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USGS1977

Paleozoic-Mesozoic boundary in the Berry Creek Quadrangle, northwestern Sierra Nevada, California

Structural and petrologic studies in the Berry Creek quadrangle at the north end of the western metamorphic belt of the Sierra Nevada have yielded new information that helps in distinguishing between the chemically similar Paleozoic and Mesozoic rocks. The distinguishing features are structural and textural and result from different degrees of deformation. Most Paleozoic rocks are strongly deformed and thoroughly recrystallized. Phenocrysts in meta volcanic rocks are granulated and drawn out into lenses that have sutured outlines. In contrast, the phenocrysts in the Mesozoic metavolcanic rocks show well-preserved straight crystal faces, are only slightly or not at all granulated, and contain fewer mineral inclusions than do those in the Paleozoic rocks. The groundmass in the Paleozoic rocks is recrystallized to a fairly coarse grained albite-epidote-amphibole-chlorite rock, whereas in the Mesozoic rocks the groundmass is a very fine grained feltlike mesh with only spotty occurrence of well-recrystallized finegrained albite-epidote-chlorite-actinolite rock. Primary minerals, such as augite, are locally preserved in the Mesozoic rocks but are altered to a mixture of amphibole, chlorite, and epidote in the Paleozoic rocks. In the contact aureoles of the plutons, and within the Big Bend fault zone, which crosses the area parallel to the structural trends, all rocks are thoroughly recrystallized and strongly deformed. Identification of the Paleozoic and Mesozoic rocks in these parts of the area was based on the continuity of the rock units in the field and on gradual changes in microscopic textures toward the plutons.

Professional Paper↗

USGS1918

Baked shale and slag formed by the burning of coal beds

The baking and reddening of large masses of strata caused by the burning of coal beds is a striking feature of the landscape in most of the great western coal-bearing areas. The general character and broader effects of the burning have been described by many writers, but the fact that in places enough heat is generated to fuse and thoroughly recrystallize the overlying shale and sandstone has received less attention. Some of the natural slags thus formed simulate somewhat abnormal igneous rocks, but others consist largely of rare and little known minerals. A wide range in the mineral composition of such slags is to be expected, depending on the composition of the original sediment and the conditions of fusion and cooling. These products of purely thermal metamorphism offer a fertile field for petrologic investigation. The writer has observed the effects produced by the burning of coal beds in several localities in Montana, particularly along upper Tongue River in the southern part of the State, in the district lying southeast of the mouth of Bighorn River, and in the Little Sheep Mountain coal field north of Miles City. A number of specimens of the rock formed have been examined under the microscope, though time has not been available for a systematic examination. The writer is greatly indebted to Mr. E. S. Larsen for assistance in the study of some of the minerals.

Montana↗

USGS1980

The Boulder Creek Batholith, Front Range, Colorado

The Boulder Creek batholith is the best known of several large Precambrian batholiths of similar rock composition that crop out across central Colorado. The rocks in the batholith belong to the calc-alkaline series and range in composition from granodiorite through quartz diorite (tonalite) to gneissic aplite. Two rock types dominate': the Boulder Creek Granodiorite, the major rock unit, and a more leucocratic and slightly younger unit herein named Twin Spruce Quartz Monzonite. Besides mafic inclusions, which occur mainly in hornblende-bearing phases of the Boulder Creek Granodiorite, there are cogenetic older and younger lenses, dikes, and small plutons of hornblende diorite, hornblendite, gabbro, and pyroxenite. Pyroxenite is not found in the batholith. The Boulder Creek Granodiorite in the batholith represents essentially two contemporaneous magmas, a northern body occurring in the Gold Hill and Boulder quadrangles and a larger southern body exposed in the Blackhawk and the greater parts of the Tungsten and Eldorado Springs quadrangles. The two bodies are chemically and mineralogically distinct. The northern body is richer in CaO and poorer in K2O, is more mafic, and has a larger percentage of plagioclase than the southern body. A crude sequence of rock types occurs from west to east in the batholith accompanied by a change in plagioclase composition from calcic plagioclase on the west to sodic on the east. Ore minerals tend to decrease, and the ratio potassium feldspar:plagioclase increases inward from the western contact of the batholith, indicating that the Boulder Creek batholith is similar to granodiorite batholiths the world over. Emplacement of the Boulder Creek batholith was contemporaneous with plastic deformation and high-grade regional metamorphism that folded the country rock and the batholith contact along west-northwest and north-northwest axes. Also, smaller satellitic granodiorite bodies tend to conform to the trends of foliation and fold axes in the country rock, suggesting that emplacement was controlled by preexisting structures in the country rock. On a gross scale, chemical equilibrium in the Boulder Creek Granodiorite is expressed by a near 1:1 ratio, or straight-line relationship in the distribution of iron, magnesium, and manganese in biotite and hornblende. General mineralogical trends in the Boulder Creek Granodiorite indicate that modal biotite, hornblende, and plagioclase tend to increase and quartz and microcline tend to decrease as CaO increases. These trends were not found in the Twin Spruce Quartz Monzonite. Differentiation is believed to have played a major role and assimilation a minor role in the development of the Boulder Creek batholith. The Boulder Creek Granodiorite is of probable mantle or lower crust origin, and, based on the scant data available, the Twin Spruce Quartz Monzonite may be of crustal origin, but the magma was extensively altered by contaminants of ambiguous origin. Mafic inclusions, possibly derived from a dioritic magma which was an early differentiate associated temporally with the Boulder Creek Granodiorite and (or) the Twin Spruce Quartz Monzonite, were in jected into the Boulder Creek Granodiorite during the mush stage and before the batholith was completely crystallized. Biotite, hornblende, and potassium feldspar were studied extensively. Their chemistry and petrology indicate a homogeneity throughout the batholith not believed possible by a casual observance of the batholithic rocks in the field. The accessory minerals, where investigated, also tend to indicate this same pervasive homogeneity.

