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 […] The post Mohr circles and stress transformation appeared first on Geological Digressions ..read more

This post deals with the materials, their rheological behaviours, and scaling as they apply to analogue structure models. Rock deformation Two of the most common structures generated by rock deformation are faults (and fractures) and folds. The rheological conditions for each are fundamentally different: faults and fractures result from brittle behaviour, whereas folds require materials […] The post Analogue structure models: Scaling the materials appeared first on Geological Digressions ..read more

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 variables. The interplay amongst these variables is complicated. System models help us organize the various processes and responses in a way that gives us some understanding about how […] The post Model dimensions and dimensional analysis appeared first on Geological Digressions ..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 post Geological models: An introduction appeared first on Geological Digressions ..read more

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 models permit quantitative analysis of a system’s properties. As noted in an earlier post, the word ‘analogue’ means the correspondence, similarity, likeness, […] The post Analogue models appeared first on Geological Digressions ..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 successio ..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 Seagras ..read more

Mangroves dominate this estuarine channel-tidal flat system. The channel is mud-bound, about 1.5 m to 2 m deep and 5 m to 6 m wide, with margins stabilized by the tangle of roots. Tidal flood waters cover the vegetated flats to depths of only a few centimetres. Each tidal cycle replenishes the supply of food and nutrients to the vegetation and the local invertebrate fauna. Whitford estuary, south Auckland. This post is part of the Lithofacies Series. Mangrove trees and forests occupy a very specific zone between fully marine and terrestrial environments in supratidal and upper intertidal re ..read more

The New Zealand mangrove Avicennia marina fringing the upper limits of tidal flats, interwoven with patches of salt marsh dominated by Salicornia. Tidal flat sediment is a mix of fine-grained sand and mud. The ruffled surface of the flats is primarily the result of burrowing crabs. Hauraki Gulf, New Zealand. This post is part of the Lithofacies Series.   Manukau Harbour on the west edge of Auckland has several large estuarine inlets fringed by verdant mangrove forests.  Residents along one of these inlets, Pahurehure Inlet, considered the mangroves an eyesore and a distraction fro ..read more

Part of a Zostera marina meadow on sandy tidal flats, Savory Island, British Columbia. Mud content here is <10%. Some leaves are 25-30 cm long. There are a few crustacean or bivalve burrows at left centre. Some criteria that help us identify fossil seagrasses in the rock record [This post is a companion to Seagrass meadows and ecosystems]   Coastal seagrasses along with mangrove communities are two of the most productive marine ecosystems in the world. Their primary ecosystem functions have been summarized in a companion post. Despite their importance in modern ecosystems and their ..read more