HKDSE Geography/E1/Sedimentary Rocks
They may be formed mechanically (clastic), or by chemical and biological means (non-clastic).
These rocks are formed from the particles of other rocks.
- Rocks are exposed on the earth's surface and are subject to denudation (erosion, weathering, mass movement, transportation) and deposition.
- The rocks are broken down into small fragments which are carried downhill by mass movement or a moving agent.
- When the energy of the moving agent is lost, the rocks will be deposited and accumulate. (At foothills, they are called colluvium; on floodplains, alluvium.)
- This is sedimentation/ deposition. The deposited fragments and grains are sediments/deposits.
- As sedimentation continues, layers of sediments pile up. Large, heavy fragments are deposited first. This is sorting.
- Rocks and grains are squeezed together tightly. The bottom layer is compressed by the weight of the layers above it. This is compaction.
- Sediments have water in the pore spaces between grains. It may contain cements iron oxide, calcite, silica. The cement glues the rocks together. This is cementation.
- Compaction and cementation transform loose sediments into sedimentary rocks. They are collectively known as lithification.
Clastic sedimentary rocks are classified according to grain size.
- Conglomerate: Rounded particles of boulders, cobbles and pebbles. Cemented by silica and very resistant to weathering.
- Breccia: Like conglomerate, but with angular particles
- Sandstone: Made, surprisingly, of sand
- Siltstone: Composed of silt
- Mudstone: Silt and some clay
- Shale: Composed of clay
The colour of the base materials/groundmass hints at the cement involved: iron oxide is red and calcite is grey.
Clastic sedimentary rocks are non-crystallline. They are a matrix of sediments.
Sedimentary rocks have strata/layers. A bedding plane is a surface at which two strata meet. They may tilt in a certain direction because of internal processes. Dip is the angle at which the bedding plane meets with the horizontal plane. Strike is the line at which the bedding plane intersects with the horizontal plane. It shows the direction of the bedding plane. It forms a right angle with the dip.
By chemical means
- A mineral is dissolved in water.
- The water may become sturated with the chemical.
- The mineral will be precipitated to form sedimentary rock.
Gypsum is the chemical precipitation of calcium carbonate.
In an arid area, water may evaporate from salt water bodies like enclosed seas. Mineral salts are left, e.g. rock salt.
By organic means
- Dead vegetation is compressed. It becomes a half-developed coal called peat.
- When it is buried deeper and for a longer time, it will turn into lignite.
- Then bituminous coal.
(The final step in coal formation involves metamorphosis, which is discussed below.)
Limestone: Made of tiny particles of calcium carbonate from the remains of the shells or skeletons of sea-dwelling organisms. Chalk is am example. Limestone is easily dissolved and very vulnerable to solution.
Traces of plants, shells or skeletons sometimes remain in organically-formed sedimentary rocks. They are fossils. They allow scientists to explore the climatic and environmental changes over time.