Stapafell is a prominent geological landmark in Iceland, recognized for its distinctive, often pyramid-like silhouette. Its unique appearance hints at a compelling geological history. This article explores the primary rock type composing Stapafell and the processes that shaped its characteristic features.
The Primary Rock of Stapafell
Stapafell is predominantly composed of basalt, a common type of extrusive igneous rock. Basalt forms when molten rock, known as lava, erupts onto the Earth’s surface and cools rapidly. This rapid cooling typically results in a fine-grained texture, meaning individual mineral crystals are too small to be seen without magnification. Basalt usually exhibits a dark gray to black coloration.
The mineral makeup of basalt primarily includes calcic plagioclase feldspar and pyroxene. Olivine can also be a significant mineral component, contributing to the rock’s overall dark hue. Basalt is abundant in volcanic regions globally, and its prevalence across Iceland underscores the country’s active geological nature.
How Basalt Forms
Basalt originates from mafic lava, which is molten rock rich in magnesium and iron. This lava has a relatively low viscosity, allowing it to flow easily across surfaces. The molten material begins deep within the Earth’s upper mantle, where intense heat and pressure create magma. During volcanic eruptions, this magma rises to the surface, becoming lava as it exits the Earth’s crust.
Upon exposure to the cooler surface environment, the mafic lava cools swiftly. This rapid cooling prevents the minerals within the lava from growing into large, discernible crystals. Instead, minerals crystallize quickly, resulting in the characteristic fine-grained texture of basalt. This process of solidification at or near the surface is termed extrusive igneous rock formation.
The Distinctive Columnar Basalt Formations of Stapafell
Stapafell’s distinctive appearance stems from columnar jointing. This process occurs as thick basaltic lava flows cool and contract. As the lava solidifies, thermal stresses cause it to crack in a systematic pattern. These cracks typically propagate from the top and bottom surfaces of the flow inwards, forming a network of fractures.
The geometry of these fractures often results in the formation of polygonal columns, most commonly hexagonal shapes. This hexagonal patterning is an efficient way for the contracting rock to relieve stress, as three fractures intersecting at 120-degree angles optimally stabilize the cooling material. The columns generally form perpendicular to the cooling surfaces of the lava flow. The conditions in Iceland, characterized by extensive and uniform lava flows, provided an ideal environment for the development of such columnar structures. These formations contribute to Stapafell’s dramatic appearance, making it a notable geological landmark.