Rocks and weathering

Question 1

Global patterns of plate boundaries

Answer 1

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.

Question 2

Types

Answer 2

Oceanic constructive, continental constructive, oceanic/continental destructive, oceanic destructive, collision and conservative.

Question 3

Oceanic constructive

Answer 3

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.

Question 4

Continental constructive

Answer 4

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.

Question 5

Oceanic/continental destructive

Answer 5

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.

Question 6

Oceanic destructive

Answer 6

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.

Question 7

Collision

Answer 7

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.

Question 8

Conservative

Answer 8

Plates slip past each other with relative horizontal movement (sinistral = left, dextral = right). Lithosphere is neither created nor destroyed. Extensive earthquakes.

Question 9

Processes and associated landforms

Answer 9

Seafloor spreading, subduction, ocean ridges, ocean trenches and volcanic island arcs.

Question 10

Seafloor spreading

Answer 10

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.

Question 11

Subduction

Answer 11

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.

Question 12

Ocean ridges

Answer 12

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.

Question 13

Ocean trenches

Answer 13

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

Question 14

Volcanic island arcs

Answer 14

Chains of volcanic islands on the continental side of an ocean trench.

Question 15

Physical weathering processes

Answer 15

Freeze-thaw, exfoliation, salt crystal growth, dilation and vegetation root action.

Question 16

Freeze-thaw weathering

Answer 16

Occurs in cold areas where ice forms as water freezes in cracks in rocks.

Question 17

Exfoliation weathering

Answer 17

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

Question 18

Salt crystal growth weathering

Answer 18

Physical disintegration due to fretting (saltwater penetrating) rock surfaces.

Question 19

Dilation weathering

Answer 19

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.

Question 20

Vegetation root action weathering

Answer 20

Roots can penetrate rocks or prevent rocks from forming/settling in a specific place.

Question 21

Chemical weathering processes

Answer 21

Hydrolysis, hydration and carbonation.

Question 22

Hydrolysis

Answer 22

Acid water breaks down rocks with feldspar mineral (such as granite).

Question 23

Hydration

Answer 23

Certain minerals absorb water - allowing them to expand and change, producing mechanical and chemical stresses. Affects shale/mudstones.

Question 24

Carbonation

Answer 24

Acid rain breaks down limestone/chalk.

Question 25

General factors affecting weathering rate

Answer 25

Rock type, rock structure, vegetation, relief and human activity.

Question 26

Rock type

Answer 26

Some minerals, cements (in sedimentary rocks) are more resistant to weathering than others.

Question 27

Rock structure

Answer 27

Differential resistance along lines of weakness and grains control water movement.

Question 28

Vegetation

Answer 28

Increased organic acid production and carbon dioxide increases carbonation. Physical weathering may reduce as temperature are moderated. Roots will increase biological weathering.

Question 29

Relief

Answer 29

Affects temperatures and exposure.

Question 30

Human activity

Answer 30

Increased weathering due to increased airborne chemical pollutants and acid rain. Vegetation removal reduces chemical/biological weathering (fewer organic acids).

Question 31

Specific factors affecting weathering

Answer 31

Temperature and rainfall

Question 32

Temperature

Answer 32

Glacial, temperate, arid/semi-arid and humid-tropical.

Question 33

Slope processes

Answer 33

Weathering, erosion, transport and deposition of the material.

Question 34

Mass movement

Answer 34

Heaves, slumps, flows, falls, slides and slip plane.

Question 35

Heaves

Answer 35

Slow movement of material where soil particles are heaved to the surface by wetting, heating and freezing of water. Occurs mainly in winter.

Question 36

Slumps

Answer 36

Weaker rocks such as clays ‘slump’ with a rotational movement along a curved slip plane.

Question 37

Flows

Answer 37

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.

Question 38

Falls

Answer 38

Occurs on steep slopes (>40°), with bare rock faces and exposed joints. Initial cause of rocks falling is erosion or weathering.

Question 39

Slides

Answer 39

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.

Question 40

Slip plane

Answer 40

Junction between two layers along a bedding line, or the joint between two rock types. Point beneath the surface where shear stress > shear strength.

Question 41

Water and sediement movement

Answer 41

Sheetwash, surface wash and rain splash,

Question 42

Sheetwash

Answer 42

Occurs when the soil’s infiltration capacity is exceeded by precipitation rate. Hortonian flow.

Question 43

Surface wash

Answer 43

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.

Question 44

Rain splash

Answer 44

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.

Question 45

Human impact on stability

Answer 45

Excavations, waste heaps, vegetation removal, traffic vibrations, footpath trampling and construction on slopes

Question 46

Strategies to modify slopes to reduce mass movements

Answer 46

Pinning, netting, grading, afforestation, gabions, drainage, grouting and shotcrete