Keck GSA Abstracts

Pillow lavas are one of the most abundant lava morphologies on Earth, but are relatively inaccessible because of their submarine or subglacial eruption environments. Our research location in a former rock quarry in southwest Iceland provides a unique opportunity to view cross-sections through well exposed pillow lavas on land. The quarry is located at the northern end of Undirhlíðar, which is a glaciovolcanic ridge on the Krisuvik fissure system, and exposes thousands of individual pillow lavas. This study uses detailed field and laboratory observations of vesicle distributions and jointing patterns to better constrain the mechanisms that control vesiculation, bubble transport, and cooling rates during emplacement of pillow lava. From detailed analysis of >40 exposed pillow cross sections, we have identified 7 fracture characteristics that make up a combination of fracture patterns within the pillow lavas. These characteristics include: short (<5 cm) fractures at the outer edge of a pillow, fractures within pillow cores, fractures between the core and the edge of a pillow, long fractures (up to 40 cm) that go through the entire pillow, ‘web’-like fractures, fractures that branch from other fractures, and curvilinear fractures that cut through bands of vesicles. The distributions of vesicles are more diverse, with at least 12 different patterns defined by characteristics including: concentric banding, moderately/highly vesicular cores, non-vesicular cores, and open cavities. We identified 6 vesicle pattern combinations in the field, and are using image analysis of nearly 50 field photographs to characterize the patterns. These characteristics will constrain physical modeling to better understand how variations in emplacement conditions (abrupt pressure changes, lava discharge rates, water infiltration along fractures) are recorded by the lavas. These pillow lavas are the only lasting record of a preexisting englacial lake presumably formed during the eruption of the lavas, so understanding the details of their textures may provide new insights into the hydrology of the enclosing ice (occurrence of syn-eruption jokulhlaups, efficiency of sub-ice drainage).

GEOCHEMICAL CONSTRAINTS ON THE MAGMATIC SYSTEM AND ERUPTIVE ENVIRONMENT OF A GLACIOVOLCANIC TINDAR RIDGE FROM UNDIRHLíðAR, SW ICELAND

WALLACE, Chloe¹, KUMPF, Benjamin¹, HEINEMAN, Rachel², LEMBO, Cara³, ORDEN, Michelle⁴, THOMPSON, Anna C⁵, ENGEN, Carl-Lars⁶, KOCHTITZKY, William⁴, POLLOCK, Meagen¹, EDWARDS, Ben⁴and HIATT, Alex¹, (1)Department of Geology, The College of Wooster, 944 College Mall, Wooster, OH 44691, (2)Department of Geology, Oberlin College, 52 West Lorain St, Oberlin, OH 44074, (3)Department of Geology, Amherst College, 11 Barrett Hill Drive, Amherst, MA 01002, (4)Department of Earth Sciences, Dickinson College, 28 N. College Street, Carlisle, PA 17013, (5)Department of Geology, Carleton College, One North College Street, Northfield, MN 55057, (6)Department of Geology, Beloit College, 700 College Street, Box 777, Beloit, WI 53511, cwallace17@wooster.edu

Glaciovolcanic tindar ridges are landforms created by the eruption of magma through fissure swarms into ice. The cores of many of these ridges comprise basaltic pillow lava, so they serve an accessible analogue for effusive mid-oceanic ridge volcanism. Furthermore, similar landforms have been identified on Mars, and thus they may also serve as models for planetary volcanic eruptions. To better understand pillow formation and effusive glaciovolcanic eruptions, we are investigating Undirhlíðar ridge, a pillow-dominated tindar on the Reykjanes Peninsula in southwest Iceland. Our detailed mapping and sampling in two rock quarries along the ridge and in the ~3 km area between the quarries show that this specific tindar ridge has had a complex eruption history. In the northern quarry (Undirhlíðar), Pollock et al. (2014) demonstrated that at least two geochemically distinct magma batches have erupted. Further trace element and isotope analyses in the southern quarry (Vatnsskarð) suggest that the ridge is fed by a heterogeneous mantle source. Isotopic Pb data show a spatially systematic linear array, which is consistent with a heterogeneous mantle mixing between depleted and enriched endmembers. The occurrence of multiple magma batches in dikes and irregular intrusions suggests that these structures are important to transporting magma within the volcanic edifice. Glassy pillow rinds were sampled for volatile analysis by FTIR in order to determine how paleo-water pressures vary along the ridge. In Undirhlíðar quarry, paleo-water pressures decrease with stratigraphic height (1.6-0.7 MPa). In Vatnsskarð quarry, paleo-water pressures show evidence of two separate eruptions, where pressure values decrease with an increase in stratigraphic height from 1.1 to 0.7 MPa over ~30 m, at which point pressure resets to 1.1 MPa and continues to decrease with elevation. When comparing the two quarries, paleo-water pressures in the upper units of Undirhlíðar and all the units in Vatnsskarð have similar values (0.7-1.1 MPa), and these are lower than the basal units of Undirhlíðar (1.2-1.6 MPa). Overall, compositional variations correlate with stratigraphy and spatial distribution along axis, suggesting that glaciovolcanic eruptions and their resulting landforms show a higher level of complexity than previously thought.

3-D

MAPPING OF QUARRY WALLS TO CONSTRAIN THE INTERNAL STRUCTURE OF A GLACIOVOLCANIC TINDAR, SW ICELAND

EDWARDS, Benjamin R.¹, POLLOCK, Meagen², KOCHTITZKY, William¹ and ENGEN, Carl-Lars³, (1)Department of Earth Sciences, Dickinson College, 28 N. College Street, Carlisle, PA 17013, (2)Department of Geology, The College of Wooster, 944 College Mall, Wooster, OH 44691, (3)Department of Geology, Beloit College, 700 College Street, Box 777, Beloit, WI 53511, edwardsb@dickinson.edu

Documentation of the internal structures of volcanoes are critical for understanding how edifices are built over time, especially for glaciovolcanoes, which have rarely formed historically and are inaccessible during eruptions. We have been unraveling the internal structure of a complex glaciovolcanic ridge (tindar) in southwestern Iceland for the past 5 years in order to better understand the sequence of events that built the ridge. Undirhlidar ridge is ~5 km long, and has been dissected by two different aggregate mines along its axis. The northern mine (Undirhlidar quarry) is inactive and has walls up to 40 m in height that fully expose several critical stratigraphic relationships including multiple sequences of separate pillow lava flows, cross-cutting dikes that locally feed overlying pillow flows, and ridge parallel, continuous massive jointed basaltic units that may be the remnants of internal lava supply networks. The second quarry, ~3 km to the southwest (Vatnsskard quarry) is presently active and continually has new exposures. This quarry only penetrates halfway through the width of the ridge but has ~500 m of exposure along strike. It also has remnants of what appears to be the internal magma distributary system, and many components clearly show evidence that they were (and some still are) open lava tubes. While both quarries contain excellent exposures, many of the structures are difficult to safely access or are inaccessible due to mining activity. In order to overcome access issues, we have used Structure-from-Motion techniques to make 3-D maps of the quarry walls. A series of overlapping pictures were taken from points constrained with D-GPS using a Trimble GeoXH data logger and external antennae. The image locations with corrected positions were imported into Photoscan software to create a point cloud representative for each quarry and to derive a Digital Elevation Model with a reported vertical resolution of less than 1 m. Field testing of a preliminary, low resolution DEM shows that measurements of dyke widths on the DEM have errors of ~5% relative to measurements on the ground. Measurements made from the field-generated DEM will provide significantly better constraints on deposit thicknesses and volume estimates compared to traditional methods of estimating unit thicknesses on vertical faces.