The visible products of weathering and erosion are the unconsolidated materials that we find around us on slopes, beneath glaciers, in stream valleys, on beaches, and in deserts. The loose collection of material is referred to as sediment, and the individual pieces that make it up are clasts. Clasts can be sand-sized and smaller (in which case they might be referred to as particles or grains), or larger than a house.
Some examples of sediments and their clasts are shown in Figure 8.14. They range widely in size and shape depending on the processes involved. If and when deposits like these are turned into sedimentary rocks, the textures of those rocks will vary significantly. When we describe sedimentary rocks that formed millions of years in the past, we can use those properties to make inferences about the conditions that existed during their formation. The properties we look at are composition, grain-size, sorting, rounding, and sphericity.
Composition refers to the mineral or minerals making up the clast. Small clasts may be single mineral grains, but larger ones can have several different mineral grains, or even several different pieces of rock within them. The composition can tell us something about what rock the sediments came from and, as you’ll learn later, it can also tell us something about the geological setting in which that rock formed.
Not all minerals have the same hardness and resistance to weathering, so some minerals tend to become more abundant than others within sediments as weathering and erosion proceed.
Quartz is one example of a mineral that is more abundant. It is highly resistant to weathering by weak acids or reaction with oxygen. This makes it unique among the minerals that are common in igneous rocks. Quartz is also very hard, so it is resistant to mechanical weathering.
In contrast, ferromagnesian minerals and feldspar are not as resistant to weathering. As weathering proceeds, they are likely to be broken into small pieces and converted into clay minerals and dissolved ions (e.g., Ca2+, Na+, K+, Fe2+, Mg2+, and H4SiO4). Ultimately this means that quartz, clay minerals, and dissolved ions are the most common products of weathering.
Whether a grain is large or small tells us something about its journey to where we’ve found it. To begin with, mechanical weathering can break off large pieces of rock. Large pieces of rock carried along by streams will bump into each other, causing smaller pieces to break off. Over time the grains get smaller and smaller still. If the grains are all very small, we can conclude that they are a long way from home.
Geologists have a specific set of definitions to describe the size of grains. Table 8.1 below is a simplified version of the scale that is used.
The scale has some of the grain sizes listed in microns (µm). There are 1000 µm in 1 mm. The particles classified as sand are what you would intuitively think of as being sand-sized, so an easy way to remember the scale is that anything smaller than sand is fine-grained, and anything larger is coarse-grained.
One other thing to notice about this scale is that the finest-grained particle is referred to as clay. While a clay-sized particle could be composed of clay minerals (and often they are), it doesn’t have to be. Any particle of that size would be referred to as clay.
Grain Size and Transportation
The grain size of sediments is not just for purposes of description. It’s also a valuable clue to the processes that have acted on those sediments, because the size of the clast determines how much energy is required to move it.
Whether or not a medium such as water or air has the ability to move a clast of a particular size and keep it moving depends on the velocity of the flow. The faster the medium flows, the larger the clasts that can be moved. Figure 8.15 shows a streambed that now contains only a trickle of water- barely enough to move particles of silt and cool puppy feet. But the velocity of water in the stream changes from season to season, as does the volume of water. All of the clasts in the streambed have been transported there by water.
Weathering can break off large fragments of rock. It can also make smaller fragments. The extent to which the grains in sediment differ in size is described by sorting.
If the grains in a sample of sediment are the same size or very nearly so, the sediments are said to be well sorted. If the grains are vary substantially in size, the sediments are poorly sorted. If they are somewhere in between, then they are moderately sorted. Examples of these are shown in the upper part of Figure 8.16. Because grains become progressively smaller as they are transported, sorting improves the further the sediments are from their source.
Rounding refers to whether clasts have sharp edges and corners or not. If the grains are rough, with lots of edges and corners, then they are referred to as angular. Grains with smooth surfaces are rounded. Grains in between can be sub-angular or sub-rounded. Examples of these are shown in the row of boxes at the bottom of Figure 8.16.
Sphericity describes whether a grain is elongate or not. Grains that are longer than they are wide (like an ellipse) have low sphericity, whereas grains that have the same diameter no matter where you measure it (like a sphere) are high sphericity. In the bottom row of boxes in Figure 8.16 the grains at the top of each box would be considered high sphericity, and the grains at the bottom would be low sphericity. Notice that a grain can be angular but still have high sphericity. It can be rounded, but still have low sphericity.
Exercise 8.3 Looking at Sand
Three samples of sand are shown below, along with information about what they contain and where they are from. Describe each sample in terms of grain size, sorting, rounding, and sphericity, and then suggest a mechanical weathering mechanism that might have produced them.
- View an example of the Wentworth scale for grain classification at http://bit.ly/UWscale. ↵
- Figure 5 of Reagan, M.K., Pearce, J.A., Petronotis, K., and the Expedition 352 Scientists, 2015, Proceedings of the International Ocean Discovery Program, Volume 352, publications.iodp.org, doi:10.14379/iodp.proc.352.102.2015 ↵