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Monogenetic volcanic fields

Volcanic activity in terrestrial settings often results in the formation of volcanic fields rather than single volcanic edi-

fices (CONNORand CONVAY2000, WALKER 2000). Volcanic fields, especially basaltic ones, are common volcanic sys-

tems on Earth (WALKER1993). They can develop as a cluster of small-volume volcanoes such as the Hopi Butte (Plate

I, 1) in Arizona (WHITE1991b), or around a central volcano, such as the volcanic field around Lamongan (Figure 5.1),

in Java (CARN2000). Monogenetic volcanic fields are those in which individual volcanoes (mainly basaltic) commonly

form during single episodes of volcanic activity, without subsequent eruptions, while the volcanic field as a whole may

be active for millions of years (WALKER 1993). Intracontinental volcanic fields commonly are characterized by low

magma supply rates over relatively long periods of time (millions of years) (TAKADA1994, CONNORet al. 2000). They

typically consist of scattered volcanic vents that are often considered to be monogenetic as they apparently never made

it into the construction stage of stratovolcanism (WALKER1993). In fact, these volcanoes are generally small in size and

in volume of accumulated eruptive products, but on closer inspection they do often show signs of multiple eruption his- tories, and therefore their architecture can be complex regardless of their small size. It is also notable, that volcanic fields

in continental settings are often associated with large shield volcanoes and lava flow fields (GREELEY1982, WALKER

1993, HASENAKA1994, NÉMETH2004).

Phreatomagmatic volcanoes in a volcanic field are commonly associated with low-lands or valleys (LORENZ 1973,

1986, LORENZand BÜCHEL1980). Magmatic explosive eruptive centres and extensive lava fields are commonly located

in elevated lands or areas with limited water availability. Lava flows are commonly confined to valleys or stopped behind syn-volcanic geomorphic barriers. Lava flows may not leave their source vent zone, forming lava lakes, or filling wide craters of phreatomagmatic volcanoes. Distribution of different styles of vents gives vital information of the syn-volcanic

landscape drainage system as well as its physiography (LORENZ and BÜCHEL 1980). Identification of widespread

phreatomagmatism in many fields suggests extensive surface and ground water availability of the region in syn-volcanic times. Volcanic fields in terrestrial settings, especially those developed in a fluvio-lacustrine basin such as the western

Pannonian Basin Mio/Pliocene fields (MARTINand NÉMETH 2004) or the Snake River Plain (GODCHAUXet al. 1992,

WOOD and CLEMENS 2004, NÉMETHand WHITE2007) volcanic fields are great volcanological interest both from vol-

canological and palaeogeomorphological point of view. The relatively long volcanic history of such volcanic fields and the adjacent lake\fluviatile environment makes them an ideal area for studying the sublacustrine, peri-lacustrine and post- lacustrine volcanism, the palaeogeomorphological evolution. Such fields have a great opportunity for developing our knowledge about eruption mechanisms resulting from magma–water interactions across the entire magma\water-ratio spectrum with the special relations to the palaeoenvironment, and the related palaeohidrology and physical characteris- tics of the pre-volcanic units.

Fundamental physical characteristics of volcanic fields that are the focus of current research include 1) the number,

type and eruption history of individual vents (CONNOR1990, SIEBEet al. 2005, VALENTINEet al. 2006); 2) the timing and

recurrence rates of the volcanic eruptions in a given volcanic field (TANAKAet al. 1986, CONDIT and CONNOR 1996,

CONVAYet al. 1998), 3) the distribution of vents and volcanic complexes (CONNOR1987), and 4) the relationship of vol-

canic fields and the volcanoes within them to tectonic features such as basins, faults and rift zones (CONNORet al. 1992,

STAMATAKOSet al. 1997, CONNORet al. 2000). In general there are three major elements to be considered in the ascent and emplacement of magma either on Earth or other planets, and each strongly depends on the physical properties and

structure of the lithosphere encountered by the magma. The three factors are (WALKER1989): 1) magma generation and

buoyancy 2) rheological boundaries in the lithosphere and 3) density boundaries in the lithosphere. In addition to these factors, the stress field (local and regional) plays an important role in controlling magma ascent which is generally relat- ed to the structural features of the lithosphere encountered by the magma.

