The Giant’s Causeway has long been presented as a classic example of cooling basalt lava forming perfect hexagonal columns, but the real story behind this landscape is far more complex and far more dramatic than the traditional explanation suggests. For almost a century, geologists believed a deep river-carved valley existed here before the lava arrived, allowing the molten basalt to pond and cool into the thick columnar-jointed structures we see today. In this video, we explore how new geological research completely overturns that long-held idea and reveals that the Causeway’s unusual thickness and structure were actually shaped by the sudden collapse of a magma chamber beneath Paleocene Ireland.
This documentary-style breakdown explains how the laterite layer beneath the Causeway basalts became the single most important clue. Laterite is an iron-rich soil formed under intense tropical weathering, and its preservation proves that no valley was carved here at all. If a river system had sliced down into the landscape, this delicate layer would have been eroded away entirely. Instead, the laterite remains intact, flat, and undisturbed, showing that the old model cannot account for the geological evidence. By analysing the dips and fractures in the Lower Basalt Formation, geologists discovered that the rocks actually tilt inward toward a central point beneath the Causeway, matching the shape of a subsidence basin rather than an eroded valley.
This video dives deep into the subsidence model, detailing how a shallow magma chamber beneath ancient Northern Ireland inflated slightly as magma entered, then catastrophically lost support when that magma drained toward an eruptive vent or migrated elsewhere within the crust. As the molten material emptied, the ground above sagged in a broad, gentle collapse. This movement fractured the overlying basalt flows and created a basin-like depression at the surface. Almost immediately afterward, a new pulse of tholeiitic basalt erupted and flowed into this freshly formed depression, forming an exceptionally thick lava pond. As this massive body of lava cooled slowly and uniformly, it developed the spectacular columnar jointing that would one day become world famous.
Link to the study used to construct this video:
Subsidence, not erosion: Revisiting the emplacement environment of the Giant’s Causeway, Northern Ireland:
https://www.sciencedirect.com/science/article/abs/pii/S0016787821000729
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The core mission of OzGeology is to make geology exciting, accessible, and inspiring for everyone. Instead of presenting rocks and earth science as dry or overly academic, OzGeology brings stories of the planet to life, revealing how every mountain, mineral, and landscape tells part of Earth’s grand adventure. The goal is to help people see the world differently, to understand the dynamic forces shaping Australia and beyond, and to spark curiosity in the next generation of geologists. Through engaging storytelling, field exploration, and clear explanations, OzGeology turns the study of our planet into a journey of discovery rather than a classroom lecture.
00:00-01:52 – A Familiar Landscape With an Unfamiliar Story
01:53-03:05 – The Basalts Beneath and the Origin of the Traditional Model
03:06-04:48 – The Evidence That Destroyed the Old Interpretation
04:49-06:58 – The New Model: Magma Chambers, Collapse, and Lava Ponding
06:59-08:33 – Why the New Interpretation Changes Everything
08:34-10:03 – Conclusion & Patreon / YouTube Member Thank You!
Credit to : OzGeology