How to… A virtual classroom

measuring dip

A virtual classroom for Earth Scientists

The posts here provide an outline of method and theory for some of the basic tasks that geologists undertake in the field and lab. They are directed primarily at undergraduate geology students, but should be useful for anyone wanting a refresher. They are not encyclopedic descriptions – refer to the many fine textbooks, and journal papers for examples of more complex geological problems.

The How to… posts are more technical than those elsewhere on this website

The list of tasks will evolve. The emphasis initially is on stratigraphy, sedimentary and volcanic strata, sedimentary basins, geochemistry and mapping, but because sedimentary strata are commonly deformed, there is an obvious cross-over to structural analysis and tectonics.

 

How to…

Common structure – mapping problems

Describing sedimentary rocks

Lithofacies

Stratigraphy – sedimentology

Sequence stratigraphic principles

The polarizing microscope

Mineralogy of sandstones

Mineralogy of carbonates

Mineralogy of evaporites

Seds in outcrop

Volcanics in outcrop

Sedimentary basins

Hydrogeology

Ancient field work

Soils and weathering

Planetary geology

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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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