1. AN EXAMINATION OF FOUR VENUSIAN SHIELD FIELDS
BY NICHOLAS J. KELLY1, NICHOLAS P. LANG1, AND BRADLEY J. THOMPSON2
1Department of Geology, Mercyhurst University, 501 East 38th Street, Erie, PA 16546
2Center for Remote Sensing, Boston University, One Silber Way, Boston, MA 02215
Chernava
Colles
Jarate
Colles
Llorona
Planitia
Ran
Colles
([2] Guest)
Flow Direction
Fracture
Wrinkle Ridges
Radar Boundary
Impact Crater
Corona
Key
+ Shield
Basement
Flows Undivided
Alternate Flows Impact Melt
Shields
Wrinkle Ridges
Shields
Wrinkle Ridges
Fractures
20km
20km
Llorona Planitia
Llorona Planitia is a shield field located near a corona in the center of the map.
The shields occur on fractures, and predate the corona flows. Shields located
outside of the flow boundaries had an average diameter of 6.2km, while shields
located within the corona flow boundaries had an average diameter of 3.1km.
The smaller average size suggests shield embayment, allowing only the peaks to
be exposed. Unembayed Embayed
Wrinkle Ridges
Shields Shields
Fractures
Fractures
45km 45km
5.0 S
0.0 N
5.0 N
5.0 S
0.0 N
5.0 N
158.0 E 167.0 E 176.0 E
158.0 E 167.0 E 176.0 E
Ran Colles
29.9 W
29.9 W
25.0 W
25.0 W
15.0 S
10.0 S
5.0 S
15.0 S
10.0 S
5.0 S
20.5 W
20.5 W
148.0 E 156.0 E 165.0 E
48.0 N
55.0 N
62.0 N
62.0 N
55.0 N
48.0 N
148.0 E 156.0 E 165.0 E
2.0 N
17.0 N
32.0 N
2.0 N
17.0 N
32.0 N
142.0 E 152.0 E 162.0 E
142.0 E 152.0 E 162.0 E
Chernava Colles
Chernava Colles was the first mapped field. The least complicated of
the map areas is composed mostly of flows undivided with little
basement exposure. Shields are more heavily concentrated
towards the center of the map, trending in a northwest direction.
Fractures also follow a similar trend. The northeast corner of the map
contains a large flow entering from the west. It is interpreted that
this material is sourced from a nearby mons which would explain
the vast flood like coverage [1]. Almost all of the wrinkle ridges on
the map occur on this flow material.
Shields located in Flow B have an average size of 4.1km and are less plentiful
than the shields located in the Flows undivided region of the map which
have an averagesize of 4.8km. The shields in Flow B are believed to have
been originally part of the main field but where embayed by the intruding
flow from the west.
Flow BFlows Undivided
Shields
60km
Shields
Fractures
Wrinkle Ridges
60km
Jurate Colles
Coronae and Planitia Materials
Abstract
Small shields represent perhaps the most dominant manifestation of volcanism on Venus.
Defined as volcanic constructs <20 km in diameter and <<1 km in height, many small
shields (or shields) typically occur in fields that are either associated with a larger volcanic
edifice (e.g. corona) or as isolated clusters. Although much work has examined the physical
characteristics of shields, the origin and history of shield fields remains unclear. With the
goal of better understanding the geologic history recorded at Venusian shield fields, we
have examined shield clusters in four regions; specifically, using Magellan Synthetic Aper-
ture Radar (SAR) imagery (~75 m/pixel) in ArcGIS 10, we geologically mapped Chernava
Colles (24°W, 10°N), Ran Colles (162°E, 0°N), Llorona Planitia (146°E, 4°N ), and Jurate Colles
(156°E, 55°N). Chernava Colles is a ~164,000km2 shield field; individual edifices ~1-2km and
are predominantly cone- shaped. Llorona Planitia hosts two fields, one with an area of
~185,000km2 and another with an area of ~22,000km2. The average edifice diameter was
4km and the construct’s displayed a flat, pancake like shape also showing up as varying
backscatter. The shield field at Ran Colles measuring ~643,000km2 200km2 contained edi-
fices averaging ~6km2 in diameter with a high width:height ratio; in fact, shields are defined
only by circular changes in backscatter in SAR imagery. The final field, Jurate Colles mea-
sures ~617,000km2 with edifices averaging ~3km in diameter containing the most defined
volcanic constructs. As a whole, each examined field hosted edifices with a morphology dis-
tinct for that field; the reason for this is unclear, but may be associated with magma viscos-
ity and/or eruption rate and style at each field – factors that may be influenced by the re-
gional geology. The average shield field are is similar to that of a corona flow area, but due
to shields residing at effective SAR resolution [5], it is difficult to quantify individual edifices’
contributions to the field and impedes deciphering histories recorded within an individual
field and requires additional means of examining shield fields. Shield field analysis may help
the ongoing research into the evolution of volcanism on the surface of Venus.
Methodology
In order to analyze shield fields on the surface of Venus, we used Magellan synthetic aperture radar imagery
(~75 m/pxl) aquired through the USGS’s Map-a-Planet site. Images were geologicaly mapped using ArcGIS 10;
geologic mapping followed the protocols of Hansen (2000). Once each field area was mapped, Adobe Illustra-
tor was used to create strat columns and organize the data in a presentable fashion. Adobe Photoshop was
also used as an auxilary tool to help crop and stretch images.
