* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Silicic Magmatism and the Volcanic–Plutonic Connection
Survey
Document related concepts
Llullaillaco wikipedia , lookup
Mount Meager massif wikipedia , lookup
Mount Pinatubo wikipedia , lookup
Mount Edziza volcanic complex wikipedia , lookup
Cascade Volcanoes wikipedia , lookup
Types of volcanic eruptions wikipedia , lookup
Mount Pelée wikipedia , lookup
Mount Vesuvius wikipedia , lookup
Cerro Azul (Chile volcano) wikipedia , lookup
Volcanology of Mars wikipedia , lookup
Transcript
Silicic Magmatism and the Volcanic–Plutonic Connection Cerro Galan ignimbrite, Argentina, a ~1,000 km3 dacite deposit from a supereruption 2 million years ago. Craig C. Lundstrom1 and Allen F. Glazner2 1811-5209/16/0012-0091$2.50 DOI: 10.2113/gselements.12.2.91 T he relationships between silicic volcanic and plutonic rocks have long puzzled geologists because the rich set of observations from petrology, geochronology, thermal modeling, geophysical techniques, and geochemistry have led to contradictory interpretations. Although compositional evolutionary trends leading to granite and rhyolite are congruent, it is not clear if rhyolites are formed by the extraction of melt from shallow crystal mushes that otherwise solidify to form granite plutons, or are derived from a greater depth in parallel with granite plutons, or are formed by processes separate from those which form granite plutons. Finding a consistent explanation for the silicic volcanic–plutonic relationship bears on important Earth science questions, including, “How is silicic continental crust formed?” and, “Can we predict supereruptions?” Bowen (1928) popularized the hypothesis that magmas more silicic than basalt are produced by fractional crystallization, but partial melting of hydrous rocks also produces silicic magma (e.g. Sisson et al. 2005). Among the terrestrial planets, regardless of how they form, silicic rocks are abundant only on Earth. Silicic magmatism contributes to society in more ways than simply providing us with dry land. To take a few examples: Many of the metals critical to modern civilization (e.g. Cu, Mo, Au, W, Sn, Pb, KEYWORDS : granite, rhyolite, continental crust, igneous petrology, magma Zn, In, Li, Nb, Ta, Be) occur in deposits formed by silicic magma“The block of granite which was an obstacle in the pathway tism; much commercially viable of the weak becomes a stepping-stone in the pathway of geothermal power production is sourced in shallow silicic systems; topographic highs on thick silicic crust (e.g. the the strong” – THOMAS CARLYLE, THEOSOPHICAL FORUM Cordillera of North and South America) trap moisture, which provides water to hundreds of millions of people; INTRODUCTION elements of biological relevance (e.g. K, P) are enriched in silicic continental crust over basaltic crust. In addition, Granites, Rhyolites, and Earth Evolution some of the world’s most spectacular scenery is carved in The Earth is unique among the terrestrial planets in having granitic rocks, whose coarse grain size and great strength an abundance of both liquid water and silicic igneous make them particularly resistant to erosion (FIG. 2). rocks at its surface. These seemingly unrelated features are linked by the phase equilibria that govern melting At the same time, silicic magmatism has the potential to end civilization (Self and Blake 2008). Past silicic super(Campbell and Taylor 1983). Earth’s continents result from (ignimthe accretion of low-density, relatively silica-rich igneous eruptions have produced pyroclastic flow deposits 3 rocks that have formed by melting and differentiation in brites) with volumes in excess of 1,000 km (Miller and Wark 2008), emitting enough magma in a matter of days the presence of water, giving the Earth its characteristic to bury an area the size of England more than 10 meters bimodal distribution of topography (FIG. 1). Continental deep. The volume of a supereruption can be 5,000 times crust sits several kilometers above basaltic oceanic crust that erupted from the 2010 event at Eyjafjallajökull volcano with the topographic offset fi lled with water. One can (Iceland), which, though relatively small, shut down air plausibly argue that the development of complex landbased life ultimately depends on the formation of conti- traffic across Europe and cost the airline industry ~US$1.7 billion. Deposits from such eruptions