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

The posts in this series focus on Earth science models, particularly the soft-rock kind. I have an abiding memory of my grandfather’s workshop. It was accessed through a trapdoor from the living room then a ladder to the basement. The entrance reflected his past as a sailor and shipwright. You knew you had arrived – the smell of cut timber and glue welcomed one to a world in miniature. Every shelf, nook and cranny filled with dozens of sailing boats in glass containers – bottles of all shapes and sizes (including a 5-gallon flagon), light bulbs, and even pharmaceutical glassware. Ships in bott ..read more

A sand-box size analogue model of thrust-fault propagation folds in layered stratigraphy, at 44% horizontal shortening. A new, embryonic thrust is forming at the frontal edge of the “thrust belt”. These experiments, although not fully scaled geometrically or dynamically, retain considerable heuristic value for students. The image was generously provided by Prof. Sandra McLaren. Analogue models are physical objects; we can touch them and observe represented systems directly. As such, they allow us to map the interactions between processes and the physical responses to those processes. Analogue ..read more

Parallel bedding in a Paleocene turbidite succession, Point San Pedro, California. The thickness of individual beds varies little along their lateral extent, at least within the confines of the outcrop; our view of bedding planes is limited to their 2D extent. The thickest bed is about 50 cm. Geologist’s shoe on the bottom right. The primacy of beds, bedding, and bedding planes Beds are the fundamental units of stratigraphy and sedimentology. They are the first things we identify and measure in outcrop, core, and borehole geophysical logs. Beds are the foundations of stratigraphic successions ..read more

Minerals are defined by their chemical composition and their crystal forms. Miller, and Miller-Bravais indices are the standard where every face on a crystal is given a unique description that in general notation is written as (hkl) and (hkil). Two of the most common forms are prisms (the tourmaline crystal on the left), and pyramids as shown in the tourmaline crystal termination and the volcanic quartz. Miller, and Miller-Bravais indices are the standard where every face on a crystal is given a unique description. Crystals are structured solids made up of an ordered arrangement of ions, atoms ..read more

Several generations of salt marsh have formed along Fundy Bay coasts during the Holocene post-glacial, eustatic rise in sea level. At this location the seaward edge is bound by marsh cliffs that define at least three platforms at different elevations, each representing marsh development at different stages of sea level rise. Vegetation here is dominated by Spartina. Small, shallow pannes, or ponds (centre right) are recharged during precipitation and spring tide flooding. This is the fourth post in a series on vegetated coastal lithofacies – see also: Seagrass meadows and ecosystems Seagrass l ..read more