USGS Science⌕ Search

Geology topics

George E. McGill

Publications and source records attributed to George E. McGill.

6 recordsLinked to original sources
USGS2005

Geologic map of Cydonia Mensae - southern Acidalia Planitia, Mars, quadrangles MTM 40007, 40012, 40017, 45007, 45012, and 45017

Cydonia Mensae lie adjacent to the boundary separating the highland terrain of western Arabia Terra from the northern lowlands (the dichotomy boundary), where this boundary is gradational rather than abrupt, as is generally the case. Cydonia Mensae are characterized by abundant knobs and mesas, most of which occur in well defined clusters that commonly are long and narrow. The adjacent lowland of Acidalia Planitia contains abundant troughs that define a crude, very large scale polygonal pattern. This pattern, and the troughs, are similar in size and spacing to those in Utopia Planitia (for example, McGill and Hills, 1992; Hiesinger and Head, 2000). The six 1:500,000-scale quadrangles mapped are MTM 40007, 40012, 40017, 45007, 45012, and 45017, which include northern Cydonia Mensae , part of southern Acidalia Planitia, and a small area of Arabia Terra highlands. These six quadrangles are combined into a single map at 1:1,000,000 scale for this publication. Issues of interest addressed by this mapping include the crustal history implied by the knobs and mesas of Cydonia Mensae , the implications of the gradational dichotomy boundary, the age of the Acidalia plains materials, the origin and age of the giant polygons, the validity of putative shorelines inferred to be present in this area, and the origin of the very abundant small cones and domes, specifically if these imply the past presence of water or ice within the plains materials. Earlier maps ( Witbeck and Underwood, 1984; Scott and Tanaka, 1986; Tanaka and others, 2003) that included the area mapped for this study addressed some or all of these issues, but these maps were more regional in coverage and were at smaller scales. In addition, new orbital data from Mars Global Surveyor and Mars Odyssey missions were available only to Tanaka and others (2003). The intent of this present study is to address the issues listed by mapping at a scale large enough to permit greater map detail and to better understand the areal distribution of the materials and features of interest.

IMAP↗
USGS2002

Geologic map transecting the highland/lowland boundary zone, Arabia Terra, Mars; quadrangles 30332, 35332, 40332, and 45332

Arabia Terra is a large region of cratered terrane extending from about 20° W. longitude eastward across the prime meridian to about 300° W. longitude for an average east-west width of about 5,000 km. The northern boundary ranges from 40° N. to 45° N.; the southern boundary is a poorly defined zone at about 0° N. Thus, the north-south width is about 2,500 km. Except for the westernmost part, Arabia Terra has an albedo higher than surrounding terranes. The four quadrangles mapped (30332, 35332, 40332, 45332) provide a north-south strip from highland terrane in the south to lowland terrane in the north. The northern portion of Arabia Terra is the type region for both fretted terrane and fretted valleys and, along with the immediately adjacent northern plains, is also the site of some of the best examples of putative flow deposits present as aprons around isolated knobs and mesas or as deposits on the floors of fretted valleys and on the lowland surface. Mass wasting, eolian erosion or deposition, glacial scouring, fluvial or shoreline erosion, deposition from an ocean, hydrovolcanism, plateau volcanism, and faulting have all been proposed to account for the topography and crater characteristics in northern Arabia Terra. Although underlain by what appears to be typical highland terrane, Arabia Terra is anomalously low, with elevations generally below the planetary reference. Probably the most important question concerning the global-scale tectonic history of Mars is the origin of the crustal dichotomy. The northern lowland is not only several kilometers lower than the southern highland, it also is surfaced by materials that are significantly younger than surface materials in the southern highland. The young surface materials in the lowland rest unconformably on basement material having an age comparable to the exposed ancient highland terrane to the south. The age of the dichotomy continues to be controversial, as does the mechanism for its formation, as reviewed by McGill and Squyres (1991). Gravity and topography data from Mars Global Surveyor, however, does appear to favor early formation due to internal processes. Because complex depositional and erosional events affected the boundary since its formation, the cause and history of these events must be unraveled before we can directly attack the fundamental question of the reason for the dichotomy.

