{{Short description|Geological formation in British Columbia}} {{Infobox Rockunit | name = Pyramid Formation | image = The Pyramid.jpg | caption = [[The Pyramid (British Columbia)|The Pyramid]] consists of Pyramid Formation [[trachyte]] | type = [[Geological formation]]<ref name="Souther">{{Cite report|last1=Souther|first1=J. G.|author-link1=Jack Souther|title=The Late Cenozoic Mount Edziza Volcanic Complex, British Columbia|work=[[Geological Survey of Canada]]|publisher=Canada Communication Group|series=Memoir 420|year=1992|pages=100, 129, 132, 133, 134, 246, 248, 249, 267|isbn=0-660-14407-7|doi=10.4095/133497|doi-access=free}}</ref> | age = {{Fossil range|1.2|0.9}} | period = Pleistocene | prilithology = [[Trachyte]], [[comendite]], [[pantellerite]]<ref name="Souther"/> | otherlithology = [[Basalt]]<ref name="Souther"/> | namedfor = [[The Pyramid (British Columbia)|The Pyramid]]<ref name="Souther"/> | namedby = [[Jack Souther|Souther]] ''et al.''<ref name="LCGU">{{cite web|url=https://weblex.canada.ca/html/012000/GSCC00053012294.html|title=Pyramid Formation|work=[[Lexicon of Canadian Geologic Units]]|publisher=[[Government of Canada]]|access-date=2023-08-04}}</ref> | year_ts = 1984<ref name="LCGU"/> | region = [[British Columbia]]<ref name="Souther"/> | country = Canada<ref name="Souther"/> | coordinates = {{coord|57.5|N|130.6|W|display=inline,title}}<ref name="Edwards">{{cite thesis|last1=Edwards|first1=Benjamin Ralph|degree=PhD|title=Field, kinetic, and thermodynamic studies of magmatic assimilation in the Northern Cordilleran Volcanic Province, northwestern British Columbia|publisher=[[University of British Columbia]]|year=1997|page=10|isbn=0-612-25005-9}}</ref> | unitof = [[Mount Edziza volcanic complex]]<ref name="Edwards"/> | subunits = | underlies = [[Ice Peak Formation]], [[Edziza Formation]]<ref name="Souther"/> | overlies = [[Nido Formation]]<ref name="Souther"/> | thickness = | extent = | area = | map = Pyramid Formation cross section.png | map_alt = Diagram showing a low dome-shaped volcano on the left, a lake in the middle and two cone-shaped volcanoes on the right. | map_caption = [[Cross section (geology)|Cross section]] showing the maximum extent of the Pyramid Formation at the end of [[Volcanism of the Mount Edziza volcanic complex#Pyramid eruptive period|Pyramid time]] }} The '''Pyramid Formation''' is a [[stratigraphic unit]] of [[Pleistocene]] age in northwestern [[British Columbia]], Canada.

==Name== The Pyramid Formation takes its name from [[The Pyramid (British Columbia)|The Pyramid]], a prominent [[pyramid]]-shaped [[lava dome]] on the northeastern flank of [[Mount Edziza]].<ref name="Souther"/><ref name="BCGN">{{cite bcgnis|id=22196|name=The Pyramid|access-date=2023-07-02}}</ref>

==Geology== Two subunits comprise the Pyramid Formation. The lower unit is a basal [[Pyroclastic rock|pyroclastic]] [[member (geology)|member]] up to {{Convert|3|m|ft|abbr=on}} thick. It contains [[trachytic]] [[pumice]], sodic [[pyroxene]] and [[alkali feldspar]]; the latter two are present in the form of [[lithic fragment (geology)|lithic]] [[clast]]s and crystals. This unit is rusty brown in colour and was deposited by a [[pyroclastic surge]]. The upper unit consists of a [[basalt]] member up to {{Convert|65|m|ft|abbr=on}} thick. It includes 6&nbsp;to 10&nbsp;individual basalt flows that reach thicknesses of {{Convert|3|–|20|m|ft|abbr=on}}. They appear to have been deposited almost immediately after the pyroclastic surge was erupted.<ref name="Souther"/> Several silicic lava domes and minor lava flows of the Pyramid Formation postdate the basalt member. Among the silicic domes are [[Sphinx Dome]], [[Pharaoh Dome]] and The Pyramid.<ref>{{cite book|last1=Smellie|first1=John L.|last2=Edwards|first2=Benjamin R.|title=Glaciovolcanism on Earth and Mars: Products, Processes and Palaeoenvironmental Significance|publisher=[[Cambridge University Press]]|year=2016|page=43|isbn=978-1-107-03739-7}}</ref>

The Pyramid Formation has a volume of {{Convert|11.4|km3|mi3|abbr=on}}, making it the eighth most voluminous [[geological formation]] of the [[Mount Edziza volcanic complex]] after the [[Little Iskut Formation]].<ref name="Souther"/><ref name="Edwards"/> It overlies the [[Nido Formation]] and underlies the [[Ice Peak Formation|Ice Peak]] and [[Edziza Formation|Edziza]] formations, all of which have formed in the last 8 million years. [[K–Ar dating]] of the Pyramid Formation has yielded ages of 1.2 ± 0.4 million years and 1.20 ± 0.03 million years for [[comenditic]] [[volcanic glass|glass]] and 0.94 ± 0.12 million years and 0.94 ± 0.05 million years for trachyte.<ref name="Souther"/>

==See also== *[[Volcanism of the Mount Edziza volcanic complex]]

==References== {{reflist}}

{{Mount Edziza volcanic complex}}

[[Category:Geologic formations of the Mount Edziza volcanic complex]] [[Category:Pleistocene British Columbia]] [[Category:Pleistocene volcanism]] [[Category:Rhyolite formations]] [[Category:Basalt formations]] [[Category:Trachyte formations]]