Background of the Educational Guidebook

Introduction

This section describes some fundamental aspects of impact craters, provides information on volcanic landforms, and describes the specific stops and general geology encountered during the field trip.

IMPACT CRATERS

Geological exploration of the Solar System has shown that impact cratering is an important geological process. Impact cratering involves the nearly instantaneous transfer of energy from an impacting object, called the bolide, to the target surface. Bolides can include meteoroids, asteroids, and comets. The velocities of these objects upon impact on Earth range from 5 to more than 40 km/sec. In the initial stage of cratering the projectile contacts the target, penetrates the surface, and generates an intense shock wave that expands radially from the point of impact. During the excavation stage, a rarefaction (or decompression) wave sets the "target" material into motion and ejects it from the impact area to form the crater. The final stage involves various modifications to the crater that are not directly attributable to the shock-wave. These include slumping of the crater wall, isostatic adjustment of the floor and rim, erosion, and infilling of the crater.

The existence of impact craters on Earth was not readily accepted by the scientific community. Even to advocates of impact cratering, prior to 1930 fewer than 10 impact structures were known on Earth. By 1966 the number had risen to only about 33. However, after intensive searches and establishment of criteria for the recognition of impact craters, by the 1980s about 100 craters and related structures had been documented as resulting from impact and an additional 100 that may be of impact origin.

Table 1 lists the criteria that are commonly used to recognize impact craters. This table is divided into three parts: factors that can be assessed via remote sensing, factors requiring geophysical observations, and observations that can be made on the ground. In practice, some combination of these factors is required to assess impact origins. In the absence of data obtained in situ on planetary surfaces, remote sensing must be employed in the assessment of crater origins.

Table One Criteria for the recognition of impact craters (modified from Dence, 1972)

Criterion	Characteristics	Reliability
Remote sensing
plan view	distinctly circular may be modified by slumping,tectonic patterns, or erosion	fair, but can be attributed toother processes
rim structure	inverted stratigraphy	definitive
central zone	floor lower than surrounding plain; may contain central uplift	fair, but can be attributed to other processes
Geophysical observations
gravity anomaly	generally negative	supportive, but not conclusive
magnetic field	variable; may be distinct anomaly over melt rock	supportive, but not conclusive
seismic velocites	generally lower in brecciated zones	supportive, but not conclusive
Ground observations
presence of meteroites	rare except in very young craters	definitive
shock metamorphism	features such as high pressure minerals, impact melt, planar shock features and shatter cones	definitive
brecciation	observed in ejecta, rim and floor of craters	may be attributed to other processes

VOLCANIC LANDFORMS

Volcanic precesses involve the generation of magma and magma-related materials, and their eruption onto planetary surfaces. Thus, volcanic structures provide direct clues to the thermal evolution and interior characteristics of planetary objects.

Basaltic volcanism is extremely important on the terrestrial planets. Basalts form the floors of ocean-basins, occur on most continents, and have erupted on Earth throughout its known geological history. Dark mare areas of the Moon are basaltic lava flows and constitute at least one-fifth of the lunar surface. Perhaps 50% or more of the martian surface is covered with basaltic materials, as may be substantial parts of Mercury. Various lines of evidence suggest that many asteroids may be basaltic. Venera and Magellan results show that many of the mountains and plains of Venus also appear to be the result of basaltic volcanism. Consequently, most considerations of planetary volcanism have focused on studies of basaltic eruptions and landforms. Concurrently, research has been conducted on basaltic volcanism on Earth to serve as a basis for comparison with other planets.

The forms of volcanoes and related terrains are the result of many complex, often interrelated parameters. As shown in Table 2, these fall into three groups: a) planetary variables, b) magma properties controlling rheology, and c) intrinsic properties of the eruptions. Planetary variables include those factors that are characteristic for a particular planet in question. For example, the vertical and horizontal "spread" of tephra in an explosive eruption is partly governed by the presence or absence of an atmosphere, the gravitational acceleration, and the escape velocity. In an airless, low-gravity environment, such as the Moon, tephra deposits would be widespread whereas on Earth, ejecta would be retarded by the presence of the atmosphere and higher gravity and would accumulate around the vent to form a cinder cone.

Table Two Factors governing the morphology of volcanic landforms (from Whitford-Stark, 1982)

Planetary variable	Controlling rheology	Properties of eruption
gravity	viscosity	eruption rate
lithostatic pressure	temperature	eruption volume
atmospheric properties	density	eruption duration
surface/subsurface liquids	compostion	vent characteristics
planetary radius	volatiles	topography
planetary composition	amount of solids	ejection velocity
temperature	yield strength
	shear strength

Magma rheology is an extremely important parameter in volcanic morphology. Fluid lavas spread more easily, leading to the emplacement of volcanic plains, and are in contrast to viscous lavas which typically form short, stubby flows which accumulate to form domes. Various characteristics of the eruption constitute the third main group of factors, including rate of effusion, total volume erupted, and characteristics of the vent.

