes. And most also have some form of central vent or summit caldera. Thus, we think that these shields forme d from basalts, much like the shield volcanoes in Hawaii. The venusian shields, however, show a map pattern which is quite different from that seen on Earth (see reference map). Namely, the shields on Venus are widely scattered, and they show no linear vo lcano chains like those on Earth. This suggests that Venus does not have active plate tectonics, and also that most volcanism on Venus is related to mantle hotspots.
This map shows the distribution of large shield volcanoes on Venus. Each red triangle marks the site of a shield volcano over 100 km in size. The background shows local surface heights across the planet. Blues denote smooth lowland plains. Greens mark reg ions 1-2 km higher than average for the planet. Reds and yellows show highlands over 3-4 km higher than the rest of the planet.
Very few large volcanoes are in the lowland plains or in the few highland regions. Rather, most of the large shields lie in the green to light yellow regions. Also note how the volcanoes are clustered into left and right sides of the map (dotted lines). T hese regions contain 2 to 4 times more volcanoes (for their size) than the planet as a whole. The reason for this grouping is not fully known, but it may reflect a cluster of closely spaced hotspots in a region of marked crustal failure. This region conta ins a large network of crisscrossing rifts and major fault zones. (Volcano distribution from data of Head et al. (1992); base map shows cycle 1, 2, 3 Magellan topography data.
This is a perspective view of the volcano Sif Mons. This volcano is about 2 km high and nearly 300 km across. It lies near the equator and is in a region called Western Eistla Regio. Note: Heights in this image are exaggerated to make the volcano stand ou t, but Sif still shows a very flat profile. This is true for most volcanoes on Venus. Also note the bright lava flows coming to the front of the image. These flows are probably very rough and blocky like aa lavas on Earth. They also are roughly 120 km lon g. To flow so far on such gentle slope suggests that these lavas were also very fluid. (Magellan Press Release Image P-37342.)
This is a perspective image of Western Eistla Regio showing two large shield volcanoes. Gula Mons is on the left, and is the largest of these shields. It is ~3.2 km high, and about 400 km across on its biggest side. Sif Mons lies in the distance on the right, and is only slightly smaller. Western Eistla Regio itself is broad dome nearly 1.5 km high and over 2000 kmwide. Similar domes are found near hotspots on Earth, and are believed to be lifted by a plume of hot rising mantle rocks. Thus, Sif and Gula seem to be hotspot volcanoes like those in Hawaii.
Where Hawaii is part of a long chain of volcanoes, however, Sif and Gula are far away from each other (over 700 km). They are also far from the nearest other shields. Gula formed at the crossing of two major rift zones, and Sif Mons may also lie on a simi lar rift. Thus, since many other large shield volcanoes also lie near large rift zones, hotspot magmas may need large crustal faults to build big volcanoes on Venus.
The arrow points to a large corona (link to Other Structures) north of Gula Mons. This flat-topped feature is 225 km across and about 600 m high. A few lava flows are seen coming from its nearer edge. (Magellan Press Release Image P-38724, JPL image MGN-7 2.)
This colorized image shows the volcano Sapas Mons. Sapas lies near the equator in Atla Regio, and is nearly 180km away from Sif and Gula Mons. Sapas is 1.5 km high and is about 120 km across. It is located on the edge of broad regional dome 1000 km across and up to 3 km high. Unlike many large shields on Venus, Sapas does not lie on a major rift structure, but it does still bury a set of NW-SE trending faults. In the upper right, lava flows also clearly embay two impact craters on the edge of the shield (a rows). Sapas shows many narrow lava flows with a range of brightnesses. Radar is brightest over rough, blocky surfaces; thus, this range of color suggests a change in volcanism over time. Namely, the youngest lavas are fairly bright, but the older lavas seem to be much smoother, especially in the band of dark lavas along the lower right edge of the volcano. (Magellan Press Release Image P-38360, JPL image MGN-51.)