cover huge areas of nents—and the origin of the continents is largely a story of magma evolution from basalt to silicic rocks, such as the world, including extensive parts of North and South America, Africa, and Australia. granite and rhyolite. Rocks formed from basaltic magma are abundant in the Many aspects of silicic magmatism remain poorly understood, including the question raised in this thematic issue terrestrial planets, and the recipe for making them is well of Elements: What is the relationship between erupted silicic known: just partially melt the planet’s mantle (Bowen 1928). The recipe for making silicic magma is less well- established. volcanic rocks, such as ignimbrites, and their intruded, slowly cooled counterparts, such as granites? This question builds from questions about silicic magmatism that have 1 Department of Geology persisted for over a century. For example, the so-called University of Illinois Urbana Champaign “room problem” arises when 1,000s of km3 of granite are Champaign, IL 61820, USA intruded into preexisting rocks; this problem was viewed E-mail: [email protected] by many during the mid-part of the 20 th century as fatal 2 Department of Geological Sciences to an intrusive origin for granitoids. As a result, an in University of North Carolina situ metamorphic (“transformist”) origin was offered Chapel Hill, NC 27599, USA E-mail: [email protected] E LEMENTS , V OL . 12, PP. 91–96 91 A PR IL 2016 0.08 Relative Area, % 0.04 0 -8 Oceans -4 AA LVC BB CC DD Continents 0 Elevation, km 4 0.16 Oceans n=16,294 0.12 Relative 0.08 Probability Continental North America 0.04 n=26,313 0 40 Representative silicic rocks from western North America. (A) Aerial view to the south of the Mono Craters, a series of rhyolite and dacite domes erupted mostly from about 40,000 to 600 years ago. These domes were erupted onto a surface largely composed of Bishop Tuff, a >600 km3 rhyolite ignimbrite erupted 760,000 years ago from the Long Valley caldera (LVC; dashed line marks the caldera’s topographic wall). The Mono chain continues to the south as the Inyo Craters, which terminate north of Mammoth Mountain (marked by star); the caldera, Mammoth Mountain, and Mono–Inyo chain all represent spatially separate foci of silicic volcanism (Hildreth 2004). (B) Cliffs of leucogranite, 2.5 km tall, are exposed in the east face of Great Trango (6,452 m) and Nameless (6,650 m) Spires, Karakoram Range (Pakistan). PHOTO COURTESY OF M IKE SEARLE. (C) The 1 km tall southeast face of El Capitan in Yosemite Valley, California (USA) exposes a complex interaction zone between several plutonic units including multiple granites (light tan and light gray) and two sets of lower-silica dikes (dark gray), all emplaced between about 107 Ma and 103 Ma (Putnam et al. 2015). It is commonly assumed that complex volcanic systems like those in the upper panel have complex plutonic systems like those in the lower panel forming below them (and vice versa), but this is a difficult assertion to test. (D) Sediments deposited in the continental interior of the western United States contain abundant volcanic ash that was presumably derived from arc volcanoes that erupted above plutonic rocks now represented by Cordilleran batholiths such as the Sierra Nevada. Here, in central Utah, the Upper Cretaceous Mancos Shale contains numerous volcanic ash layers (light bands). PHOTO COURTESY OF L AUREN B IRGENHEIER. FIGURE 2 50 60 wt % SiO2 70 80 (TOP) Distribution of elevation for oceans and continents on the Earth. Most oceanic topography lies at 3–6 km depth, whereas half of the dry land on the Earth is below 370 m elevation. ( BOTTOM ) Distribution of silica (SiO2) in igneous rocks from the ocean basins and western North America. The occurrence of abundant igneous rocks with SiO2 >55 wt% on the continents, and particularly the almost complete restriction of silicic igneous rocks (SiO2 >66 wt%) there, contributes to the relatively high elevations of the continents. ELEVATION DATA FROM E AKINS AND SHARMAN (2012); COMPOSITIONAL DATA FROM PETDB (OCEANS) AND NAVDAT (NORTH A MERICA), BOTH ACCESSIBLE THROUGH WWW.EARTHCHEM.ORG. FIGURE 1 instead (Read 1948). However, the strong correspondence between observed granites and melt compositions dictated by quartz–feldspar equilibria ended the magmatic versus metamorphic debate (Tuttle and Bowen 1958), but still left the room problem unsolved. Similarly, the