During the field trip you will have the opportunity to examine a wide variety of volcanic landforms, including a shield volcano, a dome, and a composite cone. The focus will be on those aspects of volcanic morphology that can be related to styles of volcanism, composition of magmas, and other important parameters for use in extraterrestrial comparisons.

PHOENIX TO FLAGSTAFF, GENERAL GEOLOGY: Most of the field trips head north on I-17 from Phoenix to Flagstaff and pass through the three principal physiographic regions found in Arizona (Nations & Stump, 1981). Phoenix is in the Basin and Range Province, characterized by fault-block mountains and intervening basins filled with hundreds to thousands of meters of sediments. The mountains consist predominantly of Precambrian gneisses, schists, and granites, some of which are capped with younger lava flows and other volcanic materials.

The Basin and Range Province gives way northward to the so-called Transistion Zone. "Transition" refers not only to geology and physiography, but to various biologic zones as well. Vegetation in the lower desert of the Phoenix area is characterized by saguaro cactus and ocotillo, whereas the Transition Zone includes prickly perar cactus, grasslands, and junipers. Near the community of New River, road-cuts expose light-colored, well-stratified Tertiary lake and stream sediments, some of which are cross-bedded. Climbing through the Transition Zone, I-17 crosses lava flows of Tertiary age, including those of Black Mesa, which overlay the older Precambrian rocks.

Continuing farther north, the Transition Zone gives way to the Colorado Plateau, the southern edge of which is marked by the Mogollan Rim. The Colorado Plateau is an enormous area covering the "four corners" area of Utah, Colorado, New Mexico, and Arizona. The plateau is underlain by approximately horizontal, Paleozoic to early Cretaceous rocks, most of which are well exposed in the Grand Canyon. Superimposed on the Colorado Plateau is the San Francisco volcanic plateau which consists mostly of basalt flows of the last 10 million years. The highway crosses these flows from the Verde Valley to Flagstaff.

VERDE VALLEY: The Verde River valley is a southeast-trending structural and erosional trough which is bounded on the southwest by the Black Hills and on the north and northeast by the Mogollan Rim. The Verde Valley first formed sometime after 14 million years ago, following the deposition of a sequence of volcanic rocks called the Hickie Volcanics. During this time the Black Hills were tilted southwestward along the Verde fault. Uplift of the Black Hills block exposed copper-bearing Precambrian rocks in the area of Jerome (25 km northwest of I-17). These ores were discovered by prospectors in the mid-1800s and were mined until the early 1950s.

During the Pliocene time (3-10 million years ago), the Verde Valley was dammed at the southern end (about 19 km southeast of Camp Verde) by lava flows which probably originated in the Hackberry Mountain area. The ancestral Verde Valley was dammed several times in this fashion to form large lakes in which sediments of the Verde Formation collected. These deposits vary in character from one part of the basin to another. The light-colored rocks cropping out in the axis of the valley are lakebeds whcih are mainly clayey limestone containing gypsum and other salts, as well as layers of volcanic ash.

After a period of extensive sedimentation within the lakes, the final lava dam was breached and the lake drained. Following this event, the Verde River has continued to cut downward, leaving a series of three low terraces along its course. Elephant bones have been found in deposits of one of these terraces at a height of about 8 m above the present Verde Valley.

SUNSET REST AREA: West of the Sunset rest area lies Black Canyon and the Bradshaw Mountains. The Bradshaw Mountains consist of granites intruded into metamorphic rocks, all of Precambrian age. These rocks and associated zones of mineralization were scenes of some of the earliest gold and other precious metals mining in Arizona.

Several kilometers east of Sunset rest area (across I-17) is Joe's Hill. It is among the youngest of the volcanoes and flows that consitutue Black Mesa. Dates for the lavas range in age from 10 to 11 million years (Luedke & Smith, 1978). Joe's Hill is about 5 km across and is a classic "low profile" shield volcano composed of multiple, relatively thin basalt flows. Many of these flows extend to the south and are exposed in highway cuts along I-17. Typical of low-shields, these flows are only a few meters thick and were erupted sporadically at moderate rates of effusion; duration may have been a day, to weeks, to months. Low shields like Joe's Hill typically coalesce to form a terrain termed "basaltic plains." Basaltic plains are found in many places on Earth--such as the Snake River Plain, Idaho--and have been identified on the Moon, Mars, and Venus. This style of volcanism is intermediate between flood eruptions, which produce huge, thick sheets of slowly cooling lavas, and Hawaiian-style eruptions, which produce the enormous shiel volcanoes typified by Mauna Loa (Greeley, 1982).