Professional Paper↗

USGS1930

Geologic history of the Yosemite Valley

The problem of the origin of the Yosemite Valley inherently demands a solution in quantitative terms. Its essence is, To what extent is the valley a product of glacial action, to what extent a product of stream erosion? The principal result of the investigations upon which this report is based is the determination within narrow limits of the preglacial depth of the Yosemite Valley and of other facts concerning its preglacial development which permit fairly definite estimates of the proportionate shares of work performed by stream and by glacier. The investigations comprise a detailed survey of the glacial and geomorphologic features of the Yosemite region and an equally intensive study of its rock formations, supplemented by reconnaissance work of both kinds in adjoining parts of the Sierra Nevada. The petrologic studies were made by Frank C. Calkins; the glacial and geomorphologic survey by Francois E. Matthes. Detailed mapping of the morainal system of the ancient Yosemite Glacier has served not only to determine the farthest limits reached by that glacier but to throw new light on the significance of the hanging valleys of the Yosemite region. It is reasonably certain that the glacier never extended more than about a mile beyond the site of El Portal. The hanging side valleys of the Merced Canyon below El Portal, therefore, hang not because of any glacial deepening suffered by that canyon. The explanation is offered that they hang because their streamlets have been unable to trench as rapidly as the Merced River since the rejuvenation of the Merced by the last uptilting of the Sierra Nevada. The streamlets were handicapped not only by their comparatively small volume but also by the fact that their courses trend northwestward and southeastward, substantially at right angles to the direction of the tilting, and therefore have remained essentially unsteepened, whereas the Merced's course trends southwestward, directly down the slope of the Sierra block, and therefore has been appreciably steepened. Projection of the longitudinal profiles of these hanging valleys forward to the axis of the Merced Canyon shows that they are closely accordant in height. Their profiles indicate a series of points on a former profile of the Merced with respect to which the side streams had graded their courses prior to the last uplift. This old profile can be extended upward into the glaciated part of the Merced Canyon above El Portal and even into the profoundly glaciated Yosemite Valley, accordant points being furnished by a number of hanging side valleys (due allowance being made for glacial erosion suffered by those valleys). However, not all the hanging valleys of the Yosemite region are accordant with this set. Several of them, including the upland valley of Yosemite Creek, constitute a separate set indicating another old profile of the Merced at a level 600 to 1,000 feet higher than the first. Others, including the hanging gulch of lower Bridalveil Creek, point to an old profile of the Merced about 1,200 feet lower than the first. There are thus three distinct sets of hanging valleys produced in three cycles of stream erosion. The valleys of the upper set, like those of the middle set, were left hanging as a result of rapid trenching by the Merced induced by an uplift of the range, there having been two such uplifts. Only the valleys of the lower set hang because of glacial deepening and widening of the Yosemite Valley, the cycle in which they were cut having been interrupted by the advent of the Pleistocene glaciers. They consequently indicate the preglacial depth of the Yosemite Valley. That depth, measured from the brow of El Capitan, was about 2,400 feet; measured from the rim at Glacier Point it was about 2,000 feet.

California↗

USGS2007

Geological studies of the Salmon River suture zone and adjoining areas, west-central Idaho and eastern Oregon

The papers in this volume describe petrologic, structural, and geochemical studies related to geographic areas adjacent to and including the Salmon River suture zone. We therefore start this volume by defining and giving a general description of that suture zone. The western margin of the North American continent was the setting for complex terrane accretion and large-scale terrane translation during Late Cretaceous and Eocene time. In western Idaho, the boundary that separates the Paleozoic-Mesozoic accreted oceanic, island-arc rocks on the west from Precambrian continental metamorphic and sedimentary rocks on the east is called the Salmon River suture zone (SRSZ). Readers will note that the term 'Salmon River suture zone' is used in the title of this volume and in the text of several of the papers and the term 'western Idaho suture zone' is used in several other papers in this volume. Both terms refer to the same geologic feature and reflect historical usage and custom; thus no attempt has been made by the editors to impose or demand a single term by the various authors of this volume. The suture zone is marked by strong lithologic and chemical differences. Rocks adjacent to the suture zone are characterized by high-grade metamorphism and much structural deformation. In addition, the zone was the locus of emplacement of plutons ranging in composition from tonalite to monzogranite during and after the final stages of accretion of the oceanic terrane to the North American continent. The contents of this paper consists of seven chapters.