Ushas Mons lies in the southern hemisphere on the northern part of Dione Regio. It is on the edge of several lowland plains regions, but it also is on a broad, low rise attributed to a mantle hotspot. Ushas is over 500 km across, and is about 2 km high. L ike Sapas, it shows a number of bright, rough lava flows, but it has smooth dark summit region. It is surrounded by volcanic plains with many small shields (especially in the upper left and lower right). Also note the clear north-south faults running unde r the volcano. These seem to have formed with the volcano, and may contain volcanic dikes. (Magellan Press Release Image P-42386, JPL image MGN-117.)
Theia Mons is the largest volcano on Venus. It is located in Beta Regio, and is shown here in the bottom center. Theia is over 4 km high, and its lava flows cover an area more than 800 km wide. It has an oval central caldera roughly 75 km long and 50 km w ide. The volcano also lies at the junction of 3 major rifts, one of which is seen here running off the top of the image. This rift, Devana Chasma, is over 200 km wide, but it narrows down to ~50 km as it climbs Theia Mons. It is up to 3 km deep on the fla nks of the volcano, and apparently formed throughout the growth of Theia Mons. Also note the crater in the top center (arrow). This crater has been cut by the edge of the rift, and so suggests a fairly young age of faulting.
Small shield volcanoes are very common on Venus. In fact, there may be over 100,000 of such shields less than 20 km across. These small shields usually occur in clusters, called shield fields. Over 550 shiled fields have been mapped (see Reference Map), and most of these are between 100 and 200 km across (nearly the same size as the state of Rhode Island). The shield fields are widely scattered on Venus, but they mostly occur in the lowland plains and lower upland regions. Very few are found at higher elevations or in the chaotic tessera.Many have also been partly buried by later lava plains. Thus, the shield fields seem to be fairly old, and they may have formed during the earliest stages of plains volcanism.
At slightly larger sizes, true shield volcanoes are rare. Of the 270 volcanoes between 20 and 100 km in size, only about 70 seem to be real shields. The rest are unusual features that may reflect different lava types. Most of the shields are also fairly small, less than 30-40 km across, and 25 of them show an unusual, petal-like set of bright lava flows (see images). Thus, there seem to be two distinct scales of volcanism on Venus: large and very small. These two scales may reflect differences in the nature or volume of mantle melting beneath the volcanoes.
This map shows the placement of large shield volcanoes, smaller (20-100 km) shields and small shield fields on Venus. It also maps the locations of volcanoes which have "Anemone" or flower-like lava flow patterns. Note the relative number of large shields and of shield fields, and the relative rarity of the smaller shields.
This map also shows two clear patterns of volcanism. (1) Very few of the smaller volcanoes lie at high elevations (light green to yellow). Rather, they mostly lie at lower elevations which are colored blue and dark green. (2) Many of the shield fields also seem to cluster around larger shield volcanoes. This may reflect a concentration of volcanic activity in these regions. However, the lack of shield fields in at least some regions may simply mark their burial by thick lava plains. (Data from Head et al, (1992) J. Geophys. Res. vol. 98, p. 13,153; base map of Magellan Topogaphy from NASA JPL .)
Single small shield volcano on Venus. This volcano is located in a region of fairly smooth lava plains. Note the clear summit pit/caldera, and the steep-sided cone. Differences in slope angle and the off-centered summit pit are mostly due to foreshortening in the radar data. The bright tail to the right of the shield is probably not volcanic. Rather, this tail marks a rare windstreak left by surface winds blowing around and over the volcanic shield (Magellan Press Release Image P-38810).
An example of an "Anemone" shield. It lies in southern Atla Regio, and is nearly 40 km across. The name comes from the narrow, radar bright lava flows ringing the volcano. In this example, these flows are buried by a smoother, central unit which contains a central caldera. Outside the volcano, several other bright lava flows are also seen. These flows are part of a broader volcanic complex, but they help to show off and enhance the features of this volcano. (Part of Magellan F-MIDR 10S200, centered on ~9.5S, 201E)
This image shows a typical shield field on Venus. It is about 120 km across, and it contains 51 clear shields of varying heights. There are also ~31 pits that may mark additional vents within the field. Note that the shields show no relation to the older faults in this region, while the pits seem to line up with a large graben (arrows) crossing the field. This suggests that the faults may have partially influenced the type and location of eruptions within the field. For reference, this shield field is located in Ulfrun Regio, about 3000 km east of Sapas Mons. (Part of Magellan C1-MIDR 30N225.)
