Rocks exposed at Gandalf Ridge on the northern slopes of Mt. Morning represent the earliest documented evidence of volcanism in the Erebus volcanic province and the McMurdo Volcanic Group. K/Ar dating of samples from Gandalf Ridge suggests a mid-Miocene age (15.4 m.y. to 18.73 m.y.).
The rocks range in composition from trachyandesite to quartz trachyte and pantellerite. Two different trends, one non-peralkaline and one peralkaline, are recognized. The non-peralkaline trend is represented by a trachyandesite-quartz trachyte sequence and the peralkaline trend by a trachyandesite-comenditic trachyte-comendite-pantellerite sequence. Major, trace, and rare earth element data are consistent with the trachyandesite being derived from an alkali basalt parent by fractional crystallization of olivine, clinopyroxene, feldspar, opaque oxides and apatite.
The volcanism at Mt. Morning is believed to have been associated with the fracturing caused by an extensional tectonic regime. This regime is consistent with the widespread alkaline volcanism and the known crustal thinning in the western Ross Sea. These rocks are part of the McMurdo Volcanic Group.