identification of ignimbrites as coming from giant eruptions of silicic magma also occurred relatively recently (Smith 1960). In many ways, the enigmatic relationship of silicic volcanic to plutonic rocks presented here is simply an extension of ongoing questions about the origin and mechanism for forming Earth’s silicic magmas. with volumes of several thousand km3. Batholiths around the world contain much greater volumes of plutonic rocks of comparable composition (granite and granodiorite), and individual mappable bodies of such rock (plutons) can have volumes comparable to, or greater than, those of the largest supereruptions (Frazer et al. 2014). A Brief Introduction to Silicic Rocks Field identification of a rock as volcanic or plutonic is based mainly on texture, i.e. on the size and arrangement of crystals in the rock and the proportion of crystals to glass. On Earth, low-silica basalts dominate the volcanic record, whereas plutonic rocks, or at least those exposed at the surface by tectonics and erosion, are significantly more silica-rich (FIG. 3; Daly 1914). Plutonic and volcanic rocks span virtually identical ranges in composition and mineralogy; the compositions of the vast majority fall on coincident scattered linear trends in major-element composition space and range from ~45 wt% SiO2 to ~75 wt% SiO2. The historical view of plutons is that they reflect now-frozen magma chambers that were as large as the pluton itself, a view that has been promoted in countless publications over the past century. This view led to a comfortable duality between volcanic and plutonic rocks, with plutons serving as the staging areas from which volcanic rocks are erupted. Nearly 200 years ago, Lyell (1838) drew red blobs labeled “Plutonic (Granite, & cc.)” feeding an erupting volcano (FIG. 4), and this image persists today. Silicic volcanic rocks include rhyolite and dacite and may be emplaced on the surface as lava, pyroclastic fall deposits (“volcanic ash”), and pyroclastic flow deposits (ignimbrites or welded tuffs), the latter resulting from catastrophic ground-hugging “ash hurricanes” that comprise most of the output of silicic supereruptions. Silicic eruptions range from quiet extrusion (e.g. the 2004–2008 eruptions of Mount St. Helens, Washington, USA) to supereruptions E LEMENTS Studies of silicic magmatism have benefitted greatly from advances in geochemical analysis. The ability to analyze large swaths of the periodic table down to the µg/g (or better) level reveals secrets that have led to a revolution in understanding of the genesis of rhyolite. The development of in situ methods (e.g. laser-ablation inductively coupled plasma mass spectrometry; ion probe) has provided precise geochemical and isotopic analyses of spots with 92 A PR IL 2016 dimensions on the order of 10 µm, producing spatially resolved U–Pb ages for zircon and titanite and putting the spotlight on silicic magmatism. Improvements in ultraclean lab procedures, chemical and thermal pretreatment, and precision of thermal ionization mass spectrometry now allow U–Pb dating of zircon with a precision of 0.1%. ENIGMAS Overview Here is a short list of current enigmas. Supereruptions show that huge magma bodies must exist, at least transiently, but geophysical surveys have yet to locate any present-day huge, eruptible magma bodies. Not surprisingly, this cornucopia of new data has generated far more questions than answers, with many questions surrounding the relationship between silicic volcanic and plutonic rocks. Observations from active volcanic systems lead to views that differ from those based on plutons or geophysical observations. Thermal modeling indicates that building a large body of magma requires accumulating magma at rates that are inconsistent with geophysical and geodetic data. These and other issues constitute inconsistencies that arise from studying different parts of a magmatic system with different techniques. Phase equilibria suggest that rhyolite magmas last equilibrate with a quartz–feldspar residue at shallow levels in the crust, but the plutons that should be the residue after rhyolite extraction do not have the expected complementary geochemical signature. Precise geochronology of silicic plutons requires that they grew in increments, but the boundaries between such increments can rarely be seen in the field. Thermal models indicate that magma must