This image shows a shield field at the center of a larger set of lava flows. These shields range from ~1 to ~8 km in size, and most occur in a cluster ~80 km across. Some shields occur up to 60 km outside of this cluster, however, while the lava flows extend up to 200 km from the central shield field. Since each small shield is much smaller than any of the lava flows, this shield field must indicate a change in volcanism over time. Most likely, the lava flows formed quickly during an early phase with lots of lava, and the shields then formed later when the magma supply was almost gone. (Part of Magellan C1-MIDR 15N317, centered at 13.5N, 314.5E.)
This image shows a shield field inside a corona structure. This field is roughly 125 km across, and it is surrounded by a set of younger lava plains. Note the dark unit which buries both fractures and shields in the lower left. In this case, the shield field may have formed as the first magmas slowly came up inside the corona, while the plains would have formed later during a more vigorous phase of volcanism. (Part of Magellan C1-MIDR 30N135, centered at 27.5 N, 136.5E.)
REFERENCE MAP (Channels, Flows, Calderas)
Large liquid lava flows are thought to form three types of feature on Venus. The first are large, flood-like lava flow fields or fluctus. The second are long lava channels. And the third are large calderas that have no clear shields or cones. None of these features are common on Venus, but they are still plentiful and several large examples of each type are known.
The map above shows the locations of some 53 lava flood fields. These flow fields are mostly 100 to 700 km long, but the longest are over 1000 km in length. Their widths are variable, but can reach over 300 km. In size, they are thus much like lava flood basalts on the Earth, and like the youngest mare flows on the Moon. They mostly lie near the edges of the lowland plains, and flow down into these plains. A few are located near younger large shields, but many lie at some distance from the largest shield volcanoes. It is thought that these flows may have formed near the end of plains formation.
In all nearly 200 channels have been found on Venus. Many of these channels lie in the lava flood fields or on other volcanoes. Still, nearly 50 lava channels have been found which do not lie on a larger lava flow. These channels are usually 0.5 to 1.5 km wide, but they range widely in length. Most are less than 400 km long, but some are over 700 km long. One is longer than every other known lava channel in the solar system. With a length of about 6800 km (~4200 miles), this one channel is even longer than the Nile River on Earth.
Like the Lava Flood Fields, most lava channels start near the edges of the lowland plains. Many are also found near coronas or in large groups. In general, the lava channels on Venus look much like sinuous rilles on the Moon. The longest channels, however, are not quite the same. First, they tend to be straighter and wider than the lunar rilles. Second, some show older branches and cut-off loops like those in large rivers on Earth. This suggests that some of these channels may have formed over a long time from one or more eruptions.
The last features mapped here mark sites from which large volumes of lava are believed to have erupted. These calderas are large, round to oval pits that have steep walls but very low rims. They are often surrounded by a ring of faults or graben. Calderas are thought to form when the ground falls in over an emptied, shallow magma chamber. Although part of many shield volcanoes, 86 calderas have also been found on Venus which are not. Most of these calderas are between 40 and 80 km in size, but the largest are nearly 200 km across. Most are near the shields of Atla and Beta Regio, but a few also lie in the high plains of Lakshmi Planum. Almost no calderas are found in the lowland plains. This lack suggests that shallow magma chambers did not form in the lowlands. If this is true, then lavas must have risen straight to the surface there. Thus, the lavas would have little chance to pool together or to build up in a single area over time. This could explain both plains formation and the small sizes of the lowland shields. In the highlands, however, we find both calderas and large shields. Thus, shallow magma chambers could form higher up.
