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Geology of Lava Beds National Monument Geology 72 William Hirt Department of Natural Sciences College of the Siskiyous 800 College Avenue Weed, California 96094 Hirt—Geology of Lava Beds National Monument Introduction This overview of the geology and geologic history of Lava Beds National Monument (LBNM) has been written to provide background information for College of the Siskiyous’ Geology of Lava Beds National Monument short-course (Geology 72), and to serve as a road log for the course’s field trip. The research reported here has been drawn from sources cited in references, especially works by Donnelly-Nolan and Champion (1987) and Donnelly-Nolan et al. (2007). Any errors or omissions, however, are solely the responsibility of the author. Location and Geologic Setting LBNM is located in the in eastern Siskiyou County just south of the Oregon border and southwest of the town of Tulelake (Fig. 1). The monument was established in 1935 to preserve a diverse suite of young volcanic features on the northern flank of the Medicine Lake Volcano as well as key sites related to the Modoc Indian War of 1872-1873. LBNM lies along the boundary between the High Cascade and Basin and Range geologic provinces and displays features characteristic of each. The sections that follow will describe the monument’s global geologic setting, the regional extension that defines its basic crustal structure, and the volcanic processes that have created its signature landforms. Figure 1. Map showing the location of Lava Beds National Monument, the caldera atop the Medicine Lake Volcano, and selected towns and roads (Topinka, USGS/CVO, 1997). floor volcanoes that lies offshore between Cape Mendocino and Vancouver Island. As the Pacific and Juan de Fuca Plates move apart, basalt magma generated by partial melting of the underlying asthenosphere rises to fill fractures between them and so creates new oceanic lithosphere at an average rate of a about 1 centimeter per year. Plate Tectonics Since the early 1960s geologists have realized that the distribution of most of the volcanic and seismic activity on Earth can be understood in terms of interactions between rigid plates of rock that cover the planet’s surface. These lithospheric plates are 100 to 150 km thick and consist of the crust and the cold, rigid upper mantle beneath it. The plates, in turn, lie atop a weaker partially-molten layer of the mantle that extends to a depth of about 350 km. This weak asthenosphere permits the plates to move about relatively easily, and to interact with one another along three types of boundaries: divergent, convergent, and shear. The Cascadia Subduction Zone is a fault that dips eastward beneath the continent and separates the sinking Juan de Fuca Plate from the overriding North American Plate. Heating of the subducting oceanic lithosphere causes it to release water which, in turn, triggers melting of the overlying asthenosphere. At a depth of about 100 km enough basalt melt forms that it is able to separate itself from the surrounding mantle and rise towards the surface. Most of this magma (melt + entrained crystals) lodges in or near the base of the crust where it either crystallizes or partially melts crustal rocks and mixes with them. If mixing occurs, the resulting andesite magmas may continue to rise and some of them—perhaps 10%—reach the Along the western margin of North America these boundary types are exemplified by the Juan de Fuca Ridge, Cascadia Subduction Zone, and San Andreas Fault, respectively (Fig. 2). The ridge is a chain of sea2 Hirt—Geology of Lava Beds National Monument the western margin of North America for over 100 million years. When a small part of the intervening Farallon Plate was completely subducted, the North American and Pacific Plates came into direct contact and the San Andreas developed as a result of the shear between them (Fig. 3). It has since grown to a length of 750 kilometers, and is but one strand in a broad zone of right-lateral deformation that extends eastward to the Nevada border. In southern and central California the Basin and Range Province lies to the east of this deformation zone. Although the exact mechanism that links these two features is unclear, both are apparently manifestations of the strong coupling that now exists between North America and the adjoining Pacific Plate. Across the Basin and Range the North American plate is being stretched, and has broken into a series of parallel mountain and valley blocks separated by north-trending faults. North of Cape Mendocino the San Andreas Fault ends, but the Basin and Range Province continues through northeastern California and into Oregon. The Basin and Range Province adjoins the