accumulate extremely rapidly to form a large eruptible magma body, but such accumulation rates are inconsistent with measured rates of geodetic movement. In his well-known poem “The Blind Men and the Elephant,” John Godfrey Saxe wrote: And so these men of Indostan Disputed loud and long, Each in his own opinion Exceeding stiff and strong, Though each was partly in the right, And all were in the wrong! Many plutons show systematic geochemical variations consistent with outside-in crystallization of a large magma body, but this cannot happen if the plutons are emplaced incrementally. Estimates of the proportion of partial melt in various magmatic areas derived from seismic and magnetotelluric (electrical conductivity) methods commonly show significant disagreement. In this issue of Elements, five articles cover some of the significant enigmas that confound our understanding of the Earth’s silicic rocks. Our hope is that bringing together disparate methods and viewpoints will move us all closer to a whole-elephant view of how silicic magmatic systems work. Geophysical Imaging and the Elusive Magma Chamber Magma chambers like those in FIGURE 4 must exist at some level in the crust before large eruptions. However, they may be ephemeral and, thus, difficult to capture in the present-time snapshots that geophysical methods provide. Seismology has undergone rapid advancements, including increases in computing power and the development of large organized efforts such as ORFEUS and IRIS (using the US National Science Foundation’s EarthScope USArray). New geodetic techniques, such as continuous global positioning system (GPS) and interferometric synthetic aperture radar, provide real-time data on deforming magmatic systems, thereby providing a method to examine dynamic processes. Pritchard and Gregg (2016 this issue) discuss the wide range of geophysical methods available for observing magmatic systems. 0.03 Volcanic n = 29,019 0.02 0.01 0 0.03 0.02 Plutonic n = 11,156 Although seismic images of magma chambers are commonly reported, none to date have revealed a true magma body with a high percentage of melt. In order to be mobile and capable of intrusion and extrusion, a magma must be at least 50% molten. Many seismically imaged “magma chambers” are perhaps only a few percent molten and are better called “blobby zone[s] where the velocity is low and attenuation is high” (Lees 2007). Mobile magma bodies may exist in unstudied areas, of course, but it is curious that none of the well-studied areas on Earth, including Yellowstone (Huang et al. 2015), seem to be underlain by large, high-melt fraction bodies of magma. 0.01 0 40 45 50 55 60 wt% SiO2 65 70 75 80 Relative probability histograms of silica for volcanic and plutonic rocks from western North America, using all samples for which SiO2 was reported. These arrays support field observations by showing strong biases of volcanic rocks toward low-silica compositions and plutonic rocks toward high-silica compositions. Volcanic peak at ~54 wt% SiO2 represents the Columbia River basalts (western USA), which are both widespread and oversampled relative to many other areas in the database. In spite of this compositional bias in exposure, the ranges of and correlations among compositional variables are virtually identical between volcanic and plutonic rocks (Glazner et al. 2015). DATA FROM NAVDAT (AT WWW.EARTHCHEM.ORG). FIGURE 3 E LEMENTS Another important part of the Earthscope program has been the implementation of a moving array for magnetotelluric imaging. This tomographic method provides a complementary technique to seismology and is particularly sensitive to the presence of conductive liquid, both melt and hydrothermal fluids. However, seismic and magneto- 93 A PR IL 2016 for every 10 km 3 of granite. Such residues have not been geophysically observed under batholiths, though it is possible that they have been removed by delamination. Understanding how differentiation from mafic to silicic magmas occurs is critical to understanding the volcanic– plutonic connection. Silicic intrusive suites are commonly petrologically zoned, with compositions grading from lower-silica rocks on their rims to higher-silica rocks in their interiors. For many years, this was interpreted as inward solidification of a plutonsized magma chamber (Bateman and Chappell 1979); comparable zoning of some ignimbrite deposits has been interpreted