* Active Continental Margins * Geologically active with plate boundaries leading to earthquakes, young mountain belts, and volcanoes * Ocean trench: narrow, deep trough parallel to the edge of a continent or an island arc * Deepest parts of the ocean * Mid Ocean Ridges and Trenches * Seamounts and Guyots * Seamounts: conical undersea mountains that rise 1000 meters or more above the sea floor * most are extinct volcanoes (except near mid-ocean ridge or hot spot) * chains of seamounts form aseismic ridges (no earthquakes) * Guyots: flat-topped seamounts * Cut by wave action * Reefs common around them * Reefs: wave-resistant ridges of coral, algae, and other calcareous organisms (calcium carbonate anatomical structures) * Fringing reefs: flat, table-like reefs attached directly to shore * Barrier reefs: parallel to the shore but are separated by wide, deep lagoons * Atolls: circular reefs that rim lagoons * Sea Floor Sediments * Terrigenous sediments: land-derived sediments hat have found their way to the sea floor * Comprise continental rise and abyssal plains * Pelagic sediments: settle slowly through the ocean water, and are derived from fine-grained clay (delivered primarily by wind) and skeletons of microscopic * Oceanic Crust * Recall: thinner and different composition than continental crust * Layer 1 – marine sediment of various composition and thickness * Layer 2 – pillow basalt overlying basalt dikes * Layer 3 – sill-like gabbro intrusions * XVIIII: Plate Tectonics 3/27/12 * Plate Tectonics – theory that Earth’s surface is composed of large, thick plates that move and change in size * Alfred Wegener said that continents have identical late Paleozoic rocks and fossils * Certain plant and animal fossils were found on all 5 continents, reptile fossils were found only in Brazil and south Africa * Pangeaea: supercontinent proposed by Wegener * Laurasia – northern supercontinent containing North America and Eurasia (excluding India) * Gondwanaland – southern supercontinent containing South America, Africa, India, Antarctica, and Australia * VIIII: Mass Wasting 3/29/12 * Downhill movement of masses of bedrock, rock debris, or soil, driven by the pull of gravity * Mass wasting is, with proper planning, is perhaps the most easily avoidable of all geologic hazards * Mass movements occur everywhere and cause damages that are costly, but don’t cause many deaths * Earth’s Systems * Atmosphere * Interaction between atmosphere and rocks – chemical weathering * Hydrosphere * Water is a primary contributor to landslides * Biosphere * Vegetation can sometimes act to stabilize soil * In other cases plant roots may contribute to destabilization (e.g., trees on dams) * Classification of Mass Wasting * 1. Rate of movement * 2. Type of material – solid bedrock or soil (unconsolidated material at Earth’s surface) * 3. Type of movement * Flow * Slide * Fall * Forces Associated with Mass Wasting * Gravity – the driving force for mass wasting, includes two components: * 1. Normal force: perpendicular to slope, component that ‘holds’ block in place * 2. Shear force: parallel to the slope, component that ‘pulls’ block down slope * Shear strength: resistance to movement or deformation; controlled by composition of material and water * Forces that control mass movement * Gravity is principle factor * Steepness of slope: steeper slope = larger shear force * Water: pore pressure from water decreases shear strength * Stable vs. Unstable