Tectono-Volcanic Structures
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.
Coronas
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
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
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
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.)
Atla Flow Field
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.)
Meandering Channel
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.)
Relict Channels
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.)
Braided Channel Segment
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.)
Sinuous Rilles
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.)
Small Caldera
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 Caldera
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.)
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