to reflect an inverted record of zoning in the volcanic magma chamber prior to eruption, with the top of the chamber being cooler and more evolved (McBirney 1968). Where these chemical zonings develop must be intimately interwoven with questions of the differentiation mechanisms, the heat flow, and the timescales of cooling. The classic view of the plutonic–volcanic connection goes back to Charles Lyell’s 1838 classic Elements of Geology and continues to be popular. In this model, a magma chamber is connected to the surface via a planar or pipe-like conduit, and therefore active volcanoes should overlie significant volumes of molten material where the proportion of liquid is >50%. The inability of seismic surveys to reveal such magma bodies calls this simple view into question. FIGURE 4 Because mineral–melt separation may play an important role in forming silicic magmas, the physical properties of silicic melts are critically important. As magmas become increasingly silicic, their viscosities increase dramatically because silicate tetrahedra link up and polymerize the melt. Viscosity increases from the relatively low values of a basaltic melt (<103 Pa s) to high values when a melt has a SiO2 content exceeding 60–65 wt% (>106 Pa s). Rhyolitic melt can be 4 to 10 (or more) orders of magnitude more viscous than basaltic melt, depending on the water content of the magma (FIG. 5). Because rates of mechanical separation processes (e.g. crystal settling or buoyant melt percolation) scale with viscosity, these processes must operate many orders of magnitude more slowly in rhyolitic melts than in basaltic magmas. telluric estimates of the percent partial melt in a given area commonly disagree significantly (Pritchard and Gregg 2016 this issue). Finally, advancements in geodetic monitoring and data processing have provided unprecedented images of surface deformation related to magmatic activity. The development of remote sensing methods and improvements in GPS have led to images of surface expansion and contraction of volcanoes reflecting either magma injection, hydrothermal fluctuation, or both. These signals commonly indicate activity in the system related to pulses of heat that must ultimately reflect addition of magma. How these deformation signals translate into amounts of magma added per time (flux) will always be difficult to ascertain given the short time period over which deformation is measured. Magmatic water plays a critical role in silicic systems, but details about how water accumulates, moves, and affects the various processes remain unclear. For instance, whereas eruptions unambiguously involve the generation of large amounts of gases (dominated by water), whether vapor formation occurs via decompression due to ascent or via crystallization-induced water saturation is unknown. Indeed, preeruptive water contents of melts are poorly known, and eruption triggering and decompression instead may reflect external control, such as roof collapse. Volatile component behavior remains a significant wild card in silicic rock petrogenesis. Experimental Petrology and Phase Equilibria Igneous rock suites show compositional trends that follow expectations of mineral–melt equilibria as determined by laboratory experiments. Tuttle and Bowen (1958) showed that Earth’s silicic rocks (both granitoids and silicic volcanic rocks) have bulk compositions that closely resemble hydrous melts in equilibrium with quartz and feldspar. Subsequent work has examined the effects of other major components on compositional evolution. Scaillet et al. (2016 this volume) summarize these fi ndings and delineate the change in phase assemblages that occur as a result of particular magma compositions (in this case, peraluminous, metaluminous, and peralkaline). If rhyolites represent a silicic melt extracted from a granite’s crystal residue (crystal fractionation), then granite and rhyolite compositions should be complementary—i.e. elements that are compatible in the granite crystal assemblage will be enriched in the granite and depleted in the rhyolite. However, global, regional, and local comparisons of trace element patterns fail to support this. Elements such as Sr and Ba, which are sensitive to the removal of feldspar phases, indicate no significant difference in behavior between the evolution of the two rock types (Glazner et al. 2015). Regardless of whether silicic melts are produced by extended fractional crystallization or wet partial melting of basalt, the volume ratio of crystal residue to silicic melt has to be high, generally around 10:1. Thus, the observation of large amounts of silicic plutonic rocks (FIG. 3), particularly in convergent margin settings, provides particular challenges because vast volumes of crystal residue should be left behind: for example, ~100 km3 of gabbroic residue E LEMENTS 94 A PR IL 2016 10 small magma bodies presents yet another contradiction: such bodies will cool even faster than a single large body, thus making constraints based on cooling time even more stringent. 