High Cascades near LBNM, and it is at the juncture between these two provinces that magmas have risen along north-trending fractures to build the Medicine Lake Volcano. Figure 2. Simplified tectonic map of the Pacific Northwest showing the Juan de Fuca Ridge, Cascadia Subduction Zone, and High Cascade volcanic arc. LBNM lies on the northern flank of the Medicine Lake Volcano. surface and erupt to build stratovolcanoes like Mount Shasta. Because of friction, the upper parts of the Juan de Fuca and North American Plates are often “locked” along the Cascadia Subduction Zone. Ongoing plate motion creates strain energy in this locked region until, every few hundred of years, the rocks break and the stored energy is released suddenly as an earthquake. The last major quake along the Cascadia Subduction Zone occurred in January 1700, and had a magnitude of about 9 (Satake et al., 1996). In LBNM the effects of regional extension can be seen in north-trending fault scarps, volcanic lineaments, and the flat down-dropped floor of the Tule Lake Basin. Several prominent fault scarps, such as Gillem Bluff, are exposed near the northwestern margin of the monument (Fig. 4) and are the results of young (post 200,000 year) normal faulting. In addition, volcanic vents such as those at Black Crater and Kings Rift lie along northerly alignments that suggest similar fractures exist at depth and have served as conduits for magmas to reach the surface. Finally, the floor of the Tule Lake Basin itself is a graben or down-dropped block between the mountain ranges that flank the basin. The San Andreas Fault is a shear boundary that separates the Pacific and North American Plates. The part of California that lies west of the fault is moving northwestward relative to the rest of the continent at an average rate of about 3 centimeters per year. As with the Cascadia Subduction Zone, parts of the San Andreas Fault are often locked so that strain energy accumulates for decades or centuries before the rocks break and release suddenly as an earthquake. Back Arc Volcanism Although the chemistry of its lavas suggest that the Medicine Lake Volcano is related to the High Cascades, its location on the eastern side of the arc and its eruption of mostly basalt rather than andesite lavas indicate it is not a typical subduction zone vol- Basin and Range Extension The San Andreas Fault did not exist prior to about 27 Ma. It was formed when one part of the Farallon Ridge reached the subduction zone that had marked 3 Hirt—Geology of Lava Beds National Monument Figure 4. Gillem Bluff is a scarp along which basalt lava flows at least 2 million years old have been uplifted on the western (left) side of a normal fault. masses that cannot mix with the rhyolites and the volcano erupts a bimodal assemblage of rhyolite and basalt (Fig. 6). Beneath Mount Shasta, on the other hand, “wet” basalt magmas remain molten to lower temperatures and so are able to mix with crustal rhyolites to produce lavas of intermediate composition. Although we will not see any of the rhyolitic lavas erupted from the Medicine Lake Volcano “in place” during our field trip, keep your eyes peeled for fragments of white rhyolite pumice that were scattered across the monument during the eruption of Glass Mountain 900 years ago. Volcanic rocks and their eruptive characters The eruptive behavior of lavas is strongly influenced by their temperatures, compositions, and volatile contents. Silicon and oxygen are the most abundant elements in Earth’s crust and mantle, and together they comprise about 50 to 70 weight percent of most volcanic rocks. Silicon-oxygen groups link strongly together, and if they are abundant in a melt they tend to make it very viscous (“pasty”). In fact, variations in viscosity account for three of the main differences between silica-rich (rhyolite) and silica-poor (basalt) lavas. First, because it is difficult for crystals to grow in viscous melts, cooling rhyolite lava often quenches to glass (obsidian) instead of forming crystalline rock. Second, because it is difficult for gases to escape from viscous melts, rhyolite lava typically erupts more explosively than basalt lava. The white rhyolite pumice scattered across the monument is clearly full of gas bubbles, and was blown here by eruptions that oc- Figure 3. Simplified block diagrams showing the development of the San Andreas Fault. (USGS Western Earth Surface Processes Team). cano. Along with the Newberry Volcano farther north, Medicine Lake is one of two “back arc” volcanoes in the Oregon Cascades. The relatively “dry” (waterpoor) basalt lavas it produces originate from upwelling of hot asthenosphere that circulates above the sinking Juan de Fuca Plate rather than from melting triggered by the volatiles