Grain size is the average diameter of individual grains (particles) of clastic sediments and rocks. It is also known as the particle size. Grain size represents a fundamental character of clastic sedimentary rock and one of the most important property for description of these rocks. Sizes of clastic grains reflect their weathering and erosional history, besides providing idea about their transportation motif i.e as bed load, saltation or suspension.
By the term grain size normally we mean grain diameter. This is different from the crystalline size, which refers to the
size of a single crystal inside a particle or grain. A single grain can
be composed of several crystals. Granular material can range from very
small colloidal particles, through clay, silt, sand, gravel, and
cobbles, to boulders.
Types of grain size
Clastic sedimentary rocks are named according to the characteristics of clasts (rock and mineral fragments) that comprise them. Traditionally, sediments were divided into three principal categories: gravel, sand, and mud. The latter is further divided into silt and clay, mostly based on mineralogical distinction rather than (hydro-)dynamic properties. These characteristics include grain size, shape, and sorting.
Geologists that study sediments and sedimentary rocks use the Udden-Wentworth grain-size scale for describing the sizes of the grains in these materials (Table below).
What Are the Factors That Determine Sediment Grain Size?
The size of particles is directly dependent on the type of environmental setting, transporting agent, length and time during transport, and depositional conditions, and hence it possesses significant utility as an environmental proxy.
Why is grain size important in sedimentary rocks?
(1) Obtain a deeper understanding of paleo-environmental features or modern environmental impacts, (2) Reconstruct past sedimentary transport histories, depositional conditions, or sediment provenance, or (3) Analyze in detail a catastrophic event, such as a tsunami or hurricane deposit, for example.
How to estimate grain size:
Several methods for measuring the grain size of siliciclastic particles
are available, with the choice of methods depending upon the purpose of study/analysis, sizes of
the particles and their state of consolidation (Table).
In the field
In the field, we can estimate grain size through a visual comparison with grain size charts like the one I have drawn, below. These charts are normally centered on the arenitic grain sizes, since sand grains are difficult to measure, whereas for gravels we can use rulers or measuring tapes.
|The handy USGS chart above shows a comparison of grain size classes|
Cobble cam is relatively a new autocorrelation technique developed by
Rubin (2004) to measure grain-size of fluvial and coastal gravel bars
using digital photographs (Warrick et al., 2009). This technique uses
digital photos that are obtained from ~1 m above the ground surface of a
landform containing granule- to cobble-sized sediment (e.g., a gravel
bar). The cobble cam is calibrated with physical measurements of the
intermediate and long dimensions of clasts in the field that are
compared to the short and long axes, respectively, measured from the
digital photographic images.
In The laboratory
This