10 nh yd ro u Viscosity, Pa s s 108 106 A 104 102 s Ba 1 50 alt wt% +5 R h l yo Other studies from volcanic rocks document widely varying timescales based on fi ne-scale crystal observations. In situ U–Pb and U-series dates within a zircon can vary by hundreds of thousands of years. Thus, the small volume secondary ion mass spectrometry (SIMS) analyses contrast sharply with the single grain isotope dilution thermal ionization mass spectrometry (ID-TIMS) method in terms of the obtained temporal information; this difference is attributed to the time interval that each method samples. These observations point to longer timescales and have led to suggestions that volcanic systems may spend significant time as a mostly crystal mush that becomes rejuvenated by mafic magma underplating at the time of eruption. Finally, at the same time, diffusion profi les and modeling can indicate day to week timescales of magma residence: for instance, trace element profi les suggest that the Bishop Tuff underwent a significant heating event just days before its eruption (Wark et al. 2007). ite O H2 60 wt% SiO2 70 80 Another intriguing age-related observation is that large caldera systems erupt in a periodic fashion. For instance, Yellowstone has had three major eruptions occurring at ~650 ky intervals. Compositional data through time should show the effects of the erupted magma repeatedly sampling an evolving (and depleting) source if melt were extracted from residual crystals during each cycle. But no depletion signal is seen. If these series of rhyolites reflect melt extraction from a crystal mush, then stirring and homogenization with new magma must occur to hide this depletion process. Viscosity calculated along a basalt–rhyolite mixing line. Viscosity of anhydrous rhyolite is 9 orders of magnitude greater than anhydrous basalt; adding 5 wt% water to the system decreases this ratio to about 4.5 orders of magnitude. BASALT AND RHYOLITE COMPOSITIONS FROM USSLER AND G LAZNER (1989); VISCOSITY MODEL OF G IORDANO ET AL . (2008). FIGURE 5 Geochronologic Timescales Recent U–Pb geochronology observations have led to a paradigm shift about how silicic intrusions form. Whereas compositional zoning of the Tuolumne Intrusive Suite in Yosemite (California, USA) led Bateman and Chappell (1979) to suggest inward crystallization of a pluton-sized magma chamber, precise dating has shown that this set of intrusions crystallized over 8–10 My, greatly exceeding the thermal cooling time (~105 y) of such a magma chamber (Coleman et al. 2004). This led to the idea that plutons may form by small increments over timescales on the order of 105 –10 6 years. Coleman et al. (2016 this issue) show that through the use of multiple geochronology systems, detailed time–temperature histories of plutons can be determined. GLOSSARY Dacite: Relatively high-silica volcanic rock containing ~65–70 wt% SiO2 . Granite, granitoids: Plutonic rocks composed predominantly of quartz, alkali feldspar, and plagioclase feldspar, typically in subequal amounts. Ignimbrite: A deposit formed from a pyroclastic flow. Magma: Although the defi nition of this fundamental geologic substance is disturbingly vague, the American Geosciences Institute Glossary of Geology defi nition is appropriate here: “naturally occurring molten or partially molten rock material, generated within the Earth and capable of intrusion and extrusion.” Comparing the geochronology of volcanic and plutonic systems shows important similarities and differences. On the one hand, the million-year timescales of pluton emplacement contrast sharply with the much shorter timescale for volcanic processes occurring in similar-sized magma bodies. For instance, ages for single zircons from the large supereruption that produced the