The Bridge River Cones , sometimes referred to as the Lillooet Cones and Salal Creek Cones , is the name given to a volcanic field located on the north flank of the upper Bridge River , about 40 km (25 mi) west of the town of Gold Bridge . The cones are in the lee of the Lillooet Icecap and sit astride a group of passes between the Bridge River, which flows W-E to their south, and the Lord River, which flows north to the Taseko Lakes in the Chilcotin District .
7-524: The Bridge River Cones consist of small trachybasaltic and basaltic eruptive centers. Sham Hill ( 50°54′N 123°31′W / 50.90°N 123.51°W / 50.90; -123.51 ), a 60 m (197 ft) high steep-sided volcanic plug , is the oldest volcano in the field with a potassium-argon date of one million years. The plug is approximately 300 m (984 ft) wide and its bare glaciated surface, strewn with glacial erratics , consists of large subhorizontal columns formed within
14-461: A high content of alkali metal oxides . Minerals in trachybasalt include alkali feldspar , calcic plagioclase , olivine , clinopyroxene and likely very small amounts of leucite or analcime . An aphanitic (fine-grained) igneous rock is classified as trachybasalt when it has a silica content of about 49% and a total alkali metal oxide content of about 6%. This places trachybasalt in the S1 field of
21-474: A small basaltic stratovolcano with a potassium-argon date of 0.6 million years, was constructed on the Bridge River upland when neighboring valleys were filled by ice. Where distal flows violated on the glaciers a marginal meltwater lake was created in which less than 150 m (492 ft) of interbedded hyaloclastite , debris flows , and lacustrine tuff were deposited. The youngest volcanic rocks in
28-499: The TAS diagram . Trachybasalt is further divided into sodium-rich hawaiite and potassium-rich potassic trachybasalt , with wt% Na 2 O > K 2 O + 2 for hawaiite. The intrusive equivalent of trachybasalt is monzonite. Trachybasalt is not defined on the QAPF diagram , which classifies crystalline igneous rock by its relative content of feldspars and quartz . However,
35-416: The U.S. Geological Survey defines trachybasalt as a mafic volcanic rock (composed of over 35% mafic minerals) in which the quartz - feldspar - feldspathoid fraction of the rock is less than 20% quartz and less than 10% feldspathoid, and in which plagioclase is between 65% and 90% of the total feldspar content. Trachybasalt is common in continental volcanism and is also found on some ocean islands. It
42-409: The Bridge River volcanic field lie east of Tuber Hill and are remnants of valley-filling basalt flows. The age of these valley-filling basalt flows is unknown but the presence of unconsolidated glacial until under the flows suggest they are less than 1,500 years old. Trachybasalt Trachybasalt is a volcanic rock with a composition between trachyte and basalt . It resembles basalt but has
49-522: The central conduit of an eroded stratovolcano . The Salal Glacier volcanic complex ( 50°47′N 123°23′W / 50.79°N 123.38°W / 50.79; -123.38 ), with a potassium-argon date of 0.97 to 0.59 million years, contains subaerial tephra and thin scoriaceous flows in the upper part of the pile are surrounded by ice-ponded flows up to 100 m (328 ft) thick. Tuber Hill ( 50°56′N 123°26′W / 50.93°N 123.44°W / 50.93; -123.44 ),
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