This change in volcanism with height is thought to be an effect of air pressure. In the lowlands, air pressures are very high, nearly 100 times those on Earth. Thus, lowland lavas are much like seafloor lavas on Earth. They are very dense and contain almost no gas bubbles. In the highlands, air pressures are lower, and this lets some gas bubbles form on eruption. This makes highland rocks less dense and can allow lavas to pool below the surface. If the lavas pool in one place over many eruptions, then a large shield can be built up over time. However, no such build up can occur if the pool does not fill up again after the eruptions. Instead, a caldera should form like those mapped here.
REFERENCE MAP (Tectonic/Volcanic Features)
Volcanism is also part of some very complex features on Venus. These features mix both lavas and faulting; thus, they are called TECTONO-VOLCANIC structures. They differ from volcanoes in two ways. First, volcanoes often form on older rifts or faults, but they do not cause this faulting. Second, most volcanoes are just large piles of lava. In contrast, the tectono-volcanic structures are thought to form by faulting over rising magmas. Also, lavas make up only a small part of these structures. Due to differences in faulting, three types are found on Venus.
Some 200 to 300 coronas are known, of which 175 are mapped above. These are large, round to oval shaped features with a distinct ring of faults or ridges. They often have a flat, raised or down-dropped center and an outer moat-like trough. Lava plains and small shields are found in both the centers and the moats, and pancake domes are very common as well. Coronas range in size from about 100 km to nearly 1000 km, but most are 200 to 250 km across.
Coronas are thought to form over small mantle plumes. First, rising magmas and heat lift the surface. These plumes also feed local eruptions, but they are too small for a long string of eruptions. Thus, the uplifted surface is not fully buried, and a complex mix of faults and lavas is formed. With later cooling, the uplift then sinks to yield the down-dropped centers seen in the oldest coronas.
Arachnoids are smaller cousins of the coronas. Like coronas, they have a round ring of faults or ridges, but these rings lie inside a set of radial ridges. The rings range from about 50 km to 200 km in size, with the outer ridges running out another 200 to 400 km. Over 250 arachnoids have been mapped, and they tend to cluster near both coronas and other arachnoids. Also, like the coronas, arachnoids are rarely found in the lowest plains. Instead, most lie just above the lowland plains (i.e., the green map areas).
Arachnoids look like coronas and form near coronas. Thus, they are thought to form in much the same way as coronas. They are smaller than most coronas, however, and they tend to show fewer lavas. Thus, they probably formed over smaller plumes. Since smaller plumes should have less magma and should cause less uplift, this model seems to fit the facts. However, the lack of lava flows also suggests that there are more intrusions in arachnoids than in coronas. Indeed, it has been suggested that the radial ridges may be large dikes. In this case, these dikes could drain magmas away from the plume and limit the eruption of lavas at the surface.
Novas show fewer signs of real volcanism than the coronas or the arachnoids. Instead, they show a starburst-like pattern of faults and a broad, dome- like uplift. Some of these faults seem to feed lava flows, but such flows are not common. About 50 Novas have been mapped, with sizes ranging from about 50 km to 300 km. Most are between 150 and 200 km across, and thus are the same size as many of the arachnoids. Although rare, novas tend to occur near large volcanoes or near groups of coronas and arachnoids. They are seldom found alone or in the lowland plains. 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. Given their size and shape, they may mark an early stage of uplift over small mantle plumes. If this is true, then these novas may turn into arachnoids or coronas in a few million years.
Mylitta Fluctus is one of the largest lava flow fields on Venus. It is about 1000 km long (600 miles) by 460 km wide. Thus, it covers an area slightly larger than the state of Arizona (300,000 square km). It lies on the southern edge of Lavinia Planitia, and drops some 2000 meters from south to north. Note the large crater which is partly flooded in the southeast (arrows).