Idaho, Oregon, Washington↗

USGS2008

Identification and evolution of the juvenile component in 2004-2005 Mount St. Helens ash

Petrologic studies of volcanic ash are commonly used to identify juvenile volcanic material and observe changes in the composition and style of volcanic eruptions. During the 2004-5 eruption of Mount St. Helens, recognition of the juvenile component in ash produced by early phreatic explosions was complicated by the presence of a substantial proportion of 1980-86 lava-dome fragments and glassy tephra, in addition to older volcanic fragments possibly derived from crater debris. In this report, we correlate groundmass textures and compositions of glass, mafic phases, and feldspar from 2004-5 ash in an attempt to identify juvenile material in early phreatic explosions and to distinguish among the various processes that generate and distribute ash. We conclude that clean glass in the ash is derived mostly from nonjuvenile sources and is not particularly useful for identifying the proportion of juvenile material in ash samples. High Li contents (>30 μg/g) in feldspars provide a useful tracer for juvenile material and suggest an increase in the proportion of the juvenile component between October 1 and October 4, 2004, before the emergence of hot dacite on the surface of the crater on October 11, 2004. The presence of Li-rich feldspar out of equilibrium (based on Liplagioclase/melt partitioning) with groundmass and bulk dacite early in the eruption also suggests vapor enrichment in the initially erupted dacite. If an excess vapor phase was, indeed, present, it may have provided a catalyst to initiate the eruption. Textural and compositional comparisons between dome fault gouge and the ash produced by rockfalls, rock avalanches, and vent explosions indicate that the fault gouge is a likely source of ash particles for both types of events. Comparison of the ash from vent explosions and rockfalls suggests that the fault gouge and new dome were initially heterogeneous, containing a mixture of conduit and crater debris and juvenile material, but became increasingly homogeneous, dominated by juvenile material, by early January 2005.

Washington↗

USGS2008

Trace element and Pb isotope composition of plagioclase from dome samples from the 2004-2005 eruption of Mount St. Helens, Washington

We report the results of in-situ laser ablation ICP–MS analyses of anorthite content, trace-element (Li, Ti, Sr, Ba, La, Pr, Ce, Nd, Eu, Pb) concentrations, and Pb-isotope compositions in plagioclase from eight dome-dacite samples collected from the 2004-5 eruption of Mount St. Helens and, for comparison, from three dome samples from 1981-85. For 2004-5 samples, plagioclase phenocrysts range in composition from An 30 to An 80 , with the majority An 42 -An 65 . With the exception of Li, the range of trace-element abundances in plagioclase phenocrysts is largely constant in material erupted between October 2004 and April 2005 and is broadly consistent with the 1983-85 dome samples. Anomalously high Li contents in the early stage of the eruption are thought to reflect addition of Li to the upper part of the magma chamber immediately before eruption (within ~1 year) by transfer of an alkali-enriched, exsolved vapor from deep within the magma chamber. Other trace elements show significant correlations (at >99 percent confidence limits) with anorthite content in plagioclase phenocrysts--Ba, light rareearth elements (LREE), and Pb show positive correlations, whereas Ti and Sr correlate negatively. Variations in plagioclase-melt partitioning as a function of anorthite content cannot explain trace-element variations--in particular predicting trends for Ti and Sr opposite to those observed. A simple model involving closed-system fractional crystallization of plagioclase + hypersthene + amphibole + oxides largely reproduces the observed trends. The model requires no gain or loss of plagioclase and is consistent with the lack of europium anomalies in bulk dacite samples. Analytical traverses within individual plagioclase phenocrysts support this model but also point to a diversity of melt compositions present within the magma storage zone in which plagioclase crystallized. Plagioclase crystals from gabbronorite inclusions in three dacite samples have markedly different trace-element and Pbisotope compositions from those of plagioclase phenocrysts, despite having a similar range of anorthite contents. Inclusions show some systematic differences from each other but typically have higher Ti, Ba, LREE, and Pb and lower Sr and have lower 208 Pb/ 206 Pb and 207 Pb/ 206 Pb ratios than coexisting plagioclase phenocrysts. The compositions of plagioclase from inclusions cannot be related to phenocryst compositions by any reasonable petrologic model. From this we suggest that they are unlikely to represent magmatic cumulates or restite inclusions but instead are samples of mafic Tertiary basement from beneath the volcano.

