Rocks and weathering Flashcards
Global patterns of plate boundaries
The movement of the plates is caused by the convection currents in the molten magma. These puzzle pieces are called tectonic plates, and the edges of the plates are called the plate boundaries. These plates are always moving.
Types
Oceanic constructive, continental constructive, oceanic/continental destructive, oceanic destructive, collision and conservative.
Oceanic constructive
Rising convection lifts lithosphere creating a ridge. Extensional forces cause stretching and a fissure. Fissure opens and exposed magma fills gap, then cools and solidifies. MAGMA DOES NOT OVERFLOW TO FORM TOPOGRAPHIC HIGH.
Continental constructive
Less vigorous pull so no clean break. Pulled thin creating fractures, faults develop and central block slides down creating a rift valley which may fill with water.
Oceanic/continental destructive
Plates forced together and oceanic plate subducts since denser. In the Benioff zone, crustal melting occurs, and resultant magma forced through cracks – to form volcanoes. Subducting plate drags down crustal material to form an ocean trench.
Oceanic destructive
Plates forced together, older plate subducts as is denser. Forced 100 miles below and melts – producing magma chambers. Lower density magma rises through cracks allowing volcanic eruptions.
Collision
Powerful collision between two continental plates. Both densities are lower than the mantle’s, so prevented subduction. Some subduction occurs as lithosphere breaks free. Crust fragments are trapped in collision zone, cause deformation. Intense compression results in folding.
Conservative
Plates slip past each other with relative horizontal movement (sinistral = left, dextral = right). Lithosphere is neither created nor destroyed. Extensive earthquakes.
Processes and associated landforms
Seafloor spreading, subduction, ocean ridges, ocean trenches and volcanic island arcs.
Seafloor spreading
Creates oceanic crust, explained by palaeomagnetism – where lava cools and retains the magnetic polarity of Earth at the time of cooling. Slow spreading is a result of the ridge being fed by small, discontinuous magma chambers.
Subduction
Where denser plate (density similar to asthenosphere) is pushed into upper mantle. Subduction continues once initiated, driven by the weight of the plate – subducted side remains cooler and therefore denser than surrounding mantle.
Ocean ridges
Occur at divergent boundaries. Ridges are a series of parallel ridges, with a central double ridge separated by a ridge valley. As a result of tensions and stretching a central block may fall.
Ocean trenches
Found at subduction zones. Long, narrow, asymmetric (steep side towards land mass) depressions in the ocean floor (6000-11000m). Found next to land and island arcs – common in Pacific Ocean
Volcanic island arcs
Chains of volcanic islands on the continental side of an ocean trench.
Physical weathering processes
Freeze-thaw, exfoliation, salt crystal growth, dilation and vegetation root action.
Freeze-thaw weathering
Occurs in cold areas where ice forms as water freezes in cracks in rocks.
Exfoliation weathering
Occurs in hot desert with large diurnal energy range (40°C to below freezing), rocks heat via conduction, only outer layers expand as rock is a poor heat conductor
Salt crystal growth weathering
Physical disintegration due to fretting (saltwater penetrating) rock surfaces.
Dilation weathering
Overlying rocks are removed by erosion (unloading), or if a glacier load is removed, underlying rocks expand as under reduced pressure, fractures form parallel to the surface, producing pseudo-bedding planes. Deeper down, cracks are less prominent. Most broken = close to surface.
Vegetation root action weathering
Roots can penetrate rocks or prevent rocks from forming/settling in a specific place.
Chemical weathering processes
Hydrolysis, hydration and carbonation.
Hydrolysis
Acid water breaks down rocks with feldspar mineral (such as granite).
Hydration
Certain minerals absorb water - allowing them to expand and change, producing mechanical and chemical stresses. Affects shale/mudstones.
Carbonation
Acid rain breaks down limestone/chalk.
General factors affecting weathering rate
Rock type, rock structure, vegetation, relief and human activity.
Rock type
Some minerals, cements (in sedimentary rocks) are more resistant to weathering than others.
Rock structure
Differential resistance along lines of weakness and grains control water movement.
Vegetation
Increased organic acid production and carbon dioxide increases carbonation. Physical weathering may reduce as temperature are moderated. Roots will increase biological weathering.
Relief
Affects temperatures and exposure.
Human activity
Increased weathering due to increased airborne chemical pollutants and acid rain. Vegetation removal reduces chemical/biological weathering (fewer organic acids).
Specific factors affecting weathering
Temperature and rainfall
Temperature
Glacial, temperate, arid/semi-arid and humid-tropical.
Slope processes
Weathering, erosion, transport and deposition of the material.
Mass movement
Heaves, slumps, flows, falls, slides and slip plane.
Heaves
Slow movement of material where soil particles are heaved to the surface by wetting, heating and freezing of water. Occurs mainly in winter.
Slumps
Weaker rocks such as clays ‘slump’ with a rotational movement along a curved slip plane.
Flows
More continuous, smoother form of slump. Occurs in deeply weathered clay, and if particle size is the same or smaller than a grain of sand.
Falls
Occurs on steep slopes (>40°), with bare rock faces and exposed joints. Initial cause of rocks falling is erosion or weathering.
Slides
When an entire mass of material moves along a slip plane. Can be rock/landslides, or a rotational slide. Material holds its shape until hitting the slope bottom.
Slip plane
Junction between two layers along a bedding line, or the joint between two rock types. Point beneath the surface where shear stress > shear strength.
Water and sediement movement
Sheetwash, surface wash and rain splash,
Sheetwash
Occurs when the soil’s infiltration capacity is exceeded by precipitation rate. Hortonian flow.
Surface wash
Unchanneled (sheet like) flow of water over soil’s surface. Some high/low velocity sections may develop. Transports material dislodged by rain splash, by eroding a uniform layer of soil. Produces rills.
Rain splash
Raindrops have an erosive effect. This effect is most prominent on slopes with inclines between 33° and 45° at the start of a rainfall even, when the soil is loose.
Human impact on stability
Excavations, waste heaps, vegetation removal, traffic vibrations, footpath trampling and construction on slopes
Strategies to modify slopes to reduce mass movements
Pinning, netting, grading, afforestation, gabions, drainage, grouting and shotcrete
