3. SHEET FLOWS, COLLAPSE PITS, AND LAVA LAKES OF THE RIFT VALLEY

It has long been known that pillow basalts are a dominant feature of oceanic volcanism on midocean ridges. Recent studies in the Cayman Trough, East Pacific Rise, and Galapagos Rift valley have shown that sheet flows are also an important component, especially on ridges with intermediate and fast opening rates. This paper describes the surface forms of such sheet flows observed in the Galapagos Rift valley. The principal types are lobate, smooth, rippled, and wrinkled, as well as hackly and jumpled forms. Collapse pits of a wide range of sizes are common; they appear to result from subsidence of the lava as a consequence of distal spreading of the flow and/or headward drainback into the subsurface plumbing system. Lava pillars are numerous around collapse pits, and current evidence suggests that they most likely are spiracles produced by water trapped below an advancing flow and rising through it to the surface. From Hawaiian analogs we conclude that sheet flows can be considered a submarine equivalent of surface-fed pahoehoe, while the pillow basalts are analogous to subaerial tube-fed pahoehoe. We view the difference between sheet flows and pillow basalts to be the result of different degrees of channelization and rates of deliverymore » of lava to flow fronts, associated with differences in duration and rate of erruption. In this model the sheet flows and pillowed ridges of the Galapagos Rift are the analogs of the early and the late products of an Hawaiian erruptive event, respectively. The sheet represent early, brief but voluminous eruptions. These are followed by more sustained, slower but steadier eruptive phases that produce pillow basalts after an internal plumbing system has been well established.e of pillows on slow spreading ridges and the sheet flow and pillow complexes on faster spreading ridges.« less

[1]  K. Macdonald Near-bottom magnetic anomalies, asymmetric spreading, oblique spreading, and tectonics of the Mid-Atlantic Ridge near lat 37°N , 1977 .

[2]  D. W. Peterson,et al.  Flow of Lava into the Sea, 1969–1971, Kilauea Volcano, Hawaii , 1973 .

[3]  J. Fink,et al.  Ropy pahoehoe: Surface folding of a viscous fluid , 1978 .

[4]  P. Lonsdale Structural geomorphology of a fast-spreading rise crest: The East Pacific Rise near 3°25′S , 1977 .

[5]  R. Holcomb Preliminary map showing products of eruptions, 1962-1974, from the Upper East Rift Zone of Kilauea Volcano, Hawaii , 1976 .

[6]  Robert D. Ballard,et al.  Manned Submersible Observations in the FAMOUS Area: Mid-Atlantic Ridge , 1975, Science.

[7]  William R. Normark,et al.  Delineation of the main extrusion zone of the East Pacific Rise at lat 21°N , 1976 .

[8]  David L. Williams,et al.  Submarine Thermal Springs on the Gal�pagos Rift , 1979, Science.

[9]  J. G. Jones Pillow Lava and Pahoehoe , 1968, The Journal of Geology.

[10]  J. Francheteau,et al.  Some characteristics of the Rift Valley in the Atlantic Ocean near 36° 48′ north , 1974 .

[11]  R. Hey Tectonic evolution of the Cocos-Nazca spreading center , 1977 .

[12]  James M. Hall,et al.  Photographic Atlas of the Mid-Atlantic Ridge Rift Valley , 1978 .

[13]  P. Lonsdale Abyssal pahoehoe with lava coils at the Galapagos rift , 1977 .

[14]  J. G. Jones Pillow Lava and Pahoehoe: A Reply , 1969, The Journal of Geology.

[15]  D. Swanson Pahoehoe Flows from the 1969–1971 Mauna Ulu Eruption, Kilauea Volcano, Hawaii , 1973 .

[16]  R. Allmendinger,et al.  The Galapagos Rift at 86°W: 1. Regional morphological and structural analysis , 1979 .

[17]  T. H. Andel,et al.  The Galapagos Rift at 86°W: 2. Volcanism, structure, and evolution of the Rift Valley , 1979 .

[18]  J. G. Moore,et al.  Mechanism of Formation of Pillow Lava , 1975 .

[19]  P. Vogt Volcano spacing, fractures, and thickness of the lithosphere , 1974 .

[20]  R. Ballard,et al.  Inner floor of the Rift Valley: first submersible study , 1974, Nature.

[21]  Arcyana Transform Fault and Rift Valley from Bathyscaph and Diving Saucer , 1975, Science.

[22]  R. L. Nichols Flow-Units in Basalt , 1936, The Journal of Geology.

[23]  W. Bryan,et al.  Compositional variations of young basalts in the Mid-Atlantic Ridge rift valley near lat 36°49′N , 1977 .

[24]  J. Lupton,et al.  Hydrothermal plumes in the Galapagos Rift , 1977, Nature.

[25]  K. Crane Structure and Tectonics of the Galapagos Inner Rift, 86°10'W , 1978, The Journal of Geology.

[26]  Robert D. Ballard,et al.  Morphology and tectonics of the inner rift valley at lat 36°50′N on the Mid-Atlantic Ridge , 1977 .

[27]  D. L. Peck,et al.  The Formation of Columnar Joints in the Upper Part of Kilauean Lava Lakes, Hawaii , 1968 .

[28]  G. A. Macdonald Pahoehoe, aa, and block lava , 1953 .

[29]  D. L. Peck March 1965 Eruption of Kilauea Volcano and the Formation of Makaopuhi Lava Lake THOMAS L. WarnliT ANI> WLLm T. KOSmTA U. $. Geological Survey, Hawaiian Volcano Observctory Hawaii National Park, Hawaii 96718 , 1968 .