Renewed Volcanic Activity Research Articles

An Archean megacaldera complex: The Blake River Group, Abitibi greenstone belt

The Archean Blake River Group of the southern Abitibi greenstone belt is defined as a subaqueous megacaldera. Compelling evidence include: (1) a radial and concentric organization of synvolcanic mafic to intermediate dykes, (2) an overall dome geometry defined by the volcanic strata, (3) a peripheral distribution of subaqueous volcaniclastic units, (4) a zonal distribution of carbonate alteration, and (5) a distinct annular synvolcanic inner and outer ring fault pattern. Three caldera-forming events have been identified: (1) the early Misema caldera, (2) the New Senator caldera, and (3) the well-known Noranda caldera. The multi-vent BRG mafic volcanic complex, developed on a monotonous sequence of tholeiitic basalts, forming a submarine plain, and experienced a first major collapse that created the giant Misema Caldera (80 km in diameter). An endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed. Major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults. This fault system was used as a conduit for CO 2-rich hydrothermal activity. Renewed volcanic activity was associated with a resurgent central dome inside the Misema Caldera. This second collapse event created the 35 km by 14 km NW-trending New Senator Caldera. This caldera was produced by multi-step sagging after the underlying magmatic chamber migrated to the SE and formed the Flavrian-Powel Plutons. The final collapse resulted in the formation of the Noranda Caldera that generated a well-developed 070°-trending fracture pattern associated with several VMS deposits. The multiple-caldera setting provides an effective model to explain the presence of VMS mineralization along the synvolcanic fractures associated with the three collapse episodes. However, the importance of the New Senator Caldera fracture pattern is emphasized because of its role in the positioning of the giant Horne Mine deposit.

Volcanic risk perception of young people in the urban areas of Vesuvius: Comparisons with other volcanic areas and implications for emergency management

More than 600 000 people are exposed to volcanic risk in the urban areas near the volcano, Vesuvius, and may need to be evacuated if there is renewed volcanic activity. The success of a future evacuation will strongly depend on the level of risk perception and preparedness of the at-risk communities during the current period of quiescence. The volcanic risk perception and preparedness of young people is of particular importance because hazard education programs in schools have been shown to increase the clarity of risk perception and students often share their knowledge with their parents. In order to evaluate young people's risk perception and preparedness for a volcanic crisis, a multiple choice questionnaire was distributed to 400 high-school students in three municipalities located close to the volcano. The overall results suggest that despite a 60-year period of quiescence at Vesuvius, the interviewed students have an accurate perception of the level of volcanic risk. On the other hand, the respondents demonstrate a clear lack of understanding of volcanic processes and their related hazards. Also, the interviewed students show high levels of fear, poor perceived ability to protect themselves from the effects of a future eruption, and insufficient knowledge of the National Emergency Plan for Vesuvian Area (NEPVA). The latter result suggests that in comparison with volcanic crises in other regions, during a future eruption of Vesuvius, there may not be enough time to educate the large number of people living near the volcano about how to appropriately respond. The inadequate risk education and preparedness of respondents implies that a strong effort is needed to improve communication strategies in order to facilitate successful evacuations. Therefore, it is important to take advantage of the present period of quiescence at Vesuvius to improve the accuracy of risk perception of youth in local communities.

Revised stratigraphy and eruption rates of Ceboruco stratovolcano and surrounding monogenetic vents (Nayarit, Mexico) from historical documents and new radiocarbon dates