Monogenetic volcanoes are traditionally referred as those volcanoes that erupt only once during their eruptive histo- ry. They are small and occur as scoria cones (Plate I, 2), tuff cones (Plate I, 3) and rings (Plate I, 4), and maars (Plate I, 5). They form from typically short-lived single and brief eruptions. Their characteristic feature is that the duration of the eruption is usually shorter than the solidification time required for the feeding system to provide the melt for the erup- tion. This definition maybe useful to classify volcanoes that erupt mafic magmas and produce small-volume cinder cones associated with variable long lava flows. Since phreatomagmatic volcanoes such as maars and tuff rings are considered

to be the wet equivalent of scoria cones (LORENZ 1986), a general preconception of the short eruption duration of

phreatomagmatic volcanoes is accepted. Eyewitness of a few historic maar volcanic eruptions such as Ukinrek (Alaska) (KIENLEet al. 1980, SELFet al. 1980, ORTet al. 2000) and or Rininahue (Chile) (MÜLLERand VEYL1956) support this working hypothesis that such eruptions usually last a short time. Calculation of the necessary melt involvement into such eruptions commonly gave a very low proportion of primary magma that needs to create such volcanoes. This conclusion is supported by the componentry analyses of phreatomagmatic tephra in many places in various volcanic settings. To solidify such small volume of melt in the feeding system it may need less time than the a few days or weeks. Conversely, there are reports from scoria cones which show gradual transition toward composite volcanoes, and they are hard to clas-

sify in term of monogenetic and polygenetic systems (MCKNIGHTand WILLIAMS1997). On the basis of historic eruption

it is clear, that most of these volcanoes are large in volume and the deposited tephra commonly consists of a great diver- sity of eruptive products from strikingly different fragmentation history of the melt (e.g. magmatic vs. phreatomagmat- ic). Especially phreatomagmatic volcanoes are often associated with scoria cones, spatter cones and lava flows and they form together a volcanic complex. Such volcanic complexes are closely spaced individual volcanoes that individually may fulfill the requirement of the sensu stricto term monogenetic, however, to identify these features in ancient setting maybe problematic if not impossible.

Monogenetic volcanoes may form in any type of geological environment, however, their volcanic landform strong- ly depends on the water availability and therefore the environment where they erupt. In fully subaqueous environments

either in sub-lacustrine or sub-marine settings lensoidally shaped pyroclastic mounds form (WHITE 1996). These

mounds consist of flat lying pyroclastic density current deposits (WHITE2000). In shallow subaqueous environment

after the construction of a pyroclastic mound, eruption clouds and directed pyroclast jets breach the water surface and

form steep flanked tephra cones over a basal tephra mounds (BELOUSOVand BELOUSOVA2001, WHITE2001). In fully

subaerial settings (Figure 5.2) when magma interact with near surface water tables and/or very shallow lake, sea or

river water, gently dipping broad tephra rings develop (HEIKEN1971). The tephra ring edifice consists of alternating

base surge and phreatomagmatic fall (Plate I, 6) tephras (VESPERMANNand SCHMINCKE2000). When magma interact

with ground water, hole-in-the-ground, maar volcano forms (Plate I, 7) that is surrounded by flat lying tephra beds (VESPERMANNand SCHMINCKE2000). The crater floor of maars undercut the syn-eruptive surface. The maars have four

distinct part (Figure 5.3) in cross sectional view (LORENZ1986, WHITE1991b, LORENZand KURSZLAUKIS2007); 1) root

zone with intrusions that are often mixed with matrix of conduit filling volcaniclastic debris and collapsed country rock blocks; 2) a lower diatreme which is the deep subsurface zone of the conduit filling primary and intra-vent mix- ing of different origin volcaniclastic debris; 3) an upper diatreme that consists of near surface primary pyroclastic