This flow field contains many lava flows. These vary in length from 400 to 1000 km, and form ~30 km to 100 km in width. Many of these flows contain central lava channels like those seen on Hawaii. The flows seem to have formed in 6 separate eruptions, and most come from a single center in the southeast (marked source). This source is a large shield volcano that was formed by the first eruption event. The later eruptions then produced the longer flows of the main flow field. On the basis of Earth lavas, it is thought that the shield formed in about 10 to 70 years. Each of the later lava flow sets could have formed in less than 2 to 80 days.
Note: While Flood Basalts on the Earth are as large as Mylitta Fluctus, most Flood Basalts do not come from a single source. Rather, they come out of long fissures which are buried by the erupted lavas. Thus, the flow of lavas away from the shield here suggests that it may be harder for lavas to reach the surface on Venus. Since the shield lies on a major rift zone, faulting may have helped these lavas reach the surface.
(Image from Magellan C2 MIDR 60S333;1, with parts from C2 60S333;202.)
This image shows another lava flood field. This one lies on the edge of Atla Regio, and is also about 1000 km long. At its widest, it is nearly 300 km across, but it also narrows down less than 50 km in some places. In area, it is roughly the same size as state of Oklahoma (~180,000 square km).
Like Mylitta Fluctus, this flow field formed in several stages. Here, however, there is no shield volcano at the source. Rather, the lavas erupted four times from a small group of faults and graben. Like Mylitta, these faults are part of a larger rift system. After eruption, the lavas then flowed west along the edge of Atla Regio. Note how they arc around to follow the lowest ground, and then flow into a smaller rift.
(Image from Magellan C1 MIDRs 00N197 and 00N215.)
This is one part of a long lava channel in Helen Planitia. In all, the channel has a total length of almost 1200 km, but the segment shown here is only 200 km long. It is also about 2 km wide. Note how the channel snakes along in a band that is slightly brighter than the surrounding plains. This band probably formed from thin lavas that flowed over the channel's banks. In addition, a much older lava channel can also be seen. The marked pair of bright lines (arrows) seems to mark a channel that has almost totally faded away into the nearby plains.
Although well preserved, the main channel also seems to be old. First, its ends fade away into the plains lavas. This suggests that the channel has been buried in places by younger lavas. Second, the channel is cut by a swarm of ridges and faults (see upper center). Probably, the channel formed soon after the local plains, and both then saw nearly 300 million years of slow deformation.
(Press Release Image P39226, MGN-82, centered near 49S, 273E.)
This is part of another lava channel. It lies just south of Ishtar Terra, and also is about 2 km wide. It clearly shows a set of cut-off channels and islands that look much like those seen on some Earth rivers. Thus, it seems that the lavas changed their path over time much like EarthUs rivers. Clear signs of erosion are also seen inside the channel in the upper right. Thus, it looks as if the lavas cut down into older flows. These changes in the flow path are likely the result of later lava flows using an older channel. Still, they might also have formed during one very long eruption.
(Image is part of Magellan F MIDR 45N019.)
Venusian channels also form more complex systems. Here, we see one part of such a system. In the upper left are several channels that formed when lavas spilled out of a fault-bounded trough. These channels merge in the center of the image, and then run into a ridge of highlands. The lavas pooled behind this ridge, before spilling over it too to flow further east. In the process, they carved deep outlets through the highlands (see arrows) and left a number of stream-lined islands.
These islands, at both the left and right, look much like features carved by large floods on the Earth and on Mars. Thus, it is thought that the lavas in this channel behaved much like flood waters on the Earth. Given the slow speed of most Earth lavas, this in turn suggests that the lavas were not basalts. Rather, they may have been very hot mantle melts (komatiites), or possibly liquid sulfur. For reference, this image is about 250 km wide. It shows part of a 1200 km long channel which flows around the Ammavaru volcanic complex. It lies in the south near Lada Terra.
(Image is part of Magellan F MIDR 50S021, and is centered near 51S, 22E.)
This image shows shorter sinuous rilles more like those seen on the Moon. Here, the smallest rilles begin at small or middle-sized round pits. The larger rilles begin at bigger, more complex collapse zones. Note how the rilles narrow away from these sources. This suggests that the lavas slowed and cooled as they moved away from the source vent. Thus, these eruptions were likely smaller and more short lived than the eruptions that formed longer lava channels. Also note how the source pits line up with the older faults in this image. Once again, these faults probably helped control where the lavas could reach the surface.
