Geology: Earth's Structure and Composition

Elements of Structure

• Fault lines, folds, stratified layers, lines of weakness/crevasses, and holes/mottles are key elements of structure
• Variations in features of continents and oceans, depressions, valleys, sinkholes, mountains, plateaus, plains, and geosyncline are also part of structure
• Colour, shapes, embedment, mineral content, and alignment are also important structural features

Earth's Interior

• The deep interior of the Earth is still a mystery
• We have only penetrated the outermost portion with deep drilling exploration
• What we know today comes from seismic wave data or lava that has extruded onto the Earth's surface
• The Earth's interior is like a concentric series of rings, progressing from the dense and intensely hot inner core towards the brittle outer shell of the crust

Structure in Broad Sense

• Structure includes physical and chemical characteristics of rocks and soils
• Mountains, plateau, ridges, plains, and geosyncline environments are part of the broad sense of structure

Tectonic and Structural Geomorphology

• Tectonics refers to the deformation of the Earth's crust
• Twisting or tilting during rock deformation and compression can cause folds to form at different angles
• Folds can be small and viewed in hand-held specimens or large and viewed from aerial photos

Folds and Their Elements

• Folds represent wave-like bendings of rock layers of various sizes and forms
• Two types of folds are distinguished: convex folds (anticlines) and concave folds (synclines)
• In anticlines, layers are arched upwards, and their central parts often contain more ancient rocks
• In synclines, layers are warped downwards, and the central parts in them are characterized by the development of younger rocks
• Each fold has basic elements: core or internal part, curve of the fold, flanks, and fold angle
• Energy released moves out in all directions as waves, like ripples radiating outward when you drop a pebble in a pond

Earthquakes and Faults

• Earthquakes occur at plate boundaries due to internal forces within the Earth, causing the Earth's crust to bend, compress, and stretch
• Plate tectonics cause many physical features like volcanoes, earthquakes, and geologic features like faults
• A geologic fault is a fracture in the Earth's crust causing loss of cohesion and accompanied by displacement along the fracture
• Rocks can break (brittle behavior) when put under large amounts of stress
• Deep within the Earth, rocks flow plastically (ductile behavior)
• Force or stress can cause a change in shape or volume of the rock, called strain
• Depending on the direction of offset, it can be a "right-lateral offset" or a "left-lateral offset"

Folding and Faulting

• During mountain building processes, rocks can undergo folding as well as faulting
• Rocks can deform ductilely, particularly if they are subjected to heat and pressure
• Entire mountain ranges, like the Appalachians, have extensive fold systems
• Conditions of whether a rock faults or folds vary with temperature, pressure, rock composition, and strain rate
• In the same location, some rocks can fold while others fault
• Often, folding is just a precursor to faulting