More than a dozen new radiocarbon dates reconstruct the eruptive history of Ceboruco volcano. Six of these further constrain previous results for the important plinian Jala eruption, which occurred near 1060 ± 55 yr BP. A calibrated radiocarbon age of AD 990–1020 was obtained as best overlap range for all samples. Pottery fragments found directly underneath the pumice deposit indicate that this area was inhabited by human populations that witnessed the eruption. This age therefore represents an important time marker in the prehistory of this region, because an area of > 560 km2 was devastated and covered by a thickness of > 50 cm of pumice and ash fallout. After the prominent Jala eruption (VEI = 6), at least seven major lava flows and several smaller domes were issued from Ceboruco's crater. Analysis of historical documents allows us to conclude that most of these eruptions took place well before the arrival of the Spanish conquerors to this area in 1528. Ceboruco's last historic eruption (1870–1872) produced a ca. 7 km long viscous dacite lava flow. Its emplacement was accompanied by block-and-ash flow activity and deposition of ash fallout. Because a repeat of such an eruption in the future would seriously endanger the nearby population and disrupt important life-lines, a detailed discussion of eyewitness accounts and other documents, including drawings and paintings is provided. Some of these materials are made available to the broader public for the first time in the present article. Several surrounding monogenetic vents were previously dated by others using the K–Ar and the Ar–Ar methods. Because these dating methods are often not suitable for very young rocks, we also dated several Holocene monogenetic vents by the radiocarbon method. These dates together with geologic mapping allowed calculating eruption rates and recurrence intervals of different types of eruptions. Accordingly, an andesite/dacite lava flow (accompanied by block-and-ash flow activity) was erupted from Ceboruco's crater on average every 143 years during the last 1000 years. In contrast, a monogenetic eruption forming either a scoria cone or a dome has occurred in the area surrounding Ceboruco on average every 1000 years during the Holocene. During the period between AD 1000 and AD 1500 Ceboruco displayed an elevated activity experiencing a rhyodacite plinian eruption (3–4 km 3 DRE) followed by the emplacement of six andesitic/dacitic lava flows also with a total volume of 3–4 km 3. All these figures indicate that the Ceboruco volcanic region is one of the most active areas in the entire Trans-Mexican Volcanic Belt. Consequently, production of a detailed volcanic hazard map and implementation of other preventive measures aimed at reducing potential losses in case of renewed volcanic activity should be mandatory.

Simulation of syn-eruptive floods in the circumvesuvian plain (southern Italy)

During explosive eruptions the deposition of fine-grained volcanic ash fallout reduces soil permeability, favouring runoff of meteoric water and thus increasing the occurrence of catastrophic floods. A fully dynamic, two-dimensional model was used to simulate flooding scenarios in the Vesuvian area following an explosive volcanic eruption. The highest risk occurs in the catchment area of the Acerra-Nola Plain N and NE of Vesuvius. This plain has a population of 70,000 living in low-lying areas. This catchment area is vulnerable to ash fall because it lies downwind of the dominant synoptic circulation and it lacks a natural outflow toward the sea. Our numerical simulations predict dangerous scenarios, even in quiescent periods, during extreme rain events (return periods of 200 years have been considered), and a significant increase in the extent of the flooded areas due to renewed volcanic activity. Based on these simulations a hazard zonation has been proposed.

Seismic stratigraphy of Detroit Seamount, Hawaiian‐Emperor seamount chain: Post‐hot‐spot shield‐building volcanism and deposition of the Meiji drift

Detroit Seamount, one of the northernmost seamounts of the Hawaiian‐Emperor seamount chain, was formed at ca. 76 Ma. New seismic data suggest renewed volcanism as late as 25 m.y. after initial seamount formation. We use high‐resolution single‐channel seismic (SCS) data acquired over the summit of Detroit Seamount in 2001 on Ocean Drilling Program (ODP) Leg 197, supplemented by older SCS data acquired as part of the GLORIA mapping program of the U.S. Geological Survey, to characterize the seismic stratigraphy of Detroit Seamount. Volcanic edifices occur on the summit of the seamount and are older than the oldest beds of the Meiji drift (early Oligocene: ca. 34 Ma). On the basis of ash layers in ODP drill holes, we suggest the edifices were active throughout much of the Eocene (ca. 52–34 Ma), with activity possibly extending into the early Oligocene (<34 Ma). Hence the age difference between the shield‐building lavas and the postshield cones on Detroit is far greater than the shield/postshield age differences observed on the Hawaiian Islands, suggesting that renewed volcanic activity and tectonic collapse may be possible on any of the Hawaiian Islands. We confirm earlier assertions that the thick sediment cap, Oligocene and younger in age, was deposited by an ocean‐bottom current with a southeastward flow direction, along the northeast facing flank of the Emperor Seamount chain. This sediment cap, the Meiji drift, was deposited by a lower‐velocity current than many other sediment drifts. A low‐angle normal fault, dipping ∼19°, suggests topographic collapse of Detroit seamount sometime during the Eocene or late Cretaceous.