Washington↗

USGS2010

The 2006 eruption of Augustine Volcano, Alaska

Augustine Volcano, the most historically active volcano in Alaska’s Cook Inlet region, again showed signs of life in April 2005. Escalating seismic unrest, ground deformation, and gas emissions culminated in an eruption from January 11 to mid-March of 2006, the fifth major eruption in 75 years. The eruption began with a series of 13 short-lived blasts over 20 days that sent pyroclastic flows; snow, rock, and ice avalanches; and lahars down the volcano’s snow clad flanks; ash clouds drifted hundreds of kilometers downwind. Punctuated explosive activity gave way to effusion of lava and emplacement of thick block-and-ash flows on the volcano’s north flank that continued through mid-February. In mid-March renewed extrusion resulted in the building of a new, higher summit lava dome and two blocky lava flows on the north and northeast flanks of the cone. The eruption resulted in ash fall on many south-central Alaskan communities and disrupted air traffic in the region. Augustine’s frequent eruptions and relatively easy access have long drawn volcanologists to study the accumulation, ascent, and eruption of andesitic to dacitic magma. Studies of the most recent activity before 2006, in 1976 and 1986, revealed that the volcano lately produces explosive eruptions that are preceded by months of unrest and injection of new magma into a storage region in the upper several kilometers of the crust. Each of these eruptions then followed a similar progression from explosive to effusive behavior over several months. Petrologic and geophysical observations suggest that these three eruptions were triggered by similar magma mixing events and that the subsequent ascent and eruption of magma was governed by processes that were roughly constant from one eruption to the next. Geologic studies of the island show that in the more distant past parts of Augustine’s edifice have failed repeatedly, resulting in debris avalanches that entered the sea and, at least once, in 1883, caused a tsunami that hit surrounding Cook Inlet coastlines. Such edifice failures and resultant local tsunamis should be expected in the future. Recognition of Augustine’s frequent activity and hazardous nature led to the installation of a network of telemetered seismometers beginning in 1971, the establishment of a geodetic network in 1988, and the installation of other new instrumentation such as pressure sensors, broadband seismometers, and cameras by the Alaska Volcano Observatory (AVO), and the selection of Augustine for geodetic instrumentation through the EarthScope/Plate Boundary Observatory program in 2004. In addition, remote sensing techniques, such as airborne thermal imaging and the advanced spaceborne thermal emission and reflection radiometer (ASTER), provided novel and often critical information as the 2006 eruption progressed. The combination of a long-term seismic network and an array of new monitoring techniques has provided a breadth and depth of understanding of Augustine’s most recent activity that was not possible in the past. This volume contains 28 chapters reporting on a diverse suite of new scientific observations and investigations that were motivated by the 2006 eruption. Understanding the magmatic processes that drive eruptions, identifying eruptive events, tracking the movement of ash clouds, and communicating the resultant hazards to other government agencies and the public are all critical tasks for AVO, and chapters touch upon all of these topics. One goal in this compilation is to synthesize the diverse information into as complete an understanding of the magmatic and eruptive processes as possible. An equally important goal is to provide a framework for diagnosing periods of unrest and formulating forecasts of eruptions that will certainly take place at Augustine in the future. This latter goal is especially important, as Augustine’s frequent eruptive activity suggests that another eruption can be expected within the next several decades. Consequently, the investigations in this volume are intended to provide both a means to better forecast future eruptive episodes and also an opportunity to formulate and test future hypotheses for magmatic and eruptive processes. Future eruptions may follow a course similar to those observed in 1976, 1986, and 2006. However, a major perturbation that upsets conditions within the magmatic system could occur, owing perhaps to the rise of a much larger or different parental magma or to a large edifice failure similar to the 1883 sector collapse. In such events, the comprehensive study of past eruptions will provide data critical to assessing the current state of the magmatic system. In assembling this volume we have sought as consistent and accurate a portrayal of the 2006 eruption as possible. We have asked all authors to refer to the same basic eruption chronology, unless their observations and data require alternative explanations. Naturally, not all techniques or methodologies produce a completely consistent set of observations, nor do the precise conclusions in every paper support one another. We have grouped chapters on the basis of discipline. Papers that focus on specific techniques, methodology, or instrumentation are placed throughout the volume where they best fit with others that rely on their results.

Alaska↗

USGS2010

Augustine Volcano - The influence of volatile components in magmas erupted A.D. 2006 to 2,100 years before present: Chapter 16 in The 2006 eruption of Augustine Volcano, Alaska

The petrology and geochemistry of 2006 eruptive products of Augustine Volcano, Alaska, have been investigated through analyses of whole-rock samples, phenocrysts, silicate melt inclusions, and matrix glasses to constrain processes of magma evolution, eruption, and degassing. Particular attention was directed toward the concentrations and geochemical relationships involving the magmatic volatile components H 2 O, CO 2, S, and Cl. The analytical results for 2006 samples have been integrated with data for samples of Pleistocene basalt, prehistoric andesites, and 1986 andesites from Augustine to provide a broad view of volatile behavior in Augustine magmas. The observation of generally similar geochemical features for this range of eruptions indicates that magmatic and volatile degassing processes have been relatively consistent during the past 2,100 years. Augustine andesites range from low-silica to high-silica compositions and contain phenocrysts of plagioclase, orthopyroxene, and clinopyroxene, with lesser olivine, amphiboles, iron-titanium oxides, and apatite. The groundmass varies from strongly crystallized and/or oxidized to comparatively clear, microlite-poor vesicular glass. Coexisting iron-titanium oxides of 2006 rock samples, which are generally consistent with those of prior eruptive materials, indicate &fnof; O2 values of approximately NNO+1.5 to NNO+2.5 and oxide crystallization temperatures of 835 to 1,052°C. The compositions of matrix and melt-inclusion glasses range from rhyodacite to rhyolite and show relationships that reflect magma evolution involving fractional crystallization and multiple stages of mingling and/or mixing. In particular, melt inclusions of low-silica andesites express mixing of magmas with more widely varying compositions, than do melt inclusions of high-silica andesites and dacites. The melt inclusions of 2006, 1986, and prehistoric andesites contain moderate to high concentrations of H 2 O and Cl and lesser CO 2 and SO 2 . Comparing the abundances of H 2 O, CO 2 , and Cl in these melt inclusions with experimentally established volatile solubilities for felsic melts indicates that the 2006 and prehistoric samples are most consistent with the ascent of fluid-saturated magmas containing 1 weight percent of H 2 O-enriched vapor under closed-system conditions and that pressures of volatile phase exsolution range from 150 to less than 20 MPa. This closed-system behavior was maintained to quite shallow depths prior to eruption, and this pressure range is consistent with constraints derived from 2006 geodetic measurements indicating magma storage and crystallization at 4 to 6 km and upwards to near-surface depths. The magmatic fluids were relatively oxidizing and included H 2 O-enriched and HCl-, H 2 S-, S 2 -, and SO 2 ± CO 2 -bearing vapors; hydrosaline aqueous liquids largely enriched in Cl - , SO 4 2- , alkalis, and H 2 O; and moderately saline, H 2 O-poor liquids containing Cl - , SO 4 2- , and alkali elements.

