GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 176-14
Presentation Time: 9:00 AM-6:30 PM


THOMPSON, Anna C.1, ORDEN, Michelle2, LEMBO, Cara3, WALLACE, Chloe4, KUMPF, Benjamin4, HEINEMAN, Rachel5, ENGEN, Carl-Lars6, EDWARDS, Ben2, POLLOCK, Meagen4 and KOCHTITZKY, William2, (1)Department of Geology, Carleton College, One North College Street, Northfield, MN 55057, (2)Department of Earth Sciences, Dickinson College, 28 N. College Street, Carlisle, PA 17013, (3)Department of Geology, Amherst College, 11 Barrett Hill Dr, Amherst, MA 01002, (4)Department of Geology, The College of Wooster, 944 College Mall, Wooster, OH 44691, (5)Department of Geology, Oberlin College, 52 West Lorain St, Oberlin, OH 44074, (6)Department of Geology, Beloit College, 700 College Street, Box 777, Beloit, WI 53511,

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).