Gravity Settling


Gravity settling or gravity differentiation is a key geological process which can be easily demonstrated. Understanding the process helps explain sedimentary structures, determine the way up of rocks and explain planetary geology. The principle is not complicated. In a fluid, heavier material will sink to the bottom faster than lighter material (provided it has not been designed for fluid resistance, like a boat). 
A video demonstrating how to show gravity settling can be viewed here.
The principle of gravity settling is particularly relevant in sedimentary rock formation, where sediment is commonly transported by wind or water. As a river loses energy it will deposit its heaviest load first and the finer sediment will be transported further downstream, until the river can no longer carry it. The finest particles, like clay, will remain suspended for the longest time and can be carried vast distances. By studying the sediment size in fluvial rocks geologists can get an idea of the transport history of the sediment and how far the rock formed from the source. Fluctuations in the sediment sizes in the beds adjacent can indicate changes in the river energy, for example, a flooding event or seasonal changes.
Order of deposition diagram. The further from the river source the smaller the sediments will be. (twinkl, accessed 15/7/2020)

Graded bedding is a sedimentary structure which can indicate the way up of a rock – with the coarser sediments found at the bottom of the bed and a fining up sequence. This type of sedimentary structure is often associated with turbidity currents. Turbidity currents originate on the slope between the continental shelf and deep-sea basin, where unstable sediment will start to build up. An event, such as an earthquake, will cause sudden slope failure creating a submarine landslide, establishing a turbid (murky) layer of water. When this turbid layer reaches the flatter deep sea plain the current loses energy quickly and decelerates, making it drop the sediments it was carrying (heaviest first, note: very occasionally conditions result in different bedding orders). This sedimentary structure can be very useful to geologists as it can indicate if the beds have been flipped over during folding as well as giving information about the deposition environment.
Sudden slope failure can cause turbidity currents. (NOAA, accessed 15/7/2020)
Graded bedding indicating the way up of a rock with heavy sediment settling first. (Wikimedia Commons, accessed 15/7/2020)
  • This WASP activity allows students to make their own graded bedding.

Gravity differentiation is also particularly important for planetary geology and understanding Earth’s formation. During the Hadean era (the first 0.6 billion years of Earth’s history) the Earth was hot and molten. The material making up the Earth was thus able to move. Heavier material, such as iron and nickel, sunk to the centre of the Earth leaving a silicate rich crust, through density sorting. Planetary differentiation due to gravity is common on all planetary bodies and is known as the Great Iron Catastrophe (GIC) on Earth. The term catastrophe here is used in the mathematical sense of the word “a large or sudden change” rather than meaning “disaster”. We know the Earth must have a denser core than surface, as the mass of the Earth is too great to be made only of silicate rich material, which is what we find at the surface. Our knowledge of the interior of the Earth comes primarily from earthquake data and research on meteorites.

Layer
Density (g/cm 3)
Crust
2.2
Upper Mantle
2.9
Lower Mantle
3.4 – 4.4
Outer Core
12.2
Inner Core
13.1
Table showing the average density of Earth’s layers – the layers become more dense with depth.
The internal structure of the Earth. (Wikimedia Commons, accessed 15/7/2020)


Location: Australia