Kilgore Tuff (1,800 km3 from Yellowstone-track caldera; Wotzlaw et al. 2014) show a tight range: ~30 zircons vary by only ~0.1%. Thus, whereas ~10% age variation can occur in intrusions, the age range for the eruptible portion of a similar sized volcanic system appears to be much smaller. Plutonic: Referring to magma that crystallized at depth and is, therefore, relatively coarse-grained. Granite is a plutonic rock. Pyroclastic: Said of material ejected into the air during eruption, such as volcanic ash. Pyroclastic fall: Fine material from an explosive eruption that falls like snow. Pyroclastic fall material can circle the globe if fi ne enough and thrown high enough. Pyroclastic flow: A hot, fluidized, ground-hugging mass of fragmental material from an explosive eruption that moves across the ground surface at high speed. Pyroclastic flows are dominated by fi ne (<2 mm) particles of volcanic ash and are sometimes called “ash flows.” The tight age span of volcanic magma bodies becomes important when put in the context of large eruptions formed by amalgamation of many small discrete bodies (Wilson and Charlier 2016 this issue). In essence, large geochemical differences can be observed within distinct parts of supereruptions that would be eliminated if convection and mixing in a large blob of magma occurred. For instance, the same Kilgore zircons show oxygen isotope ratios that vary by over 4 per mil (Wotzlaw et al. 2014) in sharp contrast to uniformity expected for crystallization from homogeneous melt. The idea of the amalgamation of E LEMENTS Rhyolite: High-silica volcanic rock containing >~70 wt% SiO2 . Silica: The chemical component SiO2 . Silicic: Denoting a high concentration of silica (typically >66 wt%). Silicic melts are far more viscous than less-silicic ones. 95 A PR IL 2016 (Lundstrom et al. 1998). The origin of order-of-magnitude magma flux variations, if they exist, presents yet another enigma. Thermal Timescales The upper crust is a thermal boundary layer set by the Earth’s surface temperature. Thus, magmas residing within a few kilometers of the surface are rapidly cooled once emplaced. This constraint is at the root of many of the enigmas listed above, because, although observations may suggest an upper-crustal differentiation process, cooling models place strong temporal constraints on how long these processes have to operate. For instance, because of rapid upper-crustal cooling, thermal models require that magma must be added at vertical accumulation rates >10 mm/y in order to make an eruptible magma body (Schöpa and Annen 2013). Such high rates are difficult to reconcile with geodetic, geophysical, and petrological data from volcanic areas. Blundy and Annen (2016 this issue) discuss the constraints imposed by heat flow and cooling on both pluton emplacement and volcanic magma chamber growth. CONCLUDING REMARKS The volcanic–plutonic connection of silicic rocks remains mysterious, despite decades of work in a variety of fields. Current observations from petrology, geochronology, thermal modeling, geophysical techniques, and geochemistry lead to contradictory interpretations. The importance and potential impacts of understanding this connection range from discerning how Earth’s continental crust formed to predicting volcanic eruptions. As with Saxe’s blind men trying to “decipher” an elephant, deciphering the message from silicic rocks will require integrating all possible observations to fi nally reveal our silicic elephant’s true form. The magma accumulation rate required by thermal modeling to develop a large magma body in the upper crust is 1–2 orders of magnitude greater than the rate of vertical magma accumulation recorded by granite plutons determined by geochronology (Coleman et al. 2016 this issue). These melt fluxes must ultimately reflect mantle melt production rates, which depend on decompression, on addition of volatiles (H 2O), or on addition of heat, and are closely tied to plate convergence or divergence rates REFERENCES Bateman PC, Chappell BW (1979) Crystallization, fractionation, and solidification of the Tuolumne intrusive series, Yosemite National Park, California. Geological Society of America Bulletin 90: 465-482 Blundy JD, Annen CJ (2016) Crustal magmatic systems from the perspective of heat transfer. Elements 12: 115-120 Bowen NL (1928) The Evolution of the Igneous Rocks. Princeton University Press, Princeton, New