I wonder whether James R. Beerbower had any inkling that his paper on alluvial fan dynamics would seriously influence stratigraphic […] The post Self-organization, autocyclicity, and the rock record appeared first on Geological Digressions ..read more

A look at the modern settings for carbonate platform deposits on Aitutaki volcanic island and lagoon. Isolated carbonate platforms are […] The post Aitutaki’s (Cook Islands) sedimentary carapace appeared first on Geological Digressions ..read more

The highly dissected volcanic terrain of Rarotonga, viewed from the northeast towards Te Manga (the distant flat-topped summit above the cottage green roof), that boasts the highest elevation at 653 m. Prevailing scientific wisdom tells us that volcanic seamounts and oceanic islands are forged from magma hotspots fed by mantle-derived plumes. The plumes originate deep in the mantle, possibly the core-mantle boundary. Plumes are considered stationary relative to the motion of the overlying oceanic lithosphere – a condition based on the age progression observed in many seamount chains like the H ..read more

A surprising find in 2018 on the summit of Davidson Seamount in the Monterey Bay National Marine Sanctuary (S California) 3200 m depth. In amongst the basalt rubble, along with sea anemones, brittle stars (echinoderms), octocorals and sea pens were thousands of female octopuses (Muusoctopus robustus) protecting their newly hatched eggs. Water temperatures here hover around 11o C because of low-temperature seepage from hydrothermal vents and fractures. Beyond these vents the ambient water temperature is 1.6o C. There is still so much to discover about these geological entities and ecosystems. I ..read more

A fresh road-cut of airfall tephra deposits draping antecedent topography (probably latest Pleistocene or Holocene), south of Rotorua, New Zealand. Every geological outcrop tells a story. How often do you pass a road-side rock outcrop that grabs your attention, a glimpse out of the corner of your eye, but not wanting to drive into a ditch, you carry on. Worthy of a stop on the return journey, so grab a quick look at the odometer and notable landmarks. The road-cut in in this instance is a nice example of airfall pyroclastic deposits draping and modifying antecedent topography. It is only a sin ..read more

Conjugate fractures and en echelon tension gashes – indicators of brittle failure in Old Red Sandstone, Gougane Barra, County Cork, Ireland. Brittle deformation and Mohr-Coulomb failure Deformation of Earth materials takes place predominantly within two rheological domains: brittle and ductile. Deformation close to the Earth’s surface is dominated by brittle failure (faults, fractures, and joints); ductile behaviour of rock become increasingly important deeper in the crust and lithosphere mantle – exceptions include the deformation of ice and salt at shallow depths. Ductile deformation depends ..read more

A montage of stress transformation paraphernalia and rock deformation Calculating normal stress and shear stress using Mohr circles Deformation of rock, sediment, and soil is usually manifested as a change in material location, shape, or volume. This means a change in the angular relationship between the original and final configuration of the rock body relative to some coordinate system – in other words, the rock body is rotated. Deformation involves both normal (tension, compression) and shear stresses and as deformation progresses the components of stress must change; in other words, t ..read more

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. Sandbox analogue models of strike-slip basins and pop-up structures How things ha ..read more

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. This post deals with the materials, their rheological behaviours, and scaling a ..read more

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 teased out of a dimensional analysis of the relevant hydrodynamic variables. The correct scaling of geometric, dynamic, and kinematic system variables is a critical part of any useful analogue model. Most physical systems operate with a multitude of va ..read more