Rates of erosion

Cards (8)

  • A crucial element of coastal management is understanding rates of erosion, or rates of recession. The latter term means how fast a coastline is moving inland. This is influenced by many factors but the key one is lithology, or rock type. The three major rock types – igneous, sedimentary and metamorphic – erode at different rates
  • There are exceptions to the general order of resistance to erosion. Some limestones, which are actually crystalline sedimentary rocks, are erosion resistant, as are ancient sandstones that have been compressed and compacted over hundreds of millions of years. Recently erupted volcanic lava flow rocks and layers of volcanic ash or tephra (called volcaniclastic rocks) tend to be weak and easily eroded. The weakest coastal material is unconsolidated sediment.
  • Igneous rate of erosion
    • Granite, Basalt, Dolerite
    • <1 mm per year
    • Crystalline rocks - the interlocking crystals make for strong, hard erosion resistant rock
    • Often have few joints, so there are limited weaknesses that erosion can exploit
  • Metamorphic rate of erosion
    • Slate, Schist, Marble
    • 1mm to 10cm per year
    • Crystalline metamorphic rocks are resistant to erosion
    • Many metamorphic rocks exhibit a feature called foliation, where crystals are all orientated in one direction, which produces weaknesses
    • Metamorphic rocks are often folded and heavily fractured, which are weaknesses that erosion can exploit.
  • Sedimentary rate of erosion
    • Sandstone, Limestone, Shale
    • 10cm - several metres per year
    • Most sedimentary rocks are clastic and erode faster than crystalline igneous and sedimentary rocks
    • The age of sedimentary rocks is important; geologically young rocks tend to be weaker.
    • Rocks with many bedding places and fractures, such as shale, are often most vulnerable to erosion
  • Rocks and sediment play the most important role in influencing the shape of the coastal landscape, but vegetation is also important. Many coastlines are protected from erosion of unconsolidated sediment by the stabilising influence of plants. This includes: coastal sand dunes, coastal salt marshes, coastal mangrove swamps
  • Vegetation stabilises sediment in a number of ways:
    • The roots of plants bind sediment particles together making them harder to erode.
    • When submerged, plants growing in sediment provide a protective layer so the surface of the sediment is not directly exposed to moving water and therefore erosion.
    • Plants protect sediment from wind erosion by reducing wind speed at the surface due to friction with the vegetation
  • Halophytes
    • tolerate salt water, either around their roots, being submerged in salt water (at high tide) or salt spray from the sea.