(Image part of Magellan C1 MIDR 15S095, centered near 11S, 89.5E.)
This caldera shows what most calderas on Venus look like. The central hole is about 36 km across, and it is surrounded by a large set of arcuate faults. These form a bulls-eye pattern over 100 km across. Note that these faults also cut a ring of lava flows that formed before the caldera fell in. Inside the caldera, the floor is made of dark, smooth lavas that erupted after collapse. Even after these lavas, however, the floor still rose and fell a few more times. This is shown by the ring of faults inside the floor and by the faults at the caldera's center.
Impact craters also are round holes, but most craters and calderas do not look alike. First, impact craters almost never have a bulls-eye of faults outside their rims. Second, where most impact craters have sharp, raised rims, most calderas have low, rounded rims like that seen here. Third, most impact craters of this size have a clear peak rising out of the crater floor. Calderas mostly have smooth floors. Lastly, calderas often have clear lava flows on either their rims or floors. Impact craters rarely show such signs of volcanism.
(Image part of Magellan F-MIDR 05N228.)
Sacajawea is one of the largest calderas on Venus. It is roughly 150 km long by 100 km wide, and it seems to be over 1000 m deep. It lies on the high plains of Lakshmi Planum. Again, we see a large ring of fractures outside the caldera, and a smooth lava floor inside the caldera. Some of the floor lavas even bury the ring fractures in the left center. Also note how the fractures are blurred north of the caldera. This may be a region where thin lava flows have buried the older ring faults as well. A number of small domes and shields are also found in this area, and on the southeast rim. All these features suggest that Sacajawea formed from many eruption events. Indeed, Sacajawea sits on a broad (~600 km wide), low rise that may be a very flat shield-like volcano.
(Image part of Magellan C1 MIDR 60N319, centered at ~66N, 336E.)
This image shows a small, fairly simple corona. This corona has two parts. There is a central plains region roughly 150 km across, and an outer fracture ring about 45 km wide. The central plains stand slightly higher than the outside plains, while the fracture ring seems to lie in a slight trough. There are also many signs of volcanism in the area. For instance, three pancake domes lie close to the fault ring (1,2,3). There is also a string of small shields along the bottom of the image (5). And the fault ring itself is buried by plains lavas in several places (arrows). All the pancake domes are cut by some ring faults, as are the younger plains. Thus, the corona was still forming when these lavas were erupted. This mix of major faulting and volcanism is why coronas are called tectono-volcanic features.
Note: there is a fourth round feature in this corona that is not a pancake dome. Rather, its edge is a set of concentric faults that seem to have formed around a central uplift. Possibly, this marks deformation over a magma intrusion. Or maybe it is somehow related to the faulting in the outer ring.
Idem-Kuva is about 230 km across, and is a more complex corona. It lies just north of Gula Mons, and it shows a multi-ringed plan. At the center is a large plateau, about 100 km wide and 600 m high. This looks somewhat like a pancake dome, but it is much bigger. It probably formed by heating and uplift rather than by eruption. On the edge of the central high are two arcuate troughs, and these are bounded in turn by a partial ring of faults. These faults may have once ringed the entire feature, but they seem to be buried on the north by younger plains lavas. Two long, bright lava flows also run north out of the troughs and over the fault ring. These flows are about 150 and 350 km long, and they mark the clearest signs of volcanism in Idem-Kuva.
This image also shows two other features. First, there is a major line of faults running northwards into Idem-Kuva (arrows). These faults are part of a rift zone on Gula Mons, and they may have fed the two lava flows seen in Idem-Kuva. Second, the pale circle in the upper left marks an older volcanic center (Nissaba). This volcano is fairly large but very flat. It is 300 km long by 200 km wide, but it is only about 400 m high. Note how the leftmost flow out of Idem-Kuva curves around its base. This suggests that Nissaba formed before the flow. It may even have formed before Idem-Kuva.