Alaska↗

USGS2010

Geodetic constraints on magma movement and withdrawal during the 2006 eruption of Augustine Volcano: Chapter 17 in The 2006 eruption of Augustine Volcano, Alaska

For the first time in the United States, a modern geodetic network of continuously recording Global Positioning System (GPS) receivers has measured a complete eruption cycle at a stratovolcano, Augustine Volcano in Alaska, from the earliest precursory unrest through the return to background quiescence. The on-island network consisted of five continuously recording, telemetered GPS stations, four continuously recording, nontelemetered stations, and about 10 campaign bench marks. The continuous network recorded several distinct and conspicuous signals over the course of the unrest and eruption, starting with a months-long precursory inflation centered beneath the volcano at around sea level. Nearly coincident with the highest volumetric eruption rates, this inflation gave way to a more deep seated deflation that we interpret as a major withdrawal (approx. 25 million m3 of compressed magma) from a nearly cylindrical magma reservoir centered about 5 km below sea level. Detailed analysis of the geodetic time series reveals additional nuance, including the probable upward propagation of a small dike into the edifice in the 60 days or so before the onset of large-scale explosive activity. Comparisons of the geodetic data and their resulting interpretations with other data, such as earthquake hypocenters and petrologically inferred magma-pressure histories, reveal a kinematic, if not mechanical, account of the 2006 eruption that details the shape and location of the magma source region, the means and velocity of magma transport, and the establishment of a short lived volume- (or pressure-) buffering capability held within the magma reservoir. The cumulative deformation over the course of the eruption shows a large signal close in and high on the edifice that decays rapidly with distance. This pattern indicates a small permanent increase in the edifice volume (beyond that added by the surficial lava dome) but also shows that without close-in (<2.5 km from the summit) stations, the eruption might have been invisible to campaign GPS stations alone.

Alaska↗

USGS2014

Geochronology of plutonic rocks and their tectonic terranes in Glacier Bay National Park and Preserve, southeast Alaska

We have identified six major belts and two nonbelt occurrences of plutonic rocks in Glacier Bay National Park and Preserve and characterized them on the basis of geologic mapping, igneous petrology, geochemistry, and isotopic dating. The six plutonic belts and two other occurrences are, from oldest to youngest: (1) Jurassic (201.6–145.5 Ma) diorite and gabbro of the Lituya belt; (2) Late Jurassic (161.0–145.5 Ma) leucotonalite in Johns Hopkins Inlet; (3) Early Cretaceous (145.5–99.6 Ma) granodiorite and tonalite of the Muir-Chichagof belt; (4) Paleocene tonalite in Johns Hopkins Inlet (65.5–55.8 Ma); (5) Eocene granodiorite of the Sanak-Baranof belt; (6) Eocene and Oligocene (55.8–23.0 Ma) granodiorite, quartz diorite, and granite of the Muir-Fairweather felsic-intermediate belt; (7) Eocene and Oligocene (55.8–23.0 Ma) layered gabbros of the Crillon-La Perouse mafic belt; and (8) Oligocene (33.9–23.0 Ma) quartz monzonite and quartz syenite of the Tkope belt. The rocks are further classified into 17 different combination age-compositional units; some younger belts are superimposed on older ones. Almost all these plutonic rocks are related to Cretaceous and Tertiary subduction events. The six major plutonic belts intrude the three southeast Alaska geographic subregions in Glacier Bay National Park and Preserve, from west to east: (1) the Coastal Islands, (2) the Tarr Inlet Suture Zone (which contains the Border Ranges Fault Zone), and (3) the Central Alexander Archipelago. Each subregion includes rocks assigned to one or more tectonic terranes. The various plutonic belts intrude different terranes in different subregions. In general, the Early Cretaceous plutons intrude rocks of the Alexander and Wrangellia terranes in the Central Alexander Archipelago subregion, and the Paleogene plutons intrude rocks of the Chugach, Alexander, and Wrangellia terranes in the Coastal Islands, Tarr Inlet Suture Zone, and Central Alexander Archipelago subregions.

