Ch. 2, Part-IIbGeologic Structures

Geologic Structures• Generally planar features within, cross-cutting, or

disrupting the rock materials/strata (layers)

• Important in environmental geology as:– Discontinuities– Topography/landforms– Indicators of tectonics– Implications for natural hazards– Structural integrity– Planes/zones of weakness– Preferrential“pathways” for fluid/contaminants

Types of Geologic Structures

• Stratification (Layers & Layering)

• Folding/Tilting

• Faulting

• Other Structures– fractures– joints– crosscutting from forceful injections

(dikes/sills)

Layering/Strata

Tilted LayersIndicative of forces causing horizontal layers to be

disrupted (tilted)

Significance of Layering/Tilting

• Basic geologic structure

• Planar reference boundaries that define strata (boundaries between/within rock materials)

• Implications for landforms/topography?

• Potential pathways

Folds

FOLDS

Indicative of directed (compressional) forces (past or present)

Topography similar to Appalachians

FaultsExpressions of directed forces as broken/displaced strata

Strike Slip Faulting

Dip Slip Faulting

Significance of Fault & Folds• Areas of “broken and/or disrupted” crust• Usually associated with topographic features• Usually results in exposure of different types of

rock materials at surface• Indicative of past and/or present forces• Potential for environmental hazard?• Often associated with natural resources (minerals,

petroleum, etc.)• Effects on fluid pathways (as preferential

pathways or barriers)

Other Structures

• Fractures

• Joints

• Crosscutting material from forceful injections– Dikes (cross-cuts layering)– Sills (parallel to layering)

Cross-Cutting Relationships:Dikes & Sills

Jointing

Some Environmental Implications of Geologic Structures

Layering or Foliation

Summary / Review• Building blocks of rock materials: atoms,

molecules, minerals, rocks/rock materials

• Most abundant minerals are silicates

• Basic building block is the silica tetrahedra

• Rock properties determined by properties of component materials (minerals)

• Three main classes of rocks– Igneous: Formed from molten material

– Sedimentary: Clastic, chemical, organic, combinations

– Metamorphic: foliated, non-foliated

Summary / Review• Rock type provides various types of information

– Environment/setting in which they were formed– Tectonic implications– Implications for natural hazards– Physical, chemical properties– Etc.

• Geologic Structures: – Layering, tilting– Folding– Faulting– Other types (fractures, jointing, cross-cutting features)

• Implications/significance of geologic structures

Chapter (Section) Objectives• Review of some of the important mineral and rock types and

their environmental significance

– Relationships between atoms, minerals, rocks, rock materials

– Basic silicate building block(s)

– Properties of rocks & minerals

– Basic rock types, basis for classification, types of information they tell us

• Appreciation/significance of geologic structures

– Layering

– Folds

– Faults

– Other structures (joints, dikes/sills, etc.)