(Image is part of Magellan C1 MIDR 30N351.)
This arachnoid lies in the plains northwest of Atla Regio. Like a Corona, it has a clear fault ring and a central (mostly) smooth region. Unlike a Corona, it also has a set of radial ridges which arc away from the fault ring. It is also much smaller than most coronas. In this case, the fault ring is about 95 km across, and most arachnoids are about the same size. In contrast, most coronas are over 200 km across. Still, outside the fault ring, the ridges seem to run 100 to 200 km before merging with other ridge systems.
Most arachnoids show few signs of volcanism. Here, there are a few small shields in the center, but the plains lavas show the clearest signs of volcanism. Note the bright lava flows on the northwest and southeast that seem to come out of the fault ring. Also note how the dark plains arc above the arachnoid, and how they ring the crater in the bottom center (arrows). These all suggest that many large, thin lava flows were erupted as the arachnoid was forming.
(Image is part of Magellan C1 MIDR 45N223, centered near 41N, 214E.)
Arachnoids often lie in faulted plains, and their outer ridges tend to turn into regional patterns. Thus, they often look like spiders on a bright web. Indeed, the name Arachnoid means Rspider-likeS. Here, we see two arachnoids in a faulted plains region (A and B). These features are each about 120 km across, with an outer ring of ridges maybe 150 km wide. Again, there is little sign of volcanism. Only a few bright lava flows are seen in the upper left. A third feature (C) may also be an arachnoid. However, it has only a few radial ridges, and it also shows a caldera-like pattern of nested cliffs. Thus, it is harder to classify. The edge of a larger corona (arrows) also can be seen at the top of the image.
(Image part of Magellan C1 MIDR 45N011, centered near 40N, 19E.)
Unlike Coronas and Arachnoids, Novas do not have major ring features. Rather, they show a striking starburst of faults and graben. Also, there is often a clear rise at the center of these faults. This example is some 250 km across, and it lies in Themis Regio. There is no clear sign of volcanism, but a few small lava flows may run out from faults in the upper left and upper right. Still, there is a lot of volcanism nearby. Namely, this nova lies within a very long chain of coronas. Indeed, it lies just above the corona chain image in Parga Chasma. Novas mostly occur next to large shields or major corona chains. Thus, they seem to be volcanic. They probably mark an early stage of uplift and faulting that could later become a corona or a large shield volcano.
(Image is part of Magellan C1 MIDR 30S279, centered near 27S, 272E.)
Most volcanoes on Venus are shields, but a few are not. These volcanoes all formed from very thick, viscous lavas, and they fall into three types. First are the so-called "pancake" domes. Second are a related class of "tick-like" structures. And third are a few volcanoes with thick, fan-shaped or banded flows. Since most basalt lavas are very fluid and fairly thin, these volcanoes do not seem to be made from basalt. Rather, they may mark quartz-rich or granitic lavas. Still, there are theories that gas-rich, "frothy" basalts could produce such viscous lavas. Also, few of these features lie in or near the Venusian highlands. Thus, these three volcano types may differ greatly from the volcanoes seen on Earth's continents.
Most of the non-shield volcanoes on Venus are pancake domes, shown here in yellow. Like the shield volcanoes, these domes are widely scattered. They also often form small groups or clusters. Where most venusian shield fields have well over 20 small shields, however, few dome clusters have more than 5 or 6 domes. Further, the pancake domes are rarely found near shield volcanoes on Venus. Instead, many domes lie near corona structures in the lowland plains.
The ticks, shown here in red, show a pattern similar to the pancake domes. Like the pancake domes, most ticks occur in the lowland plains far from the largest shields on Venus. Many are found near the lowland coronas, and some are quite close to one or more pancake domes. The ticks also look something like the pancake domes. Thus, it is thought that many ticks are modified dome features.
