Continental basaltic volcanoes — Processes and problems

Abstract

Monogenetic basaltic volcanoes are the most common volcanic landforms on the continents. They encompass a range of morphologies from small pyroclastic constructs to larger shields and reflect a wide range of eruptive processes. This paper reviews physical volcanological aspects of continental basaltic eruptions that are driven primarily by magmatic volatiles. Explosive eruption styles include Hawaiian and Strombolian (sensu stricto) and violent Strombolian end members, and a full spectrum of styles that are transitional between these end members. The end-member explosive styles generate characteristic facies within the resulting pyroclastic constructs (proximal) and beyond in tephra fall deposits (medial to distal). Explosive and effusive behavior can be simultaneous from the same conduit system and is a complex function of composition, ascent rate, degassing, and multiphase processes. Lavas are produced by direct effusion from central vents and fissures or from breakouts (boccas, located along cone slopes or at the base of a cone or rampart) that are controlled by varying combinations of cone structure, feeder dike processes, local effusion rate and topography. Clastogenic lavas are also produced by rapid accumulation of hot material from a pyroclastic column, or by more gradual welding and collapse of a pyroclastic edifice shortly after eruptions. Lava flows interact with — and counteract — cone building through the process of rafting. Eruption processes are closely coupled to shallow magma ascent dynamics, which in turn are variably controlled by pre-existing structures and interaction of the rising magmatic mixture with wall rocks. Locations and length scales of shallow intrusive features can be related to deeper length scales within the magma source zone in the mantle. Coupling between tectonic forces, magma mass flux, and heat flow range from weak (low magma flux basaltic fields) to sufficiently strong that some basaltic fields produce polygenetic composite volcanoes with more evolved compositions. Throughout the paper we identify key problems where additional research will help to advance our overall understanding of this important type of volcanism.

Introduction

Monogenetic basaltic volcanoes are the most common type of continental volcano. They range in form from small scoria cone volcanoes where much of the material was erupted by explosive mechanisms with variable proportions of lava flows (e.g., Wood, 1980, Gutmann, 1979, Luhr and Simkin, 1993, Gutmann, 2002, Valentine et al., 2005, Martin and Nemeth, 2006, Valentine et al., 2006, Valentine et al., 2007), to small shields and chains of shields (e.g., Kuntz et al., 2002, Hughes et al., 2002a). Basaltic volcanoes are most common in extensional or convergent tectonic settings, or in intraplate settings that are near active regions (e.g., Colorado Plateau, U.S.A.), and occur as isolated features (e.g., Glazner et al., 1991), as basaltic volcanic fields (e.g., Condit et al., 1989, Connor, 1990, Camp et al., 1991, Briggs et al., 1994, Connor and Conway, 2000, Aranda-Gómez et al., 2003, Hare and Cas, 2005, Hare et al., 2005, Valentine and Perry, 2006), or in association with (often peripheral to) silicic volcanoes and calderas (e.g., Smith et al., 1970, Carn, 2000). Despite the information these volcanoes contain that is pertinent to our broader understanding of basaltic magmatism, the potential hazards they pose (e.g., Crowe, 1986, Siebe et al., 2004, Siebert and Carrasco-Núñez, 2002, Valentine, 2003, Houghton et al., 2006), and the fact that they are the most abundant subaerial volcanic feature, continental basaltic volcanoes have received relatively little attention in terms of eruption and emplacement processes compared to volcanoes with more evolved compositions.

In this paper, we review recent physical volcanological studies of monogenetic continental basaltic volcanoes and provide our perspective on key problems that need to be addressed by future research. Our focus is on “common” monogenetic basaltic volcanoes whose eruptions are driven primarily by magmatic volatiles. Many basaltic volcanoes are produced wholly or in part by explosive magma–water interaction (hydrovolcanism) to produce features such as tuff rings, maars, and tuff cones and their feeder diatremes. These have been described extensively elsewhere in the literature (e.g., Crowe and Fisher, 1973, Sheridan and Wohletz, 1981, Wohletz and Sheridan, 1983, Lorenz, 1986, White, 1991, Houghton and Schmincke, 1989, Houghton et al., 1999, Vazquez and Ort, 2006) and are therefore are not a focus in this paper.

We begin by discussing explosive eruptive processes of basaltic volcanoes and the resulting pyroclastic facies. This is followed by a review of basaltic lava emplacement processes and resulting features. Based upon observations at historic eruptions such as Parícutin (Krauskopf, 1948, Luhr and Simkin, 1993) and at prehistoric volcanoes such as Lathrop Wells (Valentine et al., 2007) it is clear that highly explosive eruptive activity can be accompanied by simultaneous lava effusion from the same subsurface plumbing system, so it is important to view the separation of pyroclastic and effusive processes in our discussion as one of convenience rather than as an inference that the two processes only occur separately in nature. Additionally, cone growth can be closely coupled to lava effusion through processes of rafting, and in turn, this process can depend upon the slope of the surface upon which a cone is built. Small shields (small in comparison to Hawaiian volcanoes, for example) are the effusive end member of the eruptive behavior discussed here, although even small shields commonly display some pyroclastic activity. Finally, we review field and theoretical aspects of the plumbing systems that feed the various types of basaltic eruption styles. Throughout, we point out issues that require additional research, with the hope that these suggested research topics will motivate work to greatly advance our understanding of continental basaltic systems.