Basalt Column Formation: Lava Cooling, Contraction, and Hexagonal Cracking
Clip title: How Does Lava Form Hexagon Columns? Author / channel: GEO GIRL URL: https://www.youtube.com/watch?v=37DXAPteKIw
Summary
The video provides a comprehensive explanation of how basalt columns, those remarkably regular, often polygonal rock formations seen in places like Iceland and Northern Ireland, are formed. The central concept is that these structures are a direct result of the cooling and subsequent contraction of hot volcanic rock, specifically basaltic lava. As lava flows cool from its extremely hot initial state (over 1000°C), the atoms within the rock vibrate less intensely, causing the material to shrink. This contraction builds up tensile stress, which eventually exceeds the rock’s strength, leading to the formation of cracks.
This cracking process is analogous to how polygonal patterns form in drying mud. However, unlike mud cracks, basalt cracks often extend vertically, forming literal columns. Basalt itself is a dark, silica-poor, and highly fluid volcanic rock, which is why it can flow easily across landscapes and is abundant in areas of seafloor spreading like the Mid-Atlantic Ridge, upon which Iceland sits. The continuous escape of heat from the cooling lava, both to the atmosphere from the top and to the ground from the bottom, causes these fractures to propagate through the rock. Initially, the crack patterns can be irregular, but as the cooling front advances and the fracture network reorganizes to efficiently distribute the contractional strain, the cracks tend to meet at angles of approximately 120 degrees, naturally producing a network dominated by hexagons.
The video further explores characteristics and nuances of columnar jointing. The length and size of the columns are determined by how the cooling front moves through the lava, with slower cooling generally producing larger columns and faster cooling resulting in smaller ones. It’s noted that while hexagons are common, columns can have varying numbers of sides, influenced by factors like initial cooling conditions, imperfections in the rock, and the thickness of the lava flow. A key takeaway is the clarification that columnar jointing is not a different type of lava (like pahoehoe or a’a, which describe flowing lava textures), but rather what happens to a stationary body of lava as it cools and contracts.
Finally, the video dispels the misconception that these columns are exclusive to basalt or even just lava. Similar columnar structures can be found in other volcanic rocks and even in magma that cooled underground, as exemplified by Devil’s Tower in Wyoming. Ultimately, these spectacular and seemingly unnatural patterns are not evidence of nature’s preference for perfect shapes, but rather a physical record of the fundamental laws of physics at work, demonstrating how heat loss dictates the cracking and patterning of cooling rock.
Video Description & Links
Description
On this immersive trip, participants will get to meet with local scientists, learn from experts, and explore the recent lava fields of Reykjanes volcanism, Þingvellir rift valley, geothermal features, magestic waterfalls, and the glacial ice that carves many of these striking features. Learn how Icelanders harness geothermal energy, monitor volcanic hazards, increase community resilience, and study the impacts of climate change on the geological systems that sustain Iceland’s ecosystems.
In this video, I go over how cooling lava sometimes fractures into remarkably geometric columns… From Iceland to Scotland, we explore the physics behind columnar jointing, why basalt often forms hexagons, what controls the size and shape of the columns, and why some lava flows develop spectacular columns while others don’t.
0:00 Basalt Hexagons 0:43 What is Basalt? 1:31 Iceland Lava Flows 2:14 Cooling Lava Cracks 3:09 Hexagons Develop as the Cracks Propagate 4:20 How do they become so tall? 5:28 How fast the lava cooled 6:10 NOT only basalt 6:41 NOT a flow structure
Tags
geology, geology crash course, earth science
Related Concepts
- basalt column — Wikipedia
- lava cooling
- thermal contraction — Wikipedia
- tensile stress — Wikipedia
- hexagonal cracking
- polygonal jointing
- volcanic rock — Wikipedia
- geology — Wikipedia
- basalt column formation
- columnar jointing — Wikipedia
- heat loss — Wikipedia
- seafloor spreading — Wikipedia
- geothermal energy — Wikipedia
- volcanic hazards — Wikipedia
- climate change impacts — Wikipedia