Finally, fan-shaped and banded flows are quite rare. They are shown on this map in purple, and most are located in one part of the southern lowland plains. Neither their setting nor their distribution shows any clear trends, so little is known about these flows. Still, they look a lot like the most viscous lava flows on Earth. Thus, they may not have formed from basalt lavas. Instead, they may mark a rare set of granitic or rhyolitic lavas on Venus.
This image shows three of the venusian pancake domes. They lie near the equator on the edge of Eistla Regio. The two larger domes are roughly 65 km (40 miles) wide, and the smaller dome is about 22 km (13 miles) wide. All three are very low, rising less than 1 km above the surrounding plains. The tops are fairly flat to slightly bowl-shaped, and they show clear patterns of small cracks and faults. Most of these cracks probably formed with the dome, but some may have formed later with the faults in the surrounding plains. Note how some features extend from plains onto the edges of the central and right hand domes (arrows).
Each dome also shows a small central pit. These pits look like the central vents found on shield volcanoes, but they seem to have formed after their respective domes. Thus, there is no sign that the central pits fed lava flows to the dome. Rather, the domes seem to have formed from one large, slow eruption of viscous lava. After the eruption, the central pits then formed on the dome as gasses escaped from the magma or as the dome cooled and shrank. (Magellan Press Release Image P-38388, JPL image MGN 53.)
This image shows a chain of pancake domes east of Alpha Regio. Each dome is roughly 25 km (15 miles) wide, and the highest are about 750 m tall. As in Eistla Regio, most of these domes show a (very) small central pit. Here, however, we see a variety of crack patterns on the domes. These probably mark small changes in the speed or conditions of eruption. Many of the domes here also overlap. Such overlaps are quite common, and most seem to mark a series of dome eruptions. In some cases, however, they may mark two domes that formed at the same time. (Magellan Press Release Image P-37125).
This feature is called the "Tick" for obvious reasons. It lies near the edge of Eistla Regio and is roughly 66 km (41 miles) across. The central round "body," however, is only about 35 km (22 miles) wide. Like the pancake domes, the ticks are broad, mostly flat features, and they often have a central pit or vent structure. The difference is that the ticks are surrounded by an array of short, radial ridges. In this case, the "head" of the tick is defined by a set of small collapse pits.
The origin of the "leg" ridges is unknown, but two options have been suggested. First, the ridges may outline avalanche scars. In this case, the tick is simply an old pancake dome with a heavily eroded rim. The second option is that the ridges mark dikes running out from the central "body." Still, it is not clear why these dikes formed ridges and not lava flows. Thus, some rim erosion may also be needed to expose the dikes in this theory. (Part of Magellan FMIDR 20S003, centered near 18.5S, 6E.)
This image shows another "tick" south of Aphrodite Terra. It is about 12 km wide, and it lies in a region of deformed plains lavas. Note the fractures which arc around this tick to form an eyelike feature. Since this feature is about 44 km long by 33 km wide, it suggests that tick formation may affect a fairly broad region. Also note the crinkled region above the eye in this image. This seems to be a lava flow that came out of the tick before the latest stage of local faulting. (Part of Magellan C1MIDR 30S189, centered near 29.5S, 184E.)
This strange volcano lies in Aino Planitia. Its central dome is roughly 100 km across, but it is only about 1 km high. Around the dome are several thick, fan-shaped lava flows and a much larger flow with a banded surface. These flows strongly suggest the eruption of very viscous lavas. First, the fan-shaped flow lobes are very thick. The measured flow heights vary between 120 and 540 m (~400 to 1700 feet). Second, the shape of these flows suggests that the lavas had a hard time flowing away from the volcano. Third, while the larger flow is much thinner, it looks a lot like the banded flows produced by viscous lavas on Earth.
Viscous lavas on Earth are rarely basalts. Hence, this volcano may mark a rare case of non-basaltic lavas on Venus. Such lavas need a lot of water to form, however, while water is scarce on Venus. Further, basalts can also form viscous lava flows in some cases. Thus, it is not yet clear what the nature of these lavas was. (Magellan Press Release Image P-39916, JPL image MGN-93).