Alaska↗

USGS2014

Two hundred years of magma transport and storage at Kīlauea Volcano, Hawai'i, 1790-2008

This publication summarizes the evolution of the internal plumbing of Kīlauea Volcano on the Island of Hawaiʻi from the first documented eruption in 1790 to the explosive eruption of March 2008 in Halemaʻumaʻu Crater. For the period before the founding of the Hawaiian Volcano Observatory in 1912, we rely on written observations of eruptive activity, earthquake swarms, and periodic draining of magma from the lava lake present in Kīlauea Caldera. After 1912 the written observations are supplemented by continuous measurement of tilting of the ground at Kīlauea’s summit and by a continuous instrumental record of earthquakes, both measurements made during 1912–56 by a single pendulum seismometer housed on the northeast edge of Kīlauea’s summit. Interpretations become more robust following the installation of seismic and deformation networks in the 1960s. A major advance in the 1990s was the ability to continuously record and telemeter ground deformation to allow its precise correlation with seismic activity before and after eruptions, intrusions, and large earthquakes. We interpret specific events in Kīlauea’s 200- year written history as steps in a broad transition from summit lava-lake activity in Kīlauea Caldera to shield building on the east rift zone. The ability of the magmatic plumbing to deliver magma to eruption is critical to the history of eruption and intrusion. When the rate of magma supply equals the rate of eruption, there is little ground deformation or intrusion. When the magma supply rate is greater than the rate of eruption, then the edifice responds through any or all of summit inflation, intrusion, increased spreading rate, and large flank earthquakes. In Kīlauea’s 200-year history we identify three regions of the volcano in which magma is stored and supplied from below. Source 1 is at 1-km depth or less beneath Kīlauea’s summit and fed Kīlauea’s summit lava lakes throughout most of the 19th century and again from 1907 to 1924. Source 1 was used up in the series of small Halemaʻumaʻu eruptions following the end of lava-lake activity in the summit collapse of 1924. Source 2 is the magma reservoir at a depth of 2–6 km beneath Kīlauea’s summit that has been imaged by seismic and deformation measurements beginning in the 1960s. This source was first identified in the summit collapses of 1922 and 1924. Source 3 is a diffuse volume of magma-permeated rock between 5 and 11 km depth beneath the east rift zone and above the near-horizontal decollement at the base of the Kīlauea edifice. Magma distribution within source 2 has been derived by combining petrologic study of the three chemically uniform summit eruptions of 1952, 1961, and 1967–68 and the east rift eruptions within this interval with both observation of migrating centers of inflation determined from leveling surveys conducted before the 1967–68 eruption and with published models of expected deformation from different source geometries. We adopt a model of concatenated magmatic plugs with nodes beneath the inflation centers. Addition of erupted and intruded volumes of the three summit magma batches yields a liquid magma volume of about 0.2 km3, with dimensions of ~1 km by 1 km by 200 m centered at about 3-km depth within source 2. Following the Halemaʻumaʻu eruption of 1967–68, the chemistry of magma coming into Kīlauea’s summit reservoir has changed frequently, and during the eruption that began in 1983, chemical changes have been subtle and continuous. In this period we interpret changes in chemistry as related to an increase in magma supply resulting from increased partial melting in an expanding mantle source volume. We know from instrumental recording of eruptions since the long Halemaʻumaʻu eruption in 1952 that stress in the edifice accumulates as magma is added underground and is relieved by eruption and by dilation of the rift zones associated with seaward movement (spreading) of Kīlauea’s south flank. During and after the last half of the 20th century, magma transfer to the rift zone has dominantly occurred from source 2. High rates of flank motion have been correlated with high rates of endogenous growth; alternatively, lower rates of motion have characterized periods when the underground magmatic plumbing was being refilled following lateral removal of magma, as well as periods when a more open magmatic plumbing favored continuous eruption. Since at least 1952, source 3 has not drained during deflations, which was apparently not the case before 1924. Triangulation and leveling conducted in 1912, 1921, and 1926, combined with post-1912 tilt measurements, identified a broad regional uplift in 1918–19 and an equally broad collapse in 1924, neither of which has been seen since. We associate these elevation changes with addition or subtraction of magma from all three magma sources, dominantly source 3. We interpret the intrusion beneath the east rift zone during the 1924 collapse to have stabilized the rift zone-south flank relationship, preventing loss of magma from source 3 in subsequent collapses. Rates of seaward spreading were low until 1952, when earthquakes in 1950 and 1951 associated with surges of magma from the hotspot triggered a large offshore south flank earthquake swarm that unlocked the south flank and enabled a greatly increased rate of seaward spreading. Magma supply rates have been derived for the entire period of study. Between 1823 and 1840, magma was supplied from source 1 at a very high rate of more than 0.2 km 3 /yr, which we interpret as recovery from a substantial draining of magma from beneath Kīlauea in 1790. Inferred magma supply rates diminished to one-tenth of that value after 1840, in part because of increase in the activity of Mauna Loa beginning in 1843. Magma supply rates between 1918 and 1924 were about 0.024 km 3 /yr, matching that of the period from 1840 to 1894. During 1950–52 the magma supply rate increased to about 0.06 km 3 /yr, in part because of the great reduction in Mauna Loa activity following its large eruption in June 1950. Following the summit eruption of 1967–68, magma supply increased further to ~0.1 km 3 /yr, and further increases to more than 0.2 km 3 /yr occurred during the east rift eruption that began in 1983. Eruption at Kīlauea’s summit took place in 1952, and eruptive activity steadily increased as increased magma supply also drove increased spreading rates. The inability of magma supply to be accommodated by a combination of eruption and spreading during the 1969–74 Mauna Ulu period stressed Kīlauea’s south flank. The stress was relieved in part by the M7.2 earthquake of 29 November 1975. That earthquake, in turn, dilated Kīlauea’s east rift zone as the south flank moved seaward, producing a favorable condition for continuous east rift eruption, which began in 1983. The 1975 earthquake also resulted in the ability of the south flank to move independently under the influence of gravity, effectively decoupling the spreading rate from changes in the magma supply rate. The continuing increase in magma supply after 1983 was instead manifested in rift dilation, increased intrusion, and ultimately in the launching of a second eruption in Halemaʻumaʻu in March 2008, the first instance in Kīlauea’s recorded history of simultaneous eruption at the summit and on the east rift zone. Kīlauea’s history can be considered in cycles of equilibrium, crisis, and recovery. The approach of a crisis is driven by a magma supply rate that greatly exceeds the capacity of the plumbing to deliver magma to the surface. Crises can be anticipated by inflation measured at Kīlauea’s summit coupled with an increase in overall seismicity, particularly manifest by intrusion and eruption in the southwest sector of the volcano. Unfortunately the nature of the crisis—for example, large earthquake, new eruption, or edifice-changing intrusion—cannot be specified ahead of time. We conclude that Kīlauea’s cycles are controlled by nonlinear dynamics, which underscores the difficulty in predicting eruptions and earthquakes.

