Physiographic Controls on Glaciovolcanism and the Cordilleran Ice Sheet in the
northern Cordilleran volcanic province, western Canada
[*B. Edwards*] (Dept of Geology, Dickinson College, Carlisle, PA, 17013, USA, ph:
717-254-8934; edwardsb@dickinson.edu); J. Osborn (Dept Geo and Geophys, U.
Calgary, Calgary, AB); J.K. Russell (Dept Earth & Ocean Science, U.B.C.,
Vancouver, BC); I.P. Skilling (Dept of Geology and Planetary Science, 200 SRCC,
University of Pittsburgh, PA); C. Evenchick (Geological Survey of Canada,
Vancouver, BC); I. Spooner (Acadia University, Halifax, Nova Scotia); K. Simpson
(Geological Survey of Canada, Vancouver, BC); B. Cameron (Dept of Geology,
University of Wisconsin-Milwaukee, Milwaukee)
Glaciovolcanism in the northern Cordilleran volcanic province of western
Canada occurs in three different physiographic settings, each displaying unique
relationships between topography, volcanism and glacial ice. The first is represented
by the type tuya locality, Tuya Butte, located on the Tanzilla Plateau in northern
British Columbia. This region comprises broad flatlands with comparatively lowlying hills, and may have been one of the centers of ice accumulation for the
Cordilleran ice-sheet (CIS). Pleistocene volcanism in this area is dominated by
basaltic glaciovolcanic eruptions. The most common glaciovolcanic landforms are
individual tuyas, which seldom range in height more than a few hundreds of meters.
Other edifice morphologies are also present, some of which are primary and others
that are possibly the products of erosion. The subdued relief on the Tanzilla Plateau
strongly influenced edifice morphologies, favoring broad-based tuyas. The
hydrological conditions in these areas likely were controlled by relatively uniform ice
thicknesses and gentle topography. Glaciovolcanic processes and landforms conform
to Icelandic and Antarctic examples described in detail by many authors.
The second type of interaction is found mainly to the east, south and
southwest of the Tanzilla Plateau, in the Cassiar Mountains and the Skeena and
Boundary ranges. Volcanism in these areas is also predominantly mafic, but the
greater local relief of these areas had a much more dramatic influence on glaciation
and glaciovolcanism. Alpine-style glaciation dominated when the CIS was absent,
and even when overwhelmed by the CIS basal ice movement was strongly influenced
by the deep, pre-glacial drainages. Accordingly, the erosional remnants of
glaciovolcanic centers tend to be smaller and discontinuous. In the Skeena Ranges,
the isolated outcrops of pillow lava and volcanic breccia are often located along the
tops of arête-like ridges, with presumably temporally-associated volcaniclastic rocks
having collected downslope.
The third type of interaction is geographically limited to three volcanic
structures: Level Mountain, Mount Edziza, and Hoodoo Mountain. All three are
within the eastern part of the Boundary Ranges physiographic province and are large
enough to support ice caps that probably influenced local ice flow, while still being
influenced by the CIS at different times. Mount Edziza and Level Mountain have
benches of older lava 1 km+ in elevations, and have been sites of volcanism long
enough that their geothermal outputs may have had an influence on CIS dynamics
much as the modern day Grimsvotn caldera is an important heat source beneath
Vatnajokull in Iceland. At Mount Edziza most of the glaciovolcanic products are
located on top of the main volcanic plateau, which is now elevated ~1000m above the
surrounding stream valleys. Mount Edziza comprises a variety of interesting mafic
glaciovolcanic products, but more uniquely, along with Hoodoo Mountain and
possibly Level Mountain, comprises some of the largest deposits of peralkaline felsic
glaciovolcanic products known. At Mount Edziza and Level Mountain, glacier
hydrology of the CIS probably was controlled by a complex interplay between
drainage on the flat plateaus beneath relatively thinner ice and drainage within
adjacent, steep valleys filled with much thicker ice.
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