deposits and 4) the crater lake setting that is an accumulation of different origin sediments in a small sedimentary basin with variable influence from background sedimentation. The above mentioned zones of a maar volcano maybe exposed in accordance to the level of erosion of the surrounding landscape (Plate II, 1). Therefore the clear identifi- cation and the geometrical consideration of the maar volcanic architecture bear vital information of the estimation of the level of erosion. Using eroded monogenetic volcanic fields to calculate long term erosion and to estimate possi- ble palaeogeography of a region are powerful tools to reconstruct syn-volcanic landscapes. Therefore such works could give vital information of a region‘s landscape evolu- tion. Maar/diatreme volcanoes are often the only sites where already eroded pre-volcanic sediments may have been preserved, thus their study may give information to understand the stratigraphy. Phreatomagmatic explosive phases occur in almost every small-volume volcanoes especially in valley settings with good water availability. Subsequent magmatic explosive eruptions, however, are commonly accompanied with such volcanoes in the wan- ing phase of the eruption history of such volcanoes (ARANDA-GOMEZ et al. 1992). This phase commonly pro- duces large scoria cones accompanied by lava effusion

commonly erupting inside the crater (Plate II, 2) (ARANDA-

GOMEZet al. 1992). Phreatomagmatic volcanic fields commonly erupted into fluvio-lacustrine basins, therefore the

recognition of great diversity of phreatomagmatic volcanoes in ancient settings almost certainly means that the syn- volcanic landscape is rather basin-like, and/or valley settings. Such an interpretation may alter the extrapolation of the estimated landscape erosion data. It also recently has been highlighted that volcanic activity may rejuvenate from time to time in an otherwise small-volume volcano that has been considered to be monogenetic. This may challenge the „monogenetic“ characteristics of such volcanism. Recognition of such processes in the preserved geological records of such small-volume volcanoes must be taken seriously in the estimation of the landscape erosion of the syn-erup- tive settings. In the next paragraphs we list a few basic considerations that may alter the estimation of the certain syn- volcanic landscape erosion level.

Lava fields, spatter cones, and scoria cones

Hawaiian to Strombolian-type eruptions build up spatter, scoria and/or cinder cones ((Plate II, 3). Eruption rate, volatile content, magma composition and temperature are the most obvious controlling factors during their eruption (HOUGHTONand SCHMINCKE1989, HOUGHTONet al. 1999, VESPERMANNand SCHMINCKE2000). 95% of observed cinder-

Figure 5.2.Schematic cross sections of the three major types of monogenetic volcanoes (after CASand WRIGHT1988: p.

377, fig. 13.17)

Figure 5.3. Cross section of a typical maar (after LORENZ2007: p. 290, fig. 1)

cone eruptions lasted less than a year in contrast to composite volcanoes formed from multiple eruptions over thousands of years — an important notion in view of the hazard assessment. Comparative morphology of scoria cones is a useful dating tool, however, new researches suggest that their erosion could be more complex. Rare basaltic Plinian eruptions are poorly known but dangerous volcanic phenomena. Shield volcanoes (Plate II, 4) are common and give the major

sources of lavas in intraplate provinces (WALKER1993). Eruptions of large Hawaiian-type volcanic centers usually relat-

ed fissure-vent systems (Plate II, 5), but in small plains-basalt province eruptions

related to central vent systems (JOHNSON

1989), but there are several examples where shield volcanoes developed along

basement fissure systems (JOHNSON1989).

The individual lavaflows associated with intracontinental volcanic fields tend to be around 5 to 10 km long however excep- tionally long lava flows are also known (CONNOR and CONVAY 2000, KILBURN 2000). The total thickness of the lava cover could reach several tens of meters and could cover significant portions of a volcanic field. Adjacent to the lava field Strombolian scoria cones and Hawaiian spatter cones are common features in a continental volcanic field. These volca- noes are commonly point sources of lava flows (Figure 5.4). Basaltic volcanic fields commonly accompanied by exten- sive lava fields ranging from aa’ to pahoe-

hoe types of lava flows (KILBURN2000).