Discussion
Shields display a multitude of morphologies throughout different areas of
Venus’surface. By mapping four separate fields, trends in these morpholo-
gies can be recorded and used for a much broader perpective of Venus’
volcanic surface. Because shields are visable at the effective resolution of
Magellan SAR, stratigraphy within a shield field is difficult to dechipher but
how a shield field fits stratigraphically into a region can often be deter-
mined. Cross cutting relationships using stuctures enabled us to establish
a rough stratigraphy for each location. Where a fracture occurs on one sur-
face and suddenly dissappears under another, we know that the overriding
surface (or flow) is the younger unit. Since the planet’s surface is covered in
these structures, a general stratigraphy can be established for the visable
units.
Structural Events
Fractures
?
?
?
?
Wrinkleridges
Shields
?
?
b- Basement. Bedrock exposed in small areas on surface
fu-Flows undivided. Lumped surface flows with undistingushable boundaries.
fA-Flow A. Distinguishable separate flow material covering the undivided
flows as well as previously emplaced fractures.
fB-Flow B. Originating from the western side of the map, a large flood like
flow believed to be Mon’s sourced.
b
fU fB
fA
Coronae and Planitia Materials Structural Events
Fractures
?
?
?
?
Wrinkleridges
Shields
?
?
b
fU
iM
?
?
ImpactCrater
b- Basement. Bedrock is exposed in various regions on the map. Ususally embayed by flows.
fu-Flows undivided. Lumped surface flows with undistingushable boundaries.
f. IM-Impact Melt. Due to high atmospheric pressure, impacts from objects such as asteriods
cause surface melting creating separate flows originating from the crater.
Coronae and Planitia Materials Structural Events
Fractures
?
?
?
?
Wrinkleridges
Shields
?
?
b
fU fC
iM
?
?
ImpactCrater
fA
b- Basement. Bedrock is exposed in various regions on the map. Usually embayed by flows.
fu-Flows undivided. Lumped surface flows with undistingushable boundaries.
fC-Flow (Corona). Flows originating from large volcanic vent in one or multiple events.
fA-Flow A. Distinguishable separate flow material featuring no fractures or wrinkle ridges.Young-
est distinguishable flow material on map aside from impact melts.
IM-Impact Melt. Due to high atmospheric pressure, impacts from objects such as asteroids cause
surface melting creating separate flows originating from the crater.
Coronae and Planitia Materials Structural Events
Fractures
?
?
?
?
Wrinkleridges
Shields
?
?
b
fU
iM
?
?
ImpactCrater
fA
Ran Colles contains three shield fields among a predominantly contractional region
of Venus. On the Western half of the map, shields form along fractures trending NE
perpendicular to the wrinkle ridges that trend in a NW direction. The youngest
shields are associated with“flow a”material that is emplaced on top of“flows undi-
vided”. The eastern half of the map shows wrinkle ridges forming a more organized
deformation system. Shields in this region also form along fractures, however they
have a more northern trend. Due to radar backscatter, the boundaries of the defor-
mation system are uncertain, and flow material is difficult to separate out.
b- Basement. Bedrock exposed in small areas on surface.
fu-Flows undivided. Lumped surface flows with undistingushable boundaries.
fA-Flow A. Distinguishable separate flow material covering the undivided flows as well as previously
emplaced fractures.
IM-Impact Melt. Due to high atmospheric pressure, impacts from objects such as asteroids cause
surface melting creating separate flows originating from the crater.
References
[1] Bender, K.C., Senske, D.A., and Geeley, R. (2000), Geologic map of the Carson quadrangle
(V-43), Venus: U.S. Geological Survey Geologic Investigations Series Map 2620. 1:5 million
scale.
[2] Guest, J.E., M.H. Bulmer, J. Aubele, K. Beratan, R. Greeley, J.W. Head, G. Michaels,
C. Weitz, and C. Wiles (1992), Small volcanic edifices and volcanism in the plains of Venus,
J. Geophys. Res., 97(E8), 15,949-15,966.
[3] Crumpler, L.S., Aubele, J.C., Senske, D.A., and 3
others (1997), Volcanos and centers of volcanism on Venus, in Bougher, S.W., Hunten, D.M.,
and Phillips, R.J., eds., Venus II: Tucson, University of Arizona Press, p. 697-756.
[4] Zimbelman, J.R. (1998) Emplacement of long lava flows on planetary surfaces, J. Geophys. Res.
103, B11, 27503-27516.
[5] Zimbelman, J.R. (2001) Image resolution and the evaluation of genetic hypotheses for
planetary landscapes, Geomorphology 37, Nos. 3-4, 179-199.
Conclusions
ing that small shield volcanism plays a significant role in the volcanic resurfacing
of Venus. However, the volume of erupted material needs constraining
shield field, an intra-field stratigraphy is difficult to construct over the entire area
of a field due to the effective resolution of Magellan SAR data [4]
that field reflect variations in either composition, effusion rate, and/or eruption
style. Geologic setting should be taken into account when interpreting shield fields
Jurate Colles is the highest latitude observed location. Dominated by flows undi-
vided, It can only be broken up into sections based on radar boundaries. Multiple
distinguishable flow areas are absent and shields are only associated with and
along fracture zones. Shields in this region are more defined, containing steeper
slopes and more obvious summit pits. This was interpreted as a composition
change in the lava flows erupted. The reasoning that basement is more heavily
exposed which can be interpreted as shallower flows covering Jurate. The base-
ment material being closer to the surface the flows undivided material could be
causing shields to erupt with a different viscosity flow, increasing their height,
slope and definition.
Acknowledgements
National Aeronautics and Space Administration (NASA)
PDAP grant awarded to Bradley J. Thompson