Volcanic eruptions with little warning: the case of Volcán Reventador's Surprise November 3, 2002 Eruption, Ecuador

Successful mitigation of a possible volcanic disaster depends upon the early detection of renewed volcanic activity. With considerable optimism, volcano observatories instrument dangerous volcanoes, with the hope of an early recognition of the reactivation of a volcano. Reventador volcano's November 3, 2002 eruption came with little warning and had a tremendous socio-economic impact. Reventador volcano, a young andesitic cone in the Eastern Cordillera of Ecuador, has had, at least, 16 eruptions between 1541 and 2002. These eruptions were characterized by small pyroclastic flows, blocky lava flows, debris flows, and limited ash falls. With the exception of a M=4 earthquake near the volcano one month earlier, only seismic activity related to the eruption onset was registered. Following only 7 hours of seismic tremor, the paroxysmal eruption began at 0912 h on November 3, 2002 with a sustained column that ascended to 17 km and five pyroclastic flows, that traveled as much as 9 km to the east. By mid-afternoon ash falls of 1-10 mm thickness began blanketing the Interandean Valley near Quito. The economic impact was significant, including severe damage to the principal petroleum pipelines, closure of schools, businesses, and Quito's airport for 10 days. It is important to conclude that for those volcanoes that are characterized by low silica, volatile-rich, fluid magmas, magma ascent can be aseismic, rapid, and without much warning. This event should serve as a clear reminder to scientists, civil defense, and authorities of the rapid onset of the eruptions of some volcanoes.

Colonization of an island volcano, Long Island, Papua New Guinea, and an emergent island, Motmot, in its caldera lake. V. Colonization by figs (Ficus spp.), their dispersers and pollinators

Abstract Aim This study considered the colonization of disturbed island ecosystems by Ficus species (Moraceae). Specifically, we examined the extent of colonization (compared with source areas), differential roles of seed dispersing birds and mammals, and the Ficus fruit characters influencing mode of colonization. Location Research was conducted on Long Island (5°20′S; 147°10′E), a volcanic island 55 km from Papua New Guinea, which erupted catastrophically in c. 1645 with probable extirpation of all life. Renewed volcanic activity in the early 1950s produced a series of temporary islands in the volcano's caldera lake. One island, Motmot, present since 1968 has persisted. Methods Long Island and Motmot were surveyed over 15 days for Ficus species and their vertebrate dispersers and pollinating wasps. The Ficus community was compared with that of mainland sources areas and Ficus species present were characterised according to their figs' size, colour, crop size and height. Observations of frugivory and literature records were used to determine Ficus dispersal guild membership. Results At least 31 Ficus species have colonized Long Island since its eruption. Evidence of pollinator wasp colonisation was found for all sixteen Ficus species observed fruiting. Thirty‐six vertebrate species occurring on Long Island are identified as potential seed dispersers. Of these, fruit bats and pigeons are likely to have been instrumental in the island's early colonization with subsequent spread being facilitated by these frugivores as well as a number of smaller birds, and an introduced cuscus. Comparisons of fruit characters and frugivore attraction between Ficus species reveal two broad dispersal guilds. Members of the first guild produce relatively large, green figs in the lower strata of the forest and attract fruit bats. The second guild includes species that attract both birds and fruit bats with generally smaller, red figs produced throughout the vertical structure of the forest. Eight Ficus species have colonized Motmot, a 31‐year‐old emergent island in the volcano's crater lake. However, only one furgivore species was recorded alive on the island. Fig seeds are likely to have arrived during rare over‐flights or roosting visits by frugivores, or in the bodies of prey brought to Motmot by raptors. We found no evidence of pollinator presence on Motmot. Most Ficus individuals on Motmot remain immature and the figs of the only three individuals observed with crops had not been pollinated. Main conclusions Their numerical abundance and frequency of fruiting make figs an important resource in regenerating ecosystems. Ficus species differ in their ability to colonise degraded habitats because of differences in the design and presentation of their figs. Fruit bats are likely to be of special importance early in succession. Once the Ficus species on Motmot start to produce fruit regularly we can expect a rapid increase in the numbers of fleshy‐fruited plant species colonising the island.

