An introduction to some basic principles of geology
that will be useful for studying geology in East Yorkshire
without too much jargon
tutor: Mike Horne FGS
for the Centre for Life Long Learning
Crystals - individual grains of a mineral or chemical with well defined flat faces meeting at angular edges.
Minerals are naturally occurring chemicals, which may be single elements or compounds, most are inorganic and solids. The "rock forming" minerals are the ones that make up common rock types and are mostly silicates (containing Silicon and Oxygen with some metals). Ores are rarer minerals that are commercially important for processing into metals etc. Gems are even rarer minerals valued because of their appearance.
Rocks - rocks are made up of grains or crystals of one or more minerals.
They can be divided into three main groups -
Igneous Rocks - have crystallised from magma (a very hot melt of minerals) and are characterised by being made up of interlocking crystals of minerals. Large crystals can grow if the magma cools slowly: the rock will be coarse grained and is known as a plutonic rock (platonic rocks are just being friendly!). An example is Granite. If it cools rapidly, as is the case with a volcanic rock, the crystals will be small or the rock may even be glassy. Basalt is an example of a fine grained, volcanic rock.
Sedimentary Rocks - rocks made of grains of sediment that have been eroded away from previously existing rocks. The grains (or clasts) may be rounded and will probably be cemented together by a mineral that has precipitated out of mineral rich water. They form on the surface of the earth in the sea, lakes, rivers and deserts. And so they may contain the preserved remains of plants and animals (fossils). They will probably show some layering (bedding) and other depositional patterns.
Metamorphic Rocks - have been altered by heat and or pressure, leading to recrystalisation. We can expect to find some well-formed crystals and some orientation of the crystals. Original structures such as bedding and fossils will have been destroyed or distorted.
Fossils - are the preserved parts of animals or plants. Normally only the hard parts such as shells and bones are preserved. Preservation of soft body tissue or plants is unusual. Trace fossils are disturbances in the sedimentary rock caused by the movement of animals - such as worm holes and footprints.
Stratigraphy and geological time:
Stratigraphy - working out the sequence in which the rocks were deposited or formed.
Three "LAWS" of stratigraphy that are used in geology and archaeology.
- The law of superposition. The rock at the bottom of the sequence is the oldest and the one at the top is the youngest. Unless the sequence has been turned upside down or some of the rocks have been intruded into the sequence.
- The law of cross-cutting relationships. If one feature cuts across another it is younger.
- Dating using fossils (or artifacts in archaeology) - if one rock contains a particular fossil or assemblage of fossils and another rock somewhere else contains the same ones - the rocks are likely to be of similar age.
Geological time units - such as Jurassic Period- may be subdivided into Early, Mid (or Middle) or Late.
Chronostratigraphy - the rocks deposited during geological time units - such as the Jurassic System - may be subdivided into Lower, Middle and Upper. [do not get confused about time units and rock-time units]
Lithostratigraphy - the sequence of rock units, classified independently of time units - such as Speeton Clay Formation.
Biostratigraphy -the units (Zones or Biozones) based on the occurrence of specific fossils - such as the Micraster coranguinum Zone. [note - fossil names are usually printed in italics or underlined when hand written except when they are the name of a zone].
All these units and zones are defined at their base (e.g. a change of rock type, or the first appearance of a specific fossil) the top does not need to be defined as it is also the base of the next unit or zone.
Geological Structures:
Horizontal bedding - sedimentary rocks are usually assumed to have been deposited horizontally.
Dip - where the strata (layers of sedimentary rocks) are no longer horizontal. The dip is measured in degrees from the horizontal.
A fold - where the strata are bent - an upwards 'U' is called a syncline, an upside down 'U' is called an anticline
A fault - where the strata have been fractured and moved, usually a vertical displacement. A normal fault is the result of stretching - there has been an overall lengthening of the rocks. A reverse fault is the result of compression - there has been a shortening of the rocks. A thrust is a low angle reverse fault. A transverse fault is one where there has been horizontal displacement.
An Unconformity represents a period of non-deposition; it is a time gap in the rock sequence.
copyright Mike Horne - 2019
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