the plate releases (Fig. 5). Basalts that rise beneath both the main and back-arc parts of the southern Cascades heat the surrounding continental crust and produce relatively cool rhyolite magmas. Because the Medicine Lake Volcano’s basalts are so water-poor, however, they solidify at higher temperatures than do the water-rich basalts produced farther west. As a result, when Medicine Lake basalts encounter crustal magmas they chill to semisolid 4 Hirt—Geology of Lava Beds National Monument Figure 5. Schematic cross-section of the Cascade arc at the latitude of LBNM. Note that magmas (stars) form just above the sinking plate (“slab”) under Mount Shasta and at a much shallower depth beneath the Medicine Lake Volcano due to mantle upwelling. After Elkins-Tanton et al. (2001). curred at Glass Mountain 25 kilometers to the south. Finally, it is difficult for viscous lava to flow downhill, so rhyolite lavas tend to build thick, steep-sided domes atop their vents rather than forming long thin flows as basalt lavas do. Figure 6. Simplified geologic map of the Medicine Lake volcano showing post-glacial volcanic vents and the extents of basalt and rhyolite flows. From Donnelly-Nolan et al. (2007). Volcanic Features Medicine Lake Shield Volcano The landscape of LBNM is dominated by basalt lava flows and tephra cones that erupted from fissures on the northern flank of the Medicine Lake volcano. This large shield volcano dominates the monument’s southern skyline (Fig. 7) and has been active for about 500,000 years (Donnelly-Nolan et al., 2007). Since the end of Pleistocene glaciations about 12,500 years ago the Medicine Lake volcano has produced sixteen eruptions of rhyolite and basalt (Fig. 6). The volume of the basalts erupted during this period has been about four times that of the rhyolites. nated from its summit or from fissures farther down its slopes. A small, partially buried caldera dominates the mountain’s summit, and was formed about 180,000 years ago during the eruption of the Tuff of Anderson Well. This ash-flow tuff is widely distributed across the northern side of the volcano and can be seen in low outcrops just west of LBMN. Lava Flows and Lava Tubes Although at least thirty separate lava flows crop out within the boundaries of LBNM just one, the Basalt of Mammoth Crater, covers nearly 70% of the monument’s area (Donnelly-Nolan et al., 2007). Most of these flows are basalts, but smaller amounts of slightly more silica-rich basaltic andesite and andesite also occur. The surface textures of the flows depend The volcano’s broad profile indicates it has been built by hundreds relatively thin basalt lava flows that origi5 Hirt—Geology of Lava Beds National Monument Figure 7. Photograph of the Medicine Lake volcano looking south. Note the low broad profile typical of a shield volcano and the small tephra cones dotting its surface. on the viscosities of the lavas which, in turn, depend on their temperatures and volatile concentrations as well as their silica contents. Smooth “ropy” surfaces develop as a soft flexible “skin” formed on top of the flows is folded if they stop moving while they are still hot and fluid. Rough “blocky” surfaces develop, on the other hand, where a thicker crust is formed and repeatedly broken as a flow continues to move even when it is cool and viscous. These surface textures will commonly change from ropy to blocky as lava moves away from its source, cools, and loses volatiles. For example, compare the texture of the Devil’s Homestead Andesite at the overlook (far from its source) with its texture at Fleener Chimneys (near its source). Some lava tubes in the monument also contain perennial ice which forms where water seeping down from the surface encounters cold air that seasonally collects in the lower parts of the tube systems. Pit Craters Mammoth Crater, the vent for the largest lava flow in the monument, is not a crater at all in the traditional sense. Rather than having been excavated by explosive eruptions, this steep-walled circular “pit” was formed by subsidence above a shallow magma reservoir whose contents drained out elsewhere down slope. Tephra Cones Volatiles dissolved in magma will escape as it reaches the surface, and if their escape is sufficiently violent they will tear the magma apart and throw the fragments through the air as tephra. This material consists of fine ash, pea-sized lapilli, and larger blocks (angular) and bombs (rounded). Tephra cones, like Schonchin Butte, are small round hills built by the accumulation of this material around a vent. Their flanks are typically loose and steep, and their vents are marked by circular craters. Although fresh basaltic tephra is black, the oxidation of iron in the hot tephra by air or volcanic gases commonly