Basalt cliffs are primarily formed through the accumulation of thick lava flows from volcanic eruptions, which then undergo cooling and subsequent erosion. These impressive geological features often display characteristic patterns, most notably hexagonal columns, which result from the specific way basaltic lava cools and contracts.
The Volcanic Origins of Basalt Cliffs
Basalt is a type of volcanic rock formed from the rapid cooling of basaltic lava. When volcanoes erupt, they release molten rock (lava) that flows across the landscape. Basaltic lava is typically less viscous than other types, allowing it to flow over large areas and form thick layers. Over many eruptions spanning thousands or millions of years, repeated lava flows can stack up, creating massive rock formations.
Think of it like building up layers of liquid concrete – each layer hardens, and over time, a substantial structure is created. In the case of basalt cliffs, these layers can reach hundreds or even thousands of feet thick.
The Role of Cooling and Contraction
A key feature often seen in basalt cliffs is columnar jointing. This striking pattern of polygonal columns (most commonly six-sided) is not carved by erosion but is a natural result of the lava cooling and shrinking.
Here’s how columnar jointing forms, incorporating insights from geological studies:
- When a thick layer of basaltic lava cools, it cools from the outside inward – the top surface and any exposed sides cool fastest.
- As the rock cools, it contracts. This contraction causes stress within the rock.
- To relieve this stress, the rock fractures. These fractures typically start perpendicular to the cooling surface.
- These chunks begin to form at the top layer of the lava, which is cooling the fastest. The fracture pattern tends to develop into a network of polygonal cracks, with the hexagonal shape being the most energy-efficient way to fill space with uniform cracks originating from cooling centers.
- As lower levels of the lava start to cool, they are also pulled into the shape under each center. This process propagates downwards through the cooling lava flow, creating the elongated columnar structures seen in cliffs like the Giant's Causeway in Northern Ireland or Devil's Postpile in California.
- Scientists also think that faster cooling, like when lava is exposed to water (e.g., a lava flow entering the ocean or a lake), may also help with the formation of these columns, potentially leading to smaller, more numerous columns.
Erosion Exposes the Cliffs
While the lava flows create the bulk of the rock mass and the cooling forms the internal structure (like columns), it is primarily erosion that carves these formations into visible cliffs.
Natural forces like:
- Waves (for coastal cliffs)
- Rivers
- Glaciers
- Wind
- Rain and chemical weathering
Gradually wear away surrounding softer rock or weaker sections of the basalt, leaving the more resistant, massive basalt layers, often with their distinctive columnar joints exposed as sheer cliff faces. The strength and structure provided by the columnar jointing often help the basalt resist erosion more effectively than unjointed rock, allowing the cliffs to stand tall.
In summary, basalt cliffs are towering structures built from layered basaltic lava flows. Their characteristic appearance, particularly columnar jointing, is a product of the lava's cooling and contraction pattern. These formations are then revealed and shaped into cliffs over long periods by erosional processes.