MONTEZUMA WELL: Montezuma Well is a sinkhole formed in the Verde Formation. The Verde Formation consists of fresh water limestones, sandstones, and volcanic ashes deposited in a series of shallow, intermittent lakes 3 to 6 million years ago. The lakes were more than 50 km long and formed as a consequence of lava flows which dammed the Verde Valley. The lime deposits were derived by weathering of the Kaibab and other limestones exposed along the edge of the Colorado Plateau.

The sinkhole is about 125 m across and 40 m deep, and formed by collapse, probably as a consequence of lowering of the water table as Beaver Creek cut downward. A spring issues more than 1000 gallons/minute into Beaver Creek and keeps the water level in the sinkhole constant.

STONEMAN LAKE: Stoneman Lake occupies a nearly perfectly circular crater 1.7 km across and about 100 m deep. The crater is formed in basalt flows of the Morman volcanic field and is flanked by cones to the east. Although by strict definition, it can be called a pit crater (which commonly form in basalts near vents), it is an isolated structure and its origin is not known. Lack of cinders and spatter around the crater suggests that is was not a vent. Its location is near the edge of the Colorado Plateau. As seen in outcrops <10 km to the west, the basalt "cap" is probably thin. The Permian-age Kaibab limestone underlies the basalts and is typified by karst features. Thus, the pit crater may have formed as a consequence of collapse into the Kaibab limestone.

MORMON LAKE: Mormon Lake occupies a depression about 10 km long by 5 km wide (Moore et al., 1960). The east side is bounded by a north-south trending fault scarp some 50 m high. The west side is flanked by various domes and ocnes, the most dominant of which is on the northwest side of the lake. Although the Mormon volcanic field consists predominantly of flat-lying basalt flows, presumably erupted from various fissures, punctuated by basaltic cinder cones, the cone northwest of the lake is a rhyodasite dome dated at 3.1 million years old. The structural feature occupied by Mormon Lake may be a volcano-tectonic depression formed by collapse over a magma chamber as the chamber emptied.

FLAGSTAFF TO METEOR CRATER: Rocks of several ages crop out within the city limits of Flagstaff. The youngest are various basalt flows which, in places, form caprock-mesas. Older rocks include the Moencopi Formation of early Triassic age and the Kaibab Formation of Permian age. The top of the Kaibab is an ancient karst surface in which the upper 20-30 meters have been modified by solution and erosion.

Various cinder cones are readily visible east of Flagstaff. Many of the cinder cones are being quarried for light-weight aggregates and road material. Most of the cinder cones contain zenoliths derived from the underlying sedimentary rocks. More than 400 cinder cones can be identified in the San Francisco volcanic field. As the highway leaves the San Francisco volcanic field to the east, it traverses the eroded surface of the Kaibab Formation. In general, the topography reflects the structure of these sedimentary formations. For example, at a distance of about 40 km from Flagstaff, a broad rise reflects an anticline. Also at this distance, the Hopi Buttes volcanic field is visible about 100 km toward the northeast. The buttes are diatremes (Shoemaker et al., 1962) which are surrounded by lake deposits and terrces that reflect the former presence of a large Pliocene lake that may have been fed by the ancestral Colorado River. Ages for the diatremes range from 2.1 to 6.7 million years old. At a distance of about 45 km from Flagstaff, the rim of Meteor Crater is visible to the east-southeast.

RATTLESNAKE CRATER: Rattlesnake Crater is a maar crater about 1.5 km across that has been mapped as part of the Tappan-age (Pleistocene) group of volcanic materials (Moore et al., 1974). The Tappan group includes most of the basaltic cinder and tuff cones visible in the eastern San Francisco volcanic field, of which Rattlesnake Crater is the best preserved. It is composed predominantly of basaltic tephra and includes zenoliths of gabbro and olivine. The rim deposits also include blocks of basalt and sedimentary rocks derived from the Kaibab, Coconino, and Supai Formatioins, all contained within a sandy, palagonitic matrix. The finely-communited quartz grains in the matrix may be derived from the underlying Coconino sandstone. From superposition relations, it would appear that the cone on the south-southeast side of the crater originated after the formation of the tuff ring.