Hawai'i↗

USGS2023

Active volcanism on the Arabian Shield—Geology, volcanology, and geophysics of northern Harrat Rahat and vicinity, Kingdom of Saudi Arabia

Active volcanic systems pose serious hazards to people and property including inundation and incineration by lava, blanketing by tephra (volcanic ash), exposure to noxious volcanic gases, and damage from shallow earthquakes triggered by ascending molten material (magma). To improve understanding of volcanism and associated seismicity on the western Arabia Plate, the Saudi Geological Survey and the U.S. Geological Survey conducted a multi-year investigation of the northern Harrat Rahat volcanic field adjacent to the city of Al Madīnah al Munawwarah, Kingdom of Saudi Arabia. Project components included creation of a high-resolution digital topographic base; interpretation of eruptive history supported by detailed geologic mapping, paleomagnetism, and abundant high-precision geochronology of volcanic deposits; assessments of eruptive styles and volcanic hazards by physical volcanology; investigation of the origins of magmas in the mantle and of their differentiation in the crust revealed by chemical and isotopic petrology; gravity and magnetotelluric surveys to reveal crustal structures and to search for magma reservoirs; and regional and local seismic tomography and analyses of seismic hazards. Project results are presented in this Professional Paper as chapters written for technical scientific audiences.

Professional Paper↗

USGS2023

The Saudi Geological Survey-U.S. Geological Survey northern Harrat Rahat project—Styles, rates, causes, and hazards of volcanism near Al Madīnah al Munawwarah, Kingdom of Saudi Arabia

Al Madīnah al Munawwarah↗

USGS2023

Lava flow emplacement in Harrat Rahat with implications for eruptions in mafic volcanic fields

Mafic volcanic fields are widespread, but few have erupted in historical times, providing limited observations of the magnitudes, dynamics, and timescales of lava flow emplacement in these settings. The Harrat Rahat volcanic field in western Saudi Arabia offers a good opportunity to study eruptions in such a setting, with a historical eruption in 1256 C.E. (654 in the year of the Hijra) and numerous well-preserved late Pleistocene lava flows. We combine historical observations and rheological and morphological analyses of the youngest flows with analytical models to reconstruct eruptive histories and lava flow emplacement conditions in Harrat Rahat. Petrologic analysis of samples for emplacement temperatures and crystallinities show cooling trends from vent to toe of ~1,140 to ~1,090 degrees Celsius (°C) at rates of 2 to 7 °C per kilometer, crystallinities increasing from 0.5 to 60 volume percent, and apparent viscosities increasing from 10 2 to 10 9 pascal seconds. High-resolution topographic data facilitate quantitative analysis of morphology and interpolation of preeruptive surfaces to measure flow thicknesses, channels, and levees, and enable calculation of eruptive volumes. Analytical models relating flow morphology to emplacement conditions are applied to estimate effusion rates. Within the suite of studied flows, minimum volume estimates range from 0.07 to 0.42 cubic kilometers dense rock equivalent, with effusion rates on the order of tens to hundreds of cubic meters per second and durations from 1 to 15 weeks. These integrated analyses quantify past lava flow emplacement conditions and dynamics in Harrat Rahat, improving our understanding and observations of fundamental parameters and controls of effusive eruptions in Harrat Rahat and other mafic volcanic fields.

northeastern Harrat Rahat↗

USGS2023

The duration and characteristics of magmatic differentiation from basalt to trachyte within the Matan volcanic center, northern Harrat Rahat, Kingdom of Saudi Arabia

