GCSE Geology Eduqas

This subject is broken down into 11 topics in 4 modules:

  1. Basic Principles 3 topics
  2. Rocks 3 topics
  3. Geological Structures and Processes 2 topics
  4. Economic and the Environment 3 topics
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  • 4
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  • 11
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  • 4,417
    words of revision content
  • 34+
    minutes of audio lessons

This page was last modified on 28 September 2024.

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Geology

Basic Principles

The Rock Record

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The Rock Record

The Rock Record

Introduction to Rock Record

  • The rock record refers to the history of Earth captured within the layers of rock.
  • This record isn't just a historical diary of events, but an interconnected archive of Earth’s geological, climatological, and biological past.
  • Each layer, or stratum, holds clues dating back millions, even billions, of years.
  • Key rock record information is found in three primary types of rock: sedimentary, igneous and metamorphic.

The Principle of Superposition

  • Principle of Superposition posits that in any undeformed sequence of sedimentary rocks, each bed is younger than the one beneath it and older than the one atop it.
  • The principle is crucial in piecing together Earth’s history as it allows geologists to determine the relative ages of rock layers and formations.

The Principle of Original Horizontality

  • The Principle of Original Horizontality states that layers of sediment are initially deposited horizontally under the influence of gravity.
  • Therefore, if we find tilted sedimentary rocks, this tilt is due to later events that disturbed the original deposition.

The Principle of Lateral Continuity

  • According to the Principle of Lateral Continuity, layers of sediment originally extend laterally in all directions.
  • This means that if a sedimentary rock layer is found in different geographical areas, it can be confirmed that they were part of the same layer.

The Principle of Cross-Cutting Relationships

  • The Principle of Cross-Cutting Relationships states that any geological feature that cuts across, or disrupts, another feature must be younger than the feature that is disrupted.
  • For example, if an igneous intrusion is found cross-cutting a layer of sedimentary rock, it can be inferred that the intrusion is younger.

Using the Rock Record to Interpret Geological History

  • By using these principles, geologists can infer the relative ages of rock strata and infer important events shaping Earth's geologic history like volcanic activity, sea level fluctuations, or plate tectonic movements.
  • Index fossils, fossils from a short-lived, geographically widespread species, can further corroborate these inferences as they help to date and correlate rock layers.
  • An understanding of the rock cycle, which describes the transformation between different types of rocks, is also invaluable to reading the rock record.
  • Geologists also use radiometric dating techniques to measure the absolute ages of rocks, adding precision to the rock record's timeline.

Course material for Geology, module Basic Principles, topic The Rock Record

Geology

Geological Structures and Processes

Geological Structures

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Geological Structures

Geological Structures

Fold Structures

  • Folds are wavy, bend-like structures in rocks due to deformation under stress.

  • This deformation can be compressional, causing rocks to fold, or tensional, causing rocks to break and displace.

  • Describing a fold includes its antiform shape (upward arch) and synform shape (downward trough).

  • The axial plane of a fold refers to the imaginary flat surface that divides the fold as symmetrically as possible.

  • The fold axis is the line of maximum curvature on a fold layer.

Fault Structures

  • Faults occur when stress on rock surpasses their internal strength, causing a failure in the form of crack, along which movement occurs.

  • Normal faults are a result of tensional stress, where hanging wall blocks drop down relative to the standing wall, mainly due to gravity.

  • Reverse faults occur due to compressive stress, pushing hanging wall blocks upwards.

  • Strike-slip faults show horizontal movement due to shear stress.

Fractures and Joints

  • Fractures happen when rocks fail due to stress, but unlike faults, there's no appreciable movement along the cracks in the rocks.

  • Joints are a form of fracture where no shear displacement occurs.

Unconformity

  • An unconformity is a 'missing time' gap in rock sequences signifying erosion or lack of deposition for a period.

  • Disconformity is a type of unconformity, a break between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

  • Angular unconformity happens when flat layers of sedimentary rocks are deposited on tilted and eroded layers.

  • Nonconformity is when sedimentary rock is deposited on eroded, older igneous or metamorphic rock.

Stratification and Bedding

  • Stratification refers to a system of stacked layers (beds) of sedimentary rock or soil. Each stratum (singular of strata) is typically different in colour, physical properties, fossil content, and reveals the kinds of environment during the time of deposition.

  • Cross-bedding is inclined sedimentary structures within a layer of rock that can indicate water currents or wind direction in the past.

Please note that these points represent only an overview of geological structures. Additional reading and practice will provide more detailed understanding and familiarity.

Course material for Geology, module Geological Structures and Processes, topic Geological Structures

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