Maar craters such as Rattlesnake Crater share many of the morphological characteristics commonly ascribed to impact craters: their floors lie below the surrounding plain, their rims are raised, and they are often circular in plan-form. This morphology is a consequence of their origin by phreatic explosives. Lack of overturned rim deposits and the other characteristics indicated in Table 1 permit their separation from impact craters. It is interesting to speculate, however, if it would be possible to identify maar craters on other planets where only remote sensing data are available.

METEOR CRATER: To make the most effective use of time at this stop, first go outside the Museum Visitor Center and listen to the taped narration on the history and geology of Meteor Crater. Then take the short visitor trail to examine features exposed along the rim. Note in particular the evidence for inverted stratigraphy in the crater rim. Finally, return to the museum and examine the various exhibits and displays. This is probably the best museum on impact craters in existence. The developers of Meteor Crater worked very closely with Eugene Shoemaker and other members of the U.S. Geological Survey, and other specialists in impact cratering mechanics.

As summarized from Shoemaker and Kieffer (1974), Meteor Crater was formed by an iron bolide about 30 m across travelling at perhaps 15 km/sec. The amount of eneergy released by the impact is estimated to be equivalent to 4 to 5 megatons of TNT. The impact occurred a few tens of thousands of years ago to form a crater about 1.2 km in diameter by 180 m deep. Despite some erosion of the general surface in this area, Meteor Crater has a pronounced raised rim, characteristic of impact craters. Uncharacteristic of impact craters, however, is its polygonal shape in plan view, as whoen on the aerial photograph. This is attributed to impact into rocks that have a prominent joint pattern. The excavation of the crater was apparently controlled along these lines of weaknesses. The floor of the crater contains deposits derived from talus from the crater walls, windblown dust, volcanic ash, and lakebed deposits.

SUNSET CRATER: This National Monument contains some of the youngest basalts in the conterminous United States. It is part of the eastern San Francisco volcanic field described by Moore and Wolfe (1976). The dominant feature is Sunset Crater, a 300-m high cinder cone that erupted in the winter of 1064-1065. Eruptions involved both Strombolian activity that spread tephra over much of the surrounding terrain and effusive activity to emplace the Bonita lava flow (Holm, 1987).

O'LEARY PEAK: O'Leary Peak is an older volcano consisting of two domes composed of rhyodacite prophyries (Moore and Wolfe, 1976). The southern dome is interpreted as the younger. Sanidine from the prophyry of the older, northwest dome has been dated at 233,000+37,000 years by the K-Ar method (Damon et al., 1974). The flanks of the volcano are mantled by basaltic tephra derived from Sunset Crater. The view from O'Leary Peak includes an overview of Sunset Crater, views of the San Francisco volcanie complex, and a view of Mt. Elden.

The San Francisco volcanic field includes the San Francisco Peaks (including Mt. Humphreys, the highest point in the state at 3959 m) and Sunset Crater, the youngest volcano, plus about 400 other vents that have been identified in the area. Volcanic activity began 10 million years ago with extrusion of basalt flows which covered the broad area from Flagstaff to the Verde Valley. The San Francisco Peaks complex is a composite cone of andesitic to rhyolitic compositions. The oldest flow is an andesitic flow which has been dated at 1.8 million years. Dacite, rhyolite and more andesite materials were extruded until about 700,000 years ago. The volcano probably reached a height of more than 4600 meters prior to collapse to form the inner valley about a half million years ago. The collapse to form the inner valley may have been analogous in some ways to the recent formation of the inner valley of Mt. St. Helens. Sugarloaf Mountain, located at the mouth of the interior valley, was erupted about 220,000 years ago. An initial phreatic eruption produced a tuff-ring into which was emplaced rhyolitic magma to form Sugarloaf Mountain. The peaks and the inner valley show evidence of glaciation. Mt. Elden is a dacite dome which eruptied about 550,000 years ago and produced a series of viscous flows and an intrusive dome.

GOVERNMENT CAVE: Government Cave is a lava tube formed in basalt flows erupted from an unknown vent or vents east of the cave entrance (Forney, 1971). The uncollapsed segment of the tube can be traced 1125 m and is more than 12 m high and 20 m wide in places. Sections of the lava tube preserve the original wall linings, high lava marks that indicate prolonged, steady flow through the tube as it drained, and remnants of the flow reflecting incomplete drainage upon cessation of the eruption.

Lava tubes indicate a Hawaiian-style of eruption (Greeley, 1987) involving moderate rates of effusion, spread over many days, weeks, or months of activity. In effect, lava tubes are extensions of the vent-conduit and aid in the emplacement of large lava flows by retarding the rate of cooling of the magma. Collapsed lava tubes have been identified on the Moon, Mars, and possibly Mercury and Venus.

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