A fundamental goal of igneous petrology is to quantify the durations required to produce evolved magmas following influx of basalt into the crust. However, in many cases, complex field relations and (or) the presence of a long-lived magmatic system make it difficult to assess how basaltic inputs relate to more evolved magmas, therefore precluding calculation of meaningful timescales. Here we present field relations, geochemistry, 40 Ar/ 39 Ar ages, and 36 Cl surface-exposure ages for volcanic rocks from the Matan volcanic center, located in the northern part of the Harrat Rahat volcanic field, in the Kingdom of Saudi Arabia. These data document a systematic and repeated temporal progression from alkali basalt to trachyte for the youngest eruptive products. From ~155–17 thousand years ago, the following eruptive sequence occurred four times: (1) alkali basalt, (2) hawaiite, mugearite, or benmoreite, and (3) trachyte. We interpret each eruptive sequence to result from injection of basalt into the crust, and its subsequent differentiation and eruption of progressively evolved magmas. We use the interval time between successive eruptions within a given sequence to calculate the timespans required to produce trachyte from alkali basalt. Differentiation from alkali basalt to intermediate compositions (hawaiite, mugearite, and benmoreite) took ≤3 thousand years (k.y.) on average. Differentiation from intermediate compositions to trachyte took a maximum of 6.7±3.6 to 22.9±1.7 k.y. Thus, the total duration of differentiation was ~10–25 k.y. Timescales presented here are independent of the processes evoked to drive differentiation because they are based solely on the ages and compositions of eruptive products from a system characterized by a simple, repeated differentiation sequence.

northern Harrat Rahat↗

USGS2001

The Quaternary and Pliocene Yellowstone Plateau volcanic field of Wyoming, Idaho, and Montana

This region of Yellowstone National Park has been the active focus of one of the Earth's largest magmatic systems for more than 2 million years. The resulting volcanism has been characterized by the eruption of voluminous rhyolites and subordinate basalts but virtually no lavas of intermediate composition. The magmatic system at depth remains active and drives the massive hydrothermal circulation for which the park is widely known. Studies of the volcanic field using geologic mapping and petrology have defined three major cycles of rhyolitic volcanism, each climaxed by the eruption of a rhyolitic ash-flow sheet having a volume of hundreds of thousands of cubic kilometers. The field also has been analyzed in terms of its magmatic and tectonic evolution, including its regional relation to the Snake River plain and to basin-range tectonic extension.

Idaho, Montana, Wyoming↗

USGS1913

The San Franciscan volcanic field, Arizona

LOCATION OF AREA The San Franciscan volcanic field, which takes its name from San Francisco Mountain, the largest volcano of the group, covers about 3,000 square miles in the north-central part of Arizona, as shown by the shaded space on the index map forming figure 1. The center of the field lies about 50 miles south of the Grand Canyon of the Colorado and the southern boundary is in part coterminous with that of the San Francisco Plateau, which forms the southwestern division of the great Colorado Plateau. The region is easily reached, for the main line of the Atchison, Topeka, & Santa Fe Railway traverses it from east to west for more than 60 miles. Flagstaff, a town of 1,500 inhabitants 10 miles south of the summit of San Francisco Mountain, is on the railroad, amid a branch line runs from Williams, 34 miles farther west, to the Grand Canyon. All the more important points of interest in the field may be reached without difficulty by wagon, and outfits may be obtained at Flagstaff. OUTLINE OF THE REPORT This report deals primarily with the volcanic phenomena of the region as determined in the field and laboratory. Chapter I contains a brief description of the geography of the field and Chapter II is devoted largely to the sedimentary formations and structure. The rest of the report Chapters III to VI—treats entirely of the various features of the volcanoes and igneous rocks, both individually and collectively. Detailed descriptions of the volcanoes and lava fields are given in Chapter III; the volcanic history of the region and its correlation with the general history of the surrounding country are presented in Chapter IV. These two chapters will presumably suffice for the general reader who may desire to become acquainted with the broader volcanic features of the region. Chapter V (Petrography) is devoted entirely to the detailed description of the individual igneous rocks of the region, as represented by a selected set of type specimens. In Chapter VI (Petrology) is presented a discussion of the igneous rocks considered collectively—that is, as a series of genetically related members. These last two chapters will be more especially interesting to petrologists, although there is considerable matter in the last chapter which may also be of interest to the general reader. EXTENT OF FIELD WORK The field work on which the report is based was carried on during the summers of 1901 to 1903, a portion of the time, however, being occupied by side trips to the Grand Canyon of the Colorado, the Verde Valley, and the Moqui Buttes. It was the original intention to study only San Francisco Mountain, but scattered observations made during the first summer at other localities, especially at Elden Mountain and Kendrick Peak, seemed to indicate that the region would repay wider study. The work was accordingly extended so as to embrace all the large cones that lie in the vicinity of San Francisco Mountain and some 2,000 square miles of the surrounding plateau country. The more detailed work was confined to the large cones and the laccoliths, as they presented the greatest variety of phenomena within the smallest space. Reconnaissance work was carried on in the surrounding country more especially for the purpose of determining the limits of the widespread basalt flows, their relation to the underlying sedimentary formations, and the character of those formations.

Arizona↗