Colonization of an island volcano, Long Island, Papua New Guinea, and an emergent island, Motmot, in its caldera lake. I. General introduction

Abstract Biogeographical context Long Island, in Vitiaz Strait, is 55 km north of New Guinea, 60 km from Umboi Island and 125 km from New Britain. After its explosive caldera‐forming eruption in about 1645, Long is being recolonized by animals and plants. Through renewed volcanic activity in the 1950s a new island emerged from Long's caldera lake, 4 km from the nearest lake shore and was recreated by eruptions in 1968. Long Island thus provides the opportunity to study a nested pair of natural colonization sequences.The geological background, eruptive history, course and results of the seventeenth century eruption, and the geographical features and climate of Long Island are summarized. Existing knowledge of Long's recolonization, confined almost entirely to surveys of its avifauna in 1933 and 1972, is reviewed. The geological history of Motmot is outlined, and published knowledge of its colonization by animals and plants from 1968 to 1988 is summarized. The 1999 expedition and aims An expedition to Long Island and Motmot in 1999 set out to investigate the hitherto little‐known flora and present vertebrate fauna of Long Island and to survey the entire flora and fauna of Motmot for comparison with the results of previous surveys. The methods used in the 1999 survey are described, and the papers setting out the results briefly introduced.

Neogene tectonics and basin fill patterns in the Hellenic outer‐arc (Crete, Greece)

Six time‐slice reconstructions in the form of palaeogeographical maps show the large‐scale tectonic and sedimentary evolution of the Hellenic outer‐arc basins in central and eastern Crete for the middle and late Miocene. The reconstructions are based on extensive field mapping and a detailed chronostratigraphy.Latest compressional features related to subduction and associated crustal thickening are poorly dated and assigned a middle Miocene age. These are possibly contemporaneous with widespread occurrence of breccia deposits all over Crete. The precise date for the onset of extension, possibly controlled by the roll‐back of the subsiding African lithosphere, remains at this point a discussion. We present circumstantial evidence to place the beginning of the roll back in the middle Miocene, during the accumulation of an arc‐parallel, westward‐draining fluvial complex. The continental succession is transgressed steadily until it is interrupted by an important tectonic event at the boundary of the middle and late Miocene (normally seen as the onset of slab roll‐back). In the earliest late Miocene a few large‐sized fault blocks along arc‐parallel normal faults subsided rapidly causing a deepening of the half‐graben basins up to approximately 900 m. About 1 Myr later, a new N020E and N100E fault system developed fragmenting the existing half‐grabens into orthogonal horst and graben structure. The development of the new fault system caused original continental regions to subside and original deep basins to emerge, which is not easy to reconcile with roll back controlled extensional processes alone. Underplating and inherited basement structure may have played here an additional role, although evidence for firm conclusions is lacking. In late Miocene times (late Tortonian, ≈7.2 Ma), the extensional outer arc basins become deformed by N075E‐orientated strike‐slip. The new tectonic regime begins with strong uplift along existing N100E fault zones, which developed about E–W‐striking topographical highs (e.g. Central Iraklion Ridge and Anatoli anticline) in about 0.4 Myr. The strong uplift is contemporaneous with abundant landsliding observed along an important N075E fault zone crossing eastern Crete and with renewed volcanic activity of the arc. The origin of the ridges may be due to active folding related to the sinistral slip.

The volcanic, sedimentologic, and paleolimnologic history of the Crater Lake caldera floor, Oregon:Evidence for small caldera evolution

Apparent phreatic explosion craters, caldera-floor volcanic cones, and geothermal features outline a ring fracture zone along which Mount Mazama collapsed to form the Crater Lake caldera during its climactic eruption about 6,850 yr B.P. Within a few years, subaerial deposits infilled the phreatic craters and then formed a thick wedge (10-20 m) of mass flow deposits shed from caldera walls. Intense volcanic activity (phreatic explosions, subaerial flows, and hydrothermal venting) occurred during this early postcaldera stage, and a central platform of subaerial andesite flows and scoria formed on the caldera floor. Radiocarbon ages suggest that deposition of Iacustrine hemipelagic sediment began on the central platform about 150 yr after the caldera collapse. This is the minimum time to fill the lake halfway with water and cover the platform assuming present hydrologic conditions of precipitation and evaporation but with negligible leakage of lake water. Wizard Island formed during the final part of the 300-yr lake-filling period as shown by its (1) upper subaerial lava flows from 0 to -70 m below present water level and lower subaqueous lava flows from -70 to -500 m and by (2) lacustrine turbidite sand derived from Wizard Island that was deposited on the central platform about 350 yr after the caldera collapse. Pollen stratigraphy indicates that the warm and dry climate of middle Holocene time correlates with the early lake deposits. Diatom stratigraphy also suggests a more thermally stratified and phosphate-rich environment associated respectively with this climate and greater hydrothermal activity during the early lake history. Apparent coarse-grained and thick-bedded turbidites of the early lake beds were deposited throughout northwest, southwest, and eastern basins during the time that volcanic and seismic activity formed the subaqueous Wizard Island, Merriam Cone, and rhyodacite dome. The last known postcaldera volcanic activity produced a subaqueous rhyodacite ash bed and dome about 4,240 yr B.P. The late lake beds with base-of-slope aprons and thin, fine-grained basin-plain turbidites were deposited during the volcanically quiescent period of the past 4,000 yr. Deposits in Crater Lake and on similar caldera floors suggest that four stages characterize the postcaldera evolution of smaller (≤10 km in diameter) terrestrial caldera lake floors: (1) initial-stage caldera collapse forms the ring fracture zone that controls location of the main volcanic eruptive centers and sedimentary basin depocenters on the caldera floor; (2) early-stage subaerial sedimentation rapidly fills ring-fracture depressions and constructs basin-floor debris fans from calderawall landslides; (3) first-stage subaqueous sedimentation deposits thick flat-lying lake turbidites throughout basins, while a thin blanket of hemipelagic sediment covers volcanic edifices that continue to form concurrently with lake sedimentation; and (4) second-stage subaqueous sedimentation after the waning of major volcanic activity and the earlier periods of most rapid sedimentation develops small sili-ciclastic basin base-of-slope turbidite aprons and central basin plains. Renewed volcanic activity or lake destruction could cause part or all of the cycle to repeat.