Jersey, 332 pp Campbell IH, Taylor SR (1983) No water, no granites; no oceans, no continents. Geophysical Research Letters 10: 1061-1064 Coleman DS, Gray W, Glazner AF (2004) Rethinking the emplacement and evolution of zoned plutons: geochronologic evidence for incremental assembly of the Tuolumne Intrusive Suite, California. Geology 32: 433-436 Coleman DS, Mills RD, Zimmerer MJ (2016) The pace of plutonism. Elements 12: 97-102 Eakins BW, Sharman GF (2012) Hypsographic curve of Earth’s surface from ETOPO1. NOAA National Geophysical Data Center, Boulder, CO Frazer RE, Coleman DS, Mills RD (2014) Zircon U-Pb geochronology of the Mount Givens Granodiorite; implications for the genesis of large volumes of eruptible magma. Journal of Geophysical Research: Solid Earth 119: 2907-2924 Giordano D, Russell JK, Dingwell DB (2008) Viscosity of magmatic liquids: a model. Earth and Planetary Science Letters 271: 123-134 Glazner AF, Coleman DS, Mills RD (2015) The volcanic-plutonic connection. In: Breitkreuz C, Rocchi S (eds) Physical Geology of Shallow Magmatic Systems: E LEMENTS ACKNOWLEDGMENTS We thank Adam Kent and Gerhard Wörner for thoughtful reviews that significantly improved the paper, and Bernie Wood for editorial handling. This work was supported by National Science Foundation grants EAR-0312691, 0336070, 0538129, and 062210 to AFG and grants EAR-1019632, 0944169 and OCE-1060754 to CCL. Dykes, Sills, and Laccoliths. Springer International Publishing, New York, pp 1-22 Hildreth W (2004) Volcanological perspectives on Long Valley, Mammoth Mountain, and Mono Craters: several contiguous but discrete systems. Journal of Volcanology and Geothermal Research 136: 169-198 Huang H-H and 5 coauthors (2015) The Yellowstone magmatic system from the mantle plume to the upper crust. Science 348: 773-776 Lees JM (2007) Seismic tomography of magmatic systems. Journal of Volcanology and Geothermal Research 167: 37-56 Lundstrom CC, Williams Q, Gill J (1998) Investigating solid mantle upwelling rates beneath mid-ocean ridges using U-series disequilibria, 1: a global approach. Earth and Planetary Science Letters 157: 151-165 Lyell C (1838) Elements of Geology. John Murray, London, 543 pp McBirney AR (1968) Compositional variations of the climactic eruption of Mount Mazama. Andesite Conference Guidebook. Department of Geology and Mineral Industries, Portland, Oregon, Bulletin 62, pp 53-56 Miller CF, Wark DA (2008) Supervolcanoes and their explosive supereruptions. Elements 4: 11-15 Pritchard ME, Gregg PM (2016) Geophysical evidence for silicic crustal melt in the continents: where, what kind, and how much? Elements 12: 121-127 Putnam R and 5 coauthors (2015) Plutonism in three dimensions: Field and geochemical relations on the southeast face of El Capitan, Yosemite National Park, California. Geosphere 11, doi:10.1130/GES01133.1 96 Read HH (1948) Granites and granites. In: Gilluly J, Read HH (eds) Origin of Granite. Geological Society of America Memoir 28: 1-19 Scaillet B, Holtz F, Pichavant M (2016) Experimental constraints on the formation of silicic magmas. Elements 12: 109-114 Schöpa A, Annen C (2013) The effects of magma flux variations on the formation and lifetime of large silicic magma chambers. Journal of Geophysical Research: Solid Earth 118: 926-942 Self S, Blake S (2008) Consequences of explosive supereruptions. Elements 4: 41-46 Sisson TW, Ratajeski K, Hankins WB, Glazner AF (2005) Voluminous granitic magmas from common basaltic sources. Contributions to Mineralogy and Petrology 148: 635-661 Smith RL (1960) Ash flows. Geological Society of America Bulletin 71: 795-841 Tuttle OF, Bowen NL (1958) Origin of granite in the light of experimental studies in the system NaAlSi3O8 KAlSi3O8 -SiO2 -H 2O. Geological Society of America Memoir 74, 153 pp Ussler W III, Glazner AF (1989) Phase equilibria along a basalt-rhyolite mixing line: implications for the origin of calc-alkaline intermediate magmas. Contributions to Mineralogy and Petrology 101: 232-244 Wark DA, Hildreth W, Spear FS, Cherniak DJ, Watson EB (2007) Pre-eruption recharge of the Bishop magma system. Geology 35: 235-238 Wilson CJN, Charlier BLA (2016) The life and times of silicic volcanic systems. Elements 12: 103-108 Wotzlaw J-F and 5 coauthors (2014) Linking rapid magma reservoir assembly and eruption trigger mechanisms at evolved Yellowstone-type supervolcanoes. Geology 42: 807-810 A PR IL 2016