5 Geological structures: primary and secondary

Why structures form

• Crustal rocks, especially sedimentary rocks, do not stay as they were deposited; internal and external forces continually give them new forms and positions. These forms are geological structures.

Primary structures

• They form in the rocks under climate and environmental conditions (drought, heat, wind and water currents) without tectonic forces or earth movements.
• Examples: cross bedding, ripple marks, graded bedding and mud cracks.
• They are common in sedimentary rocks.

Secondary (tectonic) structures

• They form from forces emitted from the Earth's interior: great fractures, cracks and violent torsions that deform the crust.
• These forces also cause earthquakes, the advance and retreat of seas, and continental drift.
• Examples: folds, faults and joints.
• Tectonic structures are important economically.