Stapafell is a visually striking, pyramid-shaped mountain dominating the landscape near the Snæfellsjökull glacier on Iceland’s Snæfellsnes Peninsula. Its distinct form results from the powerful interplay between volcanic activity and glacial ice during the last Ice Age. Understanding the mountain’s makeup requires focusing on the materials created when molten rock meets massive sheets of frozen water.
The Primary Rock Composition
The bulk of Stapafell is composed of palagonite tuff, which belongs to Iceland’s Móberg Formation. This rock is a fragmented, consolidated mixture derived from basaltic magma, not a simple lava flow. Palagonite is a yellow-brown, hydrated glass that forms when hot basaltic glass reacts chemically with water or steam. This alteration creates a relatively soft, porous rock mass characteristic of volcanic formations erupted under ice or water.
The parent material for palagonite tuff is hyaloclastite, a glassy volcanic fragment. Hyaloclastite is shattered basaltic lava that cools so rapidly upon contact with water that it forms glass shards instead of crystals. Driven by geothermal heat, these unstable glass fragments undergo palagonitization, transforming into the more stable palagonite mineral. This transformation classifies the mountain as palagonite tuff.
Origin Through Subglacial Volcanism
Stapafell was created through glaciovolcanism, or subglacial eruption, when the area was covered by a thick ice sheet during the Pleistocene epoch. Eruptions beneath glacial ice face immense pressure, preventing the explosive release of volcanic gases. Magma encounters water from the melted ice, causing a phreatomagmatic eruption where lava instantly shatters into glass fragments that accumulate around the vent.
The heat from the magma melted the surrounding ice, creating a lake contained by the ice walls. Within this meltwater lake, the glassy fragments settled and consolidated with hot steam and hydrothermal fluids. This rapid chilling and subsequent hydrothermal alteration drives the formation of palagonite tuff. The shape of these subglacial volcanoes—often flat-topped tuyas or elongated tindars—is controlled by the thickness of the confining ice sheet.
When the glaciers retreated, the mountain was exposed and heavily eroded by the retreating ice, carving its distinctive, steep-sided profile. The soft palagonite material is easily weathered, explaining the mountain’s eroded, pyramidal shape today. This geological history links Stapafell to numerous other subglacial volcanic structures across Iceland.
Defining Structural Features
While the body of Stapafell is composed of soft palagonite tuff, its recognizable form is maintained by harder, intrusive basalt structures. The magma conduits and inner intrusions that fed the eruption survived glacial and atmospheric weathering. These structures are typically dikes and sills—sheets of basalt that cut across or intruded into the surrounding tuff layers.
Dikes are discordant intrusions that cut vertically across rock layers, while sills are concordant, intruding horizontally between layers. The basalt in these intrusive bodies cooled more slowly than the surrounding tuff, allowing it to crystallize into a much denser, more resistant rock. These basaltic intrusions often exhibit columnar jointing, where the rock contracts upon cooling and fractures into regular, polygonal columns.
These hard, vertically oriented basalt columns act as a scaffold, providing the structural integrity to resist total collapse. The visible, steep cliffs and robust shape of Stapafell are a contrast between the soft, yellow-brown palagonite core and the dark, hard basaltic intrusions that reinforce it. The mountain reveals the internal structure of a volcano born beneath glacial ice.