Paleomagnetism and tectonics of Malaita, Solomon Islands

Malaita, in the Solomon Islands, is the emergent expression of the Pacific Province, a geologically distinct suspect terrane which is commonly held to be the margin of the Ontong Java Plateau, a thickened portion of Pacific plate overthrust onto the rest of the Solomon Islands Arc during a late Miocene reversal of the polarity of the arc. Subduction of the Australian plate replaced Pacific plate subduction during this event, and the Solomon Islands Arc was transferred from the Australian plate to the Pacific plate. A paleomagnetic survey of the eastern Solomon Islands included sampling in southern Malaita. Magnetostratigraphic dating gives an age of 5.8 Ma for the Haruta Calcisiltite‐Hada Calcisiltite facies change on Malaita. This facies change resulted from renewed volcanic activity following the initiation of Australian plate subduction. Paleomagnetic poles from two pre‐Pliocene units do not lie near the Pacific apparent polar wander path (APWP), as the conventional model would suggest, but rather fall on the Australian APWP, suggesting that the Pacific Province developed as a part of the Australian plate. Correction for 6.5 Ma of Pacific‐Australia relative motion produces an improved, very close fit to the Australian APWP, consistent with the transfer of Malaita, along with the rest of the arc, to the Pacific plate at about that time. Magnetic paleolatitudes from a DSDP hole on the Ontong Java Plateau, which disagree with usually accepted Pacific Tertiary motion, also fail to match the Malaitan data; the possibility that Malaita could be a part of an Ontong Java sub‐plate, somehow detached from the rest of the Pacific plate, can be discounted. Malaita and the rest of the Pacific Province arose in the fore‐arc of the originally north‐east facing arc. Collision of the Ontong Java Plateau with the trench slowed subduction of the Pacific plate and began subduction of the Australian plate below the arc. Both subduction zones may have been active for a period, beginning at about 10 Ma and ending at about 3 Ma, until Pacific plate subduction broke down, and the Pacific Province was subsequently uplifted and deformed.

Volcanic hazard assessment in the Phlegraean Fields: a contribution based on stratigraphic and historical data

Phenomena occurring since 1982 in the Phlegraean Fields, interpreted as precursors of a potential renewal of volcanic activity, have forced us to anticipate some conclusions of a volcanic-hazard study based on the reconstruction of past eruptions in the area, to serve as basis for civil defense preparedness plans. The eruptive history of the Phlegraean Fields suggests a progressive decrease with time in the strength of eruptive phenomena paralleling a migration of vents towards the center of the Phlegraean caldera. Studies concerning the volcanic risk zonation were therefore concentrated on activities during the last 4,500 years and two eruptions (Monte Nuovo and Agnano Monte Spina), that occurred in 1538 and 4,400 years B.P., respectively were selected as the «reference eruptions» from which possible eruption scenarios were drawn.