Lava flow fields bear characteristic sur- face morphological features such as tumuli (Plate II, 6), sky rise, whale back humps, lava tubes, and pressure ridges (KILBURN 2000). Such surface morpho- logical features are commonly large in dimension (tens of metres) and character- istic for eruption rate, composition, pre- flow morphological features and composi-

tion of the flow (KILBURN2000). Among

these features, tumuli, are whale-back- shaped uplifts are common in most of the pahoehoe lava flow fields, such as the

Deccan, India (DURAISWAMIet al. 2001),

Hawaii, USA (WALKER 1991), Etna, Italy

(CALVARIet al. 2003), Iceland (MATTSSON

and HOSKULDSSON 2005), or in Eastern

Australia (OLLIER1964, WILMOTHand WALKER1993). Tumuli are positive topographic features with wide range of slope

angles and surface morphological features that are common on pahoehoe lava flow fields. Commonly three types of tumuli are distinguished on shield volcanic systems, such as (1) lava-coated tumuli, (2) upper-slope tumuli and (3) flow-

lobe tumuli in accordance to their distance from their source (ROSSIand GUDMUNDSSON1996). Flow-lobe tumuli are com-

mon in the medial and distal parts of pahoehoe flow fields, whereas the other two tumulus types are more frequent in

the proximal parts of the flow fields (ROSSIand GUDMUNDSSON1996). The flow- lobe tumuli are significantly larger, have

shallower flanks, and do not have extensive outflows from the cracks in comparison to more proximal types of tumulus

(ROSSIand GUDMUNDSSON1996). They commonly group into cluster in gentle slopes of shield volcanoes, such Hawaii

(WALKER 1991). Large tumuli are comparable in size to small lava spatter cones, and therefore their recognition from morphological point of view is important to establish the eruption history of volcanic fields,. It has been demonstrated that recognition of tumuli features and their characteristic surface textures may give hint to quantify eruption duration (MATTSSONand HOSKULDSSON2005). There are thin lava foot breccias between the individual lava beds.

Figure 5.4.Interpretation of effusive and explosive eruptive processes responsible for the formation of the Lathrop Wells volcano in the Yucca Mountain, Nevada (VALENTINEet al. 2006: fig. 17)

Spatter cones consist of a near vent strongly baked, red, slightly bedded sequences with large spindle or highly vesic- ulated fluidal bombs (Plate III, 1). These deposits usually reflect strong reworking of volcaniclastics in near vent posi- tion. Spatter cones and scoria cones can build up steep spatter and agglutinate piles, that can collapse gravitationally (Figure 5.5), or driven away by moving lava flow initiated from the flank of the cone (Plate III, 2).

Strombolian scoria and spatter deposits are common in relation with maar volcanic centers. Even maar volcanic centers may produce phases of Hawaiian and Strombolian-style eruptive activity from several distinguished eruption points, leading to agglutinates or even clastogenic lavas. There are examples in the Tihany Volcano maar volcanic

complex for this kind of deposits in the northern part of the maar complex (Gödrös–Diós) (NÉMETHet al. 1999). A remnant of Strombolian scoria cone in the Füzes-tó region in the Bakony–Balaton Highland Volcanic Field, Hungary shows near vent scoriaceous volcaniclastic breccia in peperitic matrix representing water saturated slurry in the vent

during the Strombolian activity (NÉMETHand SZABÓ1998).

Magmatic explosive and/or degassing processes as result of the fragmentation of the uprising mafic magma lead-

ing the formation of scoria cones with common welded core zones (VESPERMANNand SCHMINCKE2000). The textur-

al characteristics of the pyroclasts, such as high vesicularity, fluidal shape, and the dark, often red colour (Plate III,

3) indicate a magmatic degassing and fragmentation history due to Strombolian-style explosive eruptions (JAUPART

and VERGNIOLLE 1988, VERGNIOLLEand BRANDEIS 1996, VERGNIOLLEet al. 1996, SUMNER 1998, VESPERMANN and

SCHMINCKE2000). The closely packed, slightly oriented texture of lava ash and lapilli rich pyroclastic rocks are inter-

preted to be the result of Hawaiian-style lava fountaining (THORDARSONand SELF1993, VESPERMANNand SCHMINCKE

2000, WOLFFand SUMNER2000, SUMNERet al. 2005). The common intercalation of scoria beds of scoria cones with

welded fall out deposits and/or clastogenic lava flows indicate a sudden and common change in eruption style from

Strombolian to Hawaiian and vice versa (Plate III, 4) (PARFITTand WILSON1995, PARFITTet al. 1995, WILSONet al.