IMAP↗
USGS2002

Geological map of the Kaiwan Fluctus Quadrangle (V-44), Venus

Introduction The Magellan spacecraft orbited Venus from August 10, 1990, until it plunged into the Venusian atmosphereon October 12, 1994. Magellan had the objectives of: (1) improving knowledge of the geologic processes, surface properties, and geologic history of Venus by analysis of surface radar characteristics, topography, and morphology and (2) improving knowledge of the geophysics of Venus by analysis of Venusian gravity. The Magellan spacecraft carried a 12.6-cm radar system to map the surface of Venus. The transmitter and receiver systems were used to collect three datasets: synthetic aperture radar (SAR) images of the surface, passive microwave thermal emission observations, and measurements of the backscattered power at small angles of incidence, which were processed to yield altimetric data. Radar imaging and altimetric and radiometric mapping of the Venusian surface were done in mission cycles 1, 2, and 3, from September 1990 until September of 1992. Ninety-eight percent of the surface was mapped with radar resolution of approximately 120 meters. The SAR observations were projected to a 75-m nominal horizontal resolution; these full-resolution data compose the image base used in geologic mapping. The primary polarization mode was horizontal-transmit, horizontal receive (HH), but additional data for selected areas were collected for the vertical polarization sense. Incidence angles varied from about 20? to 45?. High-resolution Doppler tracking of the spacecraft was done from September 1992 through October 1994 (mission cycles 4, 5, 6). High-resolution gravity observations from about 950 orbits were obtained between September 1992 and May 1993, while Magellan was in an elliptical orbit with a periapsis near 175 kilometers and an apoapsis near 8,000 kilometers. Observations from an additional 1,500 orbits were obtained following orbit-circularization in mid-1993. These data exist as a 75? by 75? harmonic field.

IMAP↗
USGS2001

Geologic map of the Pandrosos Dorsa Quadrangle (V-5), Venus

Introduction The Magellan spacecraft orbited Venus from August 10, 1990, until it plunged into the Venusian atmosphere on October 12, 1994. Magellan had the objectives of (1) improving knowledge of the geologic processes, surface properties, and geologic history of Venus by analysis of surface radar characteristics, topography, and morphology and (2) improving knowledge of the geophysics of Venus by analysis of Venusian gravity. The Magellan spacecraft carried a 12.6-cm radar system to map the surface of Venus. The transmitter and receiver systems were used to collect three datasets: synthetic aperture radar (SAR) images of the surface, passive microwave thermal emission observations, and measurements of the backscattered power at small angles of incidence, which were processed to yield altimetric data. Radar imaging and altimetric and radiometric mapping of the Venusian surface were done in mission cycles 1, 2, and 3, from September 1990 until September 1992. Ninety-eight percent of the surface was mapped with radar resolution of approximately 120 meters. The SAR observations were projected to a 75-m nominal horizontal resolution; these full-resolution data compose the image base used in geologic mapping. The primary polarization mode was horizontal-transmit, horizontal-receive (HH), but additional data for selected areas were collected for the vertical polarization sense. Incidence angles varied from about 20? to 45?. High-resolution Doppler tracking of the spacecraft was done from September 1992 through October 1994 (mission cycles 4, 5, 6). High-resolution gravity observations from about 950 orbits were obtained between September 1992 and May 1993, while Magellan was in an elliptical orbit with a periapsis near 175 kilometers and an apoapsis near 8,000 kilometers. Observations from an additional 1,500 orbits were obtained following orbitcircularization in mid-1993. These data exist as a 75? by 75? harmonic field.

IMAP↗
USGS2000

Geologic map of the Sappho Patera Quadrangle (V-20), Venus

The Sappho Patera quadrangle (V–20) of Venus is bounded by 0° and 30° East longitude, 0° and 25° North latitude. It is one of 62 quadrangles covering the entire planet at a scale of 1:5,000,000. The quadrangle derives its name from Sappho Patera, a large rimmed depression (diameter about 225 km) lying on top of a shield-shaped mountain named Irnini Mons. Sappho, a noted Greek poet born about 612 B.C., spent most of her life on the island of Lesbos. All of her works were burned in 1073 by order of ecclesiastical authorities in Rome and Constantinople. What little survives was discovered in 1897 as parts of papier mâché coffins in the Fayum (Durant, 1939). The Sappho Patera quadrangle includes the central portion of Eistla Regio, an elongated, moderately elevated (relief ~1 km) region extending for about 7,500 km west-northwestward from the west end of Aphrodite Terra. It is generally interpreted to be the surface manifestation of one or more mantle plumes (Phillips and Malin, 1983; Stofan and Saunders, 1990; Kiefer and Hager, 1991; Senske and others, 1992; Grimm and Phillips, 1992; Solomon and others, 1992). Eistla Regio is dominated by several large volcanic features. All or parts of four of these occur within the Sappho Patera quadrangle: the eastern flank of Gula Mons, Irnini Mons, Anala Mons, and Kali Mons. The quadrangle also includes eight named coronae: Nehalennia, Sunrta, Libera, Belet-Ili, Gaia, Asomama, Rabzhima, and Changko. A major rift extends from Gula Mons in the northwestern corner of the quadrangle to Libera Corona near the east border. East of Irnini and Anala Montes this rift is named Guor Linea; west of the montes it is named Virtus Linea. In addition to these major features, the Sappho Patera quadrangle includes numerous smaller volcanic flows and constructs, several unnamed coronae and corona-like features, a complex array of faults, fractures, and wrinkle ridges, and extensive plains that are continuous with the regional plains that constitute about 80% of the surface of Venus (Masursky and others, 1980). This area is geologically interesting because it contains examples of most globally important types of features and deposits and is an excellent area to study the temporal and genetic relations among plains, rifts, coronae, and large shield volcanoes. The temporal relations displayed in this quadrangle can provide useful constraints on models for venusian tectonic style (McGill, 1994b).

IMAP↗