Earthquakes at depth beneath volcanoes, forerunners of their activities. Application to White Island, New Zealand

Abstract The concept of a time-depth correlation between tectonic earthquakes at depth beneath some volcanoes, and their eruptions, developed by the author since 1962, has been confirmed by new observations and successful prediction of renewed volcanic activity in New Zealand. Regular earthquake migrations are observed along the Benioff zone, and volcanic eruptions are found to be related to these seismic migrations beneath the volcanoes, as follows: Therefore, in island arcs and continental margins, volcanic activity is the result of two processes occurring beneath the volcanoes: (1) a “tectonic process”, a migration of strain release along the downgoing lithosphere, of which the earthquakes are the manifestation; (2) a “magmatic process”, a relatively fast vertical ascent of magmatic material from the deep root of the volcano, where the observed shocks may be the starting signal from this level. The rate of migration of tectonic earthquakes increases with depth in the upper mantle. An empirical time relationship between the earthquakes occurring at depth beneath a volcano and its eruptions, has been successfully tested for renewed activity at White Island in New Zealand, over the period 1977–1978.

Study of Renewed Volcanic Activity in Ecuador

Study of Renewed Volcanic Activity in Ecuador

Evolution of the central Rio Grande rift, New Mexico: New potassium-argon ages

New K Ar age determinations on mid-Oligocene to Pleistocene volcanic and shallow intrusive rocks from the central Rio Grande rift permit a more detailed understanding of the tectonic and magmatic history of the rift. Initial extension in the region of the central rift may have begun prior to 27 m.y. ago. By 25 m.y. ago broad basins existed and were filling with volcaniclastic sediments derived mainly from volcanic centers in the San Juan and Questa areas. Continued tectonic activity narrowed these basins by 21-19 m.y. ago, indicated in the Santa Fe area by tilting and faulting that immediately postdate 20-m.y.-old latite. Uplift of the Sangre de Cristo, Sandia, and Nacimiento Mountains shed clastic debris of the Santa Fe Group into these basins. Early rift magmatism is characterized by an overlap of mid-Tertiary intermediate intrusive and extrusive activity, extending to 20 m.y. ago, with mafic and ultramafic volcanism, ranging from 25 to 19 m.y. Both volcanism and tectonic activity were minimal during the middle Miocene. About 13 m.y. ago renewed volcanic activity began. Tectonism commenced in the late Miocene, resulting in the present, narrow grabens. The term “Rio Grande rift” should be restricted to these grabens formed during post-mid-Miocene deformation. Widespread eruption of tholeiitic and alkali olivine basalts occurred 3-2 m.y. ago. The Rio Grande drainage system was integrated 4.5-3 m.y. ago, leading to the present erosional regime. These intervals of deformation and magmatism correspond generally with a similar sequence of events in the Basin and Range province south of the Colorado Plateau. This similarity indicates that the Rio Grande rift is not a unique structure in the southwestern U.S., and must be related to the larger context of the entire Basin and Range province.

Mid-Atlantic Ridge: Age and Spreading Rates

Fission-track dating of basaltic glass from the Mid-Atlantic Ridge gives results which are consistent with the proposal of ocean-floor spreading. Solidification ages from ~10,000 years to ~300,000 years were measured. Correlation is also possible between the magnetic anomaly patterns over the Crest Mountains at 45 degrees N and the geochronology of the outcropping basalts. Renewed volcanic activity well removed from the axis of the Mid-Atlantic Ridge has been demonstrated to have taken place in recent times.

GEOLOGY OF THE UINKARET PLATEAU, NORTHERN ARIZONA

The Uinkaret Plateau, a major block of the Colorado Plateau province in northern Arizona, is bounded on the west by the Hurricane Ledge, developed along the Hurricane fault, and on the east by scarps developed along the Toroweap and Sevier faults. Over the southern part there are extensive outpourings of lava which fall into four age groups. The oldest eruptions preceded earliest movement along the Hurricane fault and antedated cutting of the present Grand Cayon. These lavas poured out on a relatively even surface believed to have been a large pediment or series of pediments extending at least 16 miles north from the Colorado River. Extensive dissection of the oldest lavas preceded the next eruptions, which took place at considerably lower elevations. The Inner Gorge of Grand Canyon was cut to approximately its present depth and later filled to a height of at least 600 feet and perhaps 1,200 by lava floods from Toroweap Valley. The lava dams were entirely removed before later flows, pouring into the Inner Gorge from the rim of the Esplanade, again dammed the river, which has removed all but small portions of these dams. Repeated movements along the Toroweap fault have occurred in post-lowan-Wisconsin time. In the very recent past, renewed volcanic activity formed a single small cone and lava flow.