3. Rocks and Weathering Flashcards
9696 - Cambridge AS Geography
Plate Tectonics
The theory that Earth’s lithosphere (upper mantle and crust) is divided into rigid plates that move horizontally across the plastic asthenosphere below.
Size and Composition of Plates
Plates vary in size and are composed of continental crust (silica-rich) and oceanic crust (basalt-rich).
Movement of Plates
Plates move slowly (a few centimeters per year) in different directions, driven by mantle convection currents.
Global Patterns of Plate Movement
Seven major plates and numerous smaller plates make up the Earth’s surface, with distinct boundaries.
Divergent (Constructive) Boundaries
Plates move apart, allowing upwelling of mantle material to form new oceanic crust.
Key Features of Constructive Plate Boundaries
Mid-ocean ridges, volcanic activity, and rift valleys.
Conservative Boundaries
Plates slide past each other horizontally, creating friction and potential earthquakes.
Key Features of Conservative Plate Boundaries
Transform faults and mountain ranges.
Convergent (Destructive) Boundaries
Plates collide, pushing one plate under the other (subduction).
Subduction Zone Characteristics
Oceanic crust dives under continental crust or another oceanic plate, causing earthquakes, volcanic activity, and trench formation.
Seafloor Spreading
Molten rock rises at divergent boundaries, solidifying and creating new ocean floor, pushing continents apart.
Subduction
Oceanic crust sinks beneath another plate, recycling crust back into the mantle and creating deep ocean trenches.
Landforms of Subduction
Ocean trenches, volcanic island arcs (e.g., Japan).
Fold Mountain Building
Convergence can crumple and fold sediments at plate boundaries, forming mountain ranges.
Examples of Fold Mountains
The Himalayas, Alps, and Appalachians are all examples of fold mountains.
Ocean Ridges
Long, underwater mountain ranges marking divergent boundaries with volcanic activity.
Ocean Trenches
Deep, elongated depressions in the ocean floor marking convergent boundaries (subduction zones).
Volcanic Island Arcs
Chains of volcanic islands formed due to subduction, often parallel to trenches.
Weathering
The breakdown and decomposition of rocks and minerals at the Earth’s surface by physical, chemical, and biological processes.
Freeze-Thaw
Water expands as it freezes, causing cracks and fissures in rocks. Repeated freezing and thawing can break rocks apart.
Heating/Cooling
Repeated cycles of heating and cooling can cause rocks to expand and contract, weakening them and making them more susceptible to further weathering.
Salt Crystal Growth
Dissolving salts can crystallize within rock cracks, exerting pressure and causing the rock to crumble. This is common in arid environments.
Pressure Release (Dilatation)
As rocks are eroded and exposed to the surface, the pressure they were under is released. This pressure release can cause them to expand and crack.
Vegetation Root Action
Growing plant roots can exert pressure on rocks, wedging them apart and breaking them into smaller pieces.
Hydrolysis
Water reacts with minerals in rocks, breaking down their chemical structure and forming new compounds.
Hydration
Minerals absorb water molecules, expanding and weakening the rock structure.
Carbonation
Rainwater containing dissolved carbon dioxide becomes carbonic acid, which dissolves minerals like calcite (calcium carbonate) in limestone and creates new soluble compounds.
Climate’s affect on weathering
Temperature and precipitation play a major role. Warmer and wetter climates generally experience faster weathering rates.
Rock Type’s affect on weathering
Different rock types have varying susceptibility to weathering. For instance, porous rocks weather more easily than non-porous ones.
Rock Structure’s affect on weathering
Joints, cracks, and faults in rocks provide more surface area for weathering processes to attack.
Vegetation’s affect on weathering
Vegetation cover can protect rocks from physical weathering but can also promote chemical weathering through the production of organic acids.