1995). The presence of pyroclastic breccias, lapilli tuff and tuff interbeds in scoria cones are common signs of a

phreatomagmatic influence on the eruptions (HOUGHTONand HACKETT1984, HOUGHTONet al. 1991, DOUBIKand HILL

1999, HOUGHTONet al. 1999).

Scoria cones are the most common subaerial volcanic landforms on Earth and are generally considered to be a

result of mild explosive eruption of mafic magmas in a short period of time (days, weeks) (VESPERMANN and

SCHMINCKE2000), however long-lived scoria cone eruption such as Parícutin in Mexico was active for 9 years (LUHR

and SIMKIN1993). In spite of the numerous scoria cones associated with volcanic fields and central volcanoes (e.g.

along rift zones) there are only a few detailed studies that have been carried out on their architecture (MCGETCHIN

et al. 1972, CHOUET et al. 1974, MCGETCHIN et al. 1974, MCGETCHIN and SETTLE 1975, HEADand WILSON 1989,

RIEDELet al. 2003). Scoria cones inner part usually consists of welded agglutinate (Figure 5.6). Such welded part usually more resistant against erosion and can be preserved long time (Plate III, 5). Detailed analyses of deposits preserved on scoria cones lead to the clarification of the role of the shal- low seated magmatic system in the control of the

explosive eruptions of such volcanoes (HOUGHTONet

al. 1999). Among the identified parameters the vari- ations in degassing patterns, magma ascent rates and degrees of interaction with external water are thought to be responsible for sudden changes in the eruption sequence from deposits representative for

“wet” and “dry” eruption conditions (HOUGHTONet

al. 1999). In general scoria cone-forming eruptions are linked to Strombolian-type activity driven by magmatic fragmentation occurring in the near sur- face region of the open volcanic conduit (BLACKBURN and SPARKS 1976, HOUGHTON et al. 1999). Among scoria cones a great variety has been observed and described, which show gradual transi- tions between Hawaiian lava fountaining (Figure 5.7) to moderate Strombolian-type eruptions. It has been suggested that magma ascent speed is the most important factor causing such transitions, with gas content and viscosity also influencing the ascent

speed at which the transition occurs (PARFITT and

WILSON 1995, PARFITT et al. 1995). A decrease in gas content does not cause a transition from Hawaiian to

Strombolian activity, but instead causes a transition to passive effusion of vesicular lava (PARFITTand WILSON1995).

Some authors suggest that a change from Hawaiian to Strombolian-style requires a significant reduction in magma

ascent speed (PARFITTand WILSON1995). Among many of the recently identified deposits related to basaltic explo-

sive volcanism Tarawera (New Zealand) 1886 eruption is a classical example of characteristic mafic (sub)-Plinian-

style (HOUGHTON et al. 2004). “Violent Strombolian” eruptions are explosive eruptions of mafic magma charac-

terised by eruption column heights <10 km, voluminous ash production, and simultaneous lava effusion (HOUGHTON

Figure 5.6.Scoria cone structure (after VESPERMANand SCHMINCKE

2000)

1 — initial phreatomagmatic units, 2 — strombolian units with intercalated phreatomagmatic beds, 3 — strombolian eruption formed cone facies, 4 — post-strombolian talus, 5 — distal fallout tephra, CF = crater facies, UCF = upper crater facies, WF = wall facies, TS = talus slope

et al. 2004). The mechanism of gener- ation, fragmentation, transportation and deposition of ash in these erup- tions is poorly understood, however such eruption was just recently identi- fied to have occurred not only during eruption of composite volcanoes but also in single scoria cones. The best

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