Volcanism; products and processes

Volcanoes and volcanic eruptions always capture the imagination, so long as one is at a safe distance from the action. These processes have been instrumental in shaping Earth – physical and human landscapes and ancient migration routes, the origin and shaping of life, climate, plus all those subterranean processes deep in Earth’s crust and mantle. Volcanic rocks provide a window into our ancient world and worlds beyond our own. Observing volcanism in action tells us which window to open.

Geology for Kayakers #2 Huka Falls, New Zealand

 

Geology for Kayakers #4 In The Shadow of The Volcano

 

Geology for Kayakers #8 The Toba eruption – how a super volcano almost stopped humanity in its tracks

 

The brutality of Surtsey’s laboratory

 

Overture to a cave; the spectacle of jointing in ancient basalt lava flows

 

Lahars; train-wreck geology

 

Islands with attitude; the devastation wrought by collapse of oceanic volcanoes

 

Erupting mud volcanoes: we have ignition

 

Deciphering a volcano’s moods; predicting volcanic eruptions

 

Hiking with my grandson

 

The Pink and White Terraces, Magic Lanterns, and 19th century narratives

 

Mount St. Helens: 40th Anniversary

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Conjugate fractures and en echelon tension gashes – indicators of brittle failure in Old Red Sandstone, Gougane Barra, County Cork, Ireland.
Mohr-Coulomb failure criteria
A montage of stress transformation paraphernalia and rock deformation
Mohr circles and stress transformation
The Marlborough strike-slip fault array extends north from the dextral Alpine Fault transform; faults continue across Cook Strait to join the North Island Dextral Fault Belt in the Wellington region (central Aotearoa New Zealand). In Marlborough and beneath Cook Strait there are several pull-apart basins formed at releasing bend stepovers. Sandbox analogue models can help us decipher the mechanical and kinematic processes that produce structures like these. Base image from NASA – International Space Station 2003.
Strike-slip analogue models
Scaled sand-box experiments are an ideal medium to observe rock deformation that, in this example, involves synkinematic deposition during rift-like crustal extension. The choice of model materials, in addition to imposed boundary conditions such as strain rates, will determine the outcome of the experiment. Dry sand was chosen for this model because its brittle behaviour under the model conditions is a good representation of natural rock failure. Diagram modified slightly from Eisenstadt and Sims, 2005, Figure 3a.
Analogue structure models: Scaling the materials
The relationship between inertial and gravitational forces expressed by the Froude number (Fr) is reflected by the changes in surface flows and the formation-decay of stationary (standing) waves. Fr < 1 reflects subcritical (tranquil) flow; Fr>1 supercritical flow. Although the Froude number can be determined experimentally, it can also be eased out of a dimensional analysis of the relevant hydrodynamic variables.
Model dimensions and dimensional analysis
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