gives cinder cones a reddish color. Like most liquids, lavas shrink as they cool and solidify. After a flow has come to rest, it will cool inwards from its top and bottom surfaces. Shrinkage fractures develop perpendicular to these cooling surfaces and grow inward as the flow cools. These fractures split the cooling lava into polygonal columns. Look for these columnar joints in the lavas exposed near the entrance to Skull Cave. LBNM is well-known for its lava tubes, and at least 436 of these “caves” are found throughout the monument. Unlike true caves, lava tubes are formed as the fluid centers of lava flows drain away after their outer parts have solidified. Some are “surface tubes” formed from small flow lobes, whereas other larger ones are formed by the “roofing over” of lava-filled channels. Many of the monument’s large lava tubes occur in the Basalt of Mammoth Crater and can be accessed from the Cave Loop Road just southwest of the visitor center. Mushpot Cave, which has a paved trail and explanatory displays, is part of this tube system. Spatter Cones If the escape of volatiles from magma is not vigorous enough to fragment it into tephra, it will instead throw small soft masses of lava out of the vent to form spatter. Such masses only travel short distances through the air and are still hot and semi-solid when they land next to the vent. They will pile up there 6 Hirt—Geology of Lava Beds National Monument and typically partially fuse back together to form a spatter cone. Chains of these small cones commonly develop along the length of an erupting fissure during the early stages of an eruption, as at Ross Chimneys. As the eruption proceeds parts of the fissure typically become blocked and only a few of the cones remain active as the volatiles are exhausted and the “fire fountaining” phase of the eruption ends. Fleener Chimneys, at the head of the Devil’s Homestead flow, are excellent examples of spatter cones. Glass Mountain Rhyolite 885 years Field Trip Road Log LBNM northern boundary: road log begins. Note that it is illegal to disturb or collect any rocks, plants, or animals in the monument. STOP 1. Gillem Bluff: Basalt lava flows more than 2,000,000 years old are exposed in this cliff after having been uplifted on the western side of a normal fault that bounds the Tule Lake basin. This northtrending fault is typical of those that accommodate Basin and Range extension in this area. Tuff Cones and Tule Lake In the far northeastern corner of LBNM there are two well-preserved tuff cones that were built by explosive eruptions triggered when basalt lavas came into contact with the water in Tule Lake. Prisoners Rock and The Peninsula were formed about 275,000 years ago when… STOP 2. Devil’s Homestead Flow: From this overlook you can see the surface texture of the distal part of the 10,500-year old Devil’s Homestead andesite flow. By the time it reached this location the lava was relatively cool and volatile-poor, and so developed this blocky (aa) texture. [619571 E, 4627602 N, 4214 ft.] Geologic History Summary Below is a list of the estimated ages of some of the major eruptive events that have produced the features we’ll see in LBNM. Most of the dates are taken from a NPS handout prepared by Donnelly-Nolan (2000). STOP 3. Fleener Chimneys spatter cones: These spatter cones formed at the vent of the Devil’s Homestead andesite flow. Notice the local development of ropy (pahoehoe) texture here where the lava was hot and volatile rich. Also notice the small surface tube on the right side of the trail as you walk up towards the cones from the parking lot. [619172 E, 4623623 N, 4455 ft.] Gillem Bluff lavas > 2,000,000 years Medicine Lake volcano (entire history) 500,000 years Basalt of Hovey Point 450,000 years Prisoners Rock and The Peninsula 275,000 years Tuff of Anderson Well 180,000 years Schonchin Butte Andesite 62,000 years Mammoth Crater and Modoc Crater 30,000-40,000 years Fleener Chimneys, Devils Homestead Basalt 10,500 years Basalt of Black Crater 3,025 years Cinder Butte, Callahan Basaltic Andesite 1,110 years Little Glass Mountain Rhyolite 1,065 years STOP 3. Schonchin Butte tephra cone: This tephra cone and its associated lava flow formed about 62,000 years ago. A hike up to the old lookout tower on top will be rewarded with panoramic views of the monument, the Medicine Lake volcano to the south, Mount Shasta to the west, and the cone’s small summit crater. [622723 E, 4621854 N, 4806 ft.] STOP 4. LBNM Headquarters and Mushpot Cave: The visitors center has displays describing both the volcanic and human history of the monument. Be sure to look at the scale models of Mount Shasta and the Medicine Lake volcano to get a sense of the differences in size and shape between a stratovolcano and a shield volcano. Also, take the short hike south from the visitors center to Mushpot Cave and tour the well7 Hirt—Geology of Lava Beds National Monument signed interior of this short but geologically diverse lava tube. [624075 E, 4618940 N, 4760 ft.] plagioclase. Basaltic andesite: A volcanic rock that contains between 52 and 57 weight percent silica. Basaltic andesites are typically black in color and contain visible crystals of plagioclase, olivine, and augite. STOP 5: Mammoth Crater: This pit crater, which was active 30,000 to 40,000 years ago, is the source of the basalt that covers over 70% of the monument. Just across the road from the crater a short trail leads to Hidden Valley, a very large lava tube whose roof has collapsed. [621043 E, 4616559 N, 5538 ft.] Crystal: A piece of solid material within which all of the atoms are arranged in a regular, three-dimensional pattern. Some crystals have smooth external surfaces (faces) that formed as they grew from a melt or vapor. STOP 6: Skull Cave: This large lava tube was a major feeder channel for the Basalt of Modoc Crater and was formed 30,000-40,000 years old. It is named for a collection of human and animal bones that were found in the pit at the eastern end of the main chamber. A trail leads from the entrance to the top of the pit, and a metal staircase takes you to a lower level with a floor of permanent ice. [623945 E, 4020797N, 4548 ft.] Dacite: A volcanic rock that contains between 63 and 72 weight percent silica. Dacites are typically gray to pink in color and contain visible crystals of plagioclase, hypersthene, and hornblende. Debris avalanche: A dense, incoherent mixture of water, rock, and soil that flows down slope at speeds of 40 to 200 km/hr (25 to 125 mi/hr). STOP 7: Petroglyph Point and Prisoner’s Rock tuff cone: This tuff cone was formed about 275,000 years ago by hydrovolcanic activity in Tule Lake. The lake has since been diked and drained, but the wave cut terrace adjacent to the parking area and the petroglyphs cut into the side of the cliff just above the old waterline give you an idea of how deep it used to be. To the north is The Peninsula, another tuff cone of similar age that once contained a basalt lava lake. [633670 E, 4633666 N, 4069 ft.] Debris flow: A dense, incoherent mixture of water, rock, and soil that flows down slope at speed of 2 to 40 km/hr (1 to 25 mi/hr). Dike: A sheet-like body of solidified magma that cuts across layering or other structures in the surrounding rock. Dome: A relatively small, steep-sided volcano formed by pasty lava that has piled up atop its vent. Domes are typically no larger than 2 to 3 km (1 to 2 mi) in diameter and are composed of silica-rich lavas. Glossary Andesite: A volcanic rock that contains between 56 and 63 weight percent silica. Andesites are typically gray or black in color and contain visible crystals of plagioclase, augite, and hypersthene. Holocene Epoch: The period of time from 10,000 years ago to present. On Earth this corresponds to the time since the last retreat of widespread continental glaciers. Asthenosphere: Part of the Earth’s mantle that lies below the lithosphere, at depths between about 100 and 350 kilometers. Rock here is relatively soft because its high temperature and relatively low confining pressure enable a small amount of melt to form and lubricate its movement. Lava: Partially molten rock that has risen through Earth’s crust and been erupted onto the surface. Magma: partially molten rock that consists of melt with or without entrained crystals and vapor bubbles. Basalt: A volcanic rock that contains between 47 and 52 weight percent silica. Basalts are typically black and commonly contain visible crystals of olivine and Mineral: A solid natural material that has a specific 8 Hirt—Geology of Lava Beds National Monument chemical composition and a unique internal arrangement of its atoms. For example, quartz consists of silicon and oxygen atoms in a 1:2 ratio (SiO2), and these atoms are bonded together in a hexagonal structure. about 60,000 years old. Recurrence interval: The average period of time between two episodic events such as earthquakes, floods or volcanic eruptions. Mudflow: A dense suspension of fine rock fragments in water. A mudflow is a type of debris flow in which most of the sediment particles are sand-sized (2 mm across) or smaller. Rockfall: A moving mass of rock fragments that has broken loose from an outcrop and cascaded down a slope. Peridotite: A dense, coarse-grained rock that consists mostly of the magnesium and iron-silicate mineral olivine (”peridot”). Shield volcano: A broad volcano built of many thin, overlapping flows of basalt or basaltic andesite lava. Shield volcanoes differ widely in size, from several kilometers to several hundreds of kilometers in diameter and have very gently sloping flanks. Phenocryst: A relatively large crystal in a volcanic rock that grew from the surrounding melt at depth. Silicate: A compound formed of silicon and oxygen with or without other elements. Silicate minerals and the rocks they are part of make up most of Earth’s mantle and crust. Pleistocene Epoch: The period of time between 1.8 million and 10,000 years ago. On Earth this corresponds to the interval during which large continental glaciers repeatedly advanced and retreated across landmasses at high latitudes. Informally, this epoch is also called the “ice age”. Stratovolcano: A volcano composed of alternating lava flows, layers of pyroclastic material, and debris flow deposits piled up around a central vent. Stratovolcanoes are typically 10 to 30 km (6 to 20 mi) in diameter and have slopes that steepen gradually upwards towards their summits. Potassium-argon dating: A technique for determining the age of rock and mineral samples by measuring the amounts of radioactive potassium (40K) and its daughter element, a form of argon (40Ar). Because 40K decays relatively slowly, this technique usually only yields reliable ages for samples that are more than about 100,000 years old. Subduction: Process in which a plate of dense oceanic lithosphere sinks back into Earth’s interior along a dipping surface that separates it from the overriding lithosphere and asthenosphere. Pumice: A porous, glassy volcanic rock formed by the rapid expansion of gas bubbles in melt that is quenched as it is erupted. Subduction zone: A dipping surface that separates a sinking plate of oceanic lithosphere from the overriding lithosphere and asthenosphere. Pyroclastic material: Volcanic rock that has been fragmented by explosions during an eruption or by the collapse and disintegration of the flanks of domes or lava flows. Tephra: Fragmental volcanic rock formed by lava that has been blown explosively from a vent and solidified as it traveled through the air. Radiocarbon dating: A technique for determining the age of a sample of organic material (charred wood, plant roots, cloth) by measuring the rate at which the radioactive carbon (14C) it contains is decaying. Because 14C decays relatively rapidly, this technique only yields accurate ages for samples that are less than Tephra cone: A volcano composed of layers of tephra piled up around a central vent or crater. Tephra cones are typically 1 to 2 km (1 mile) in diameter and have steep (35 to 40o) slopes determined by the angle at which the loose tephra begins to slide. 9 Hirt—Geology of Lava Beds National Monument Texture: Description of how the various materials that make up a rock (crystals, glass, and fragments of other rocks) are arranged with respect to one another. Tuff: A rock made of fine volcanic tephra that has been cemented or partially fused (welded) together. 147-163. Satake, K., Shimazaki, K., Tsuji, Y., and Ueda, K., 1996, Time and size of a giant earthquake in Cascadia inferred from Japanese tsunami records of January 1700: Nature, v. 379, p. 246-248. Tuff breccia: A rock made of tephra and other pyroclastic materials of a wide range of sizes that have been cemented or partially fused together. Large fragments are angular. Volatiles: Chemical compounds and elements (such as water, H2O, and nitrogen, N2) that occur as gases at high temperatures and atmospheric pressure. Volcanic arc: A chain of volcanic peaks that stands above a subduction zone. Because Earth’s surface is spherical, subduction zones and the chains of volcanoes that develop above them are inevitably curved. Volcaniclastic material: Fragmented volcanic rock deposited by either pyroclastic or debris flows. References Donnelly-Nolan, J.M., and Champion, D.E., 1987, Geologic map of Lava Beds National Monument: U.S. Geological Survey Map I-1804, scale 1:24,000. Donnelly-Nolan, J.M., Nathenson, M., Champion, D.E., Ramsey, D. W. , Lowenstern, J. B., and Ewert, J.W., 2007, Volcano hazards assessment for Medicine Lake Volcano, Northern California, U.S. Geological Survey Scientific Investigations Report 2007-5174-A, 26 p. Elkins Tanton, L.T., Grove, T.L., and Donnelly-Nolan, J., 2001, Hot, shallow melting under the Cascades volcanic arc: Geology, v. 29, no. 7, p. 631-634. Lavine, A., 1994, Geology of Prisoners Rock and The Peninsula: Pleistocene hydrovolcanism in the Tule Lake basin, northeastern California: California Geology, v. 47, no. 4, p. 95-103. Lavine, A., and Aalto, K.R., 2002, Morphology of a crater-filling lava lake margin, The Peninsula tuff cone, Tule Lake National Wildlife Refuge, California: implications for the formation of peperite textures: Journal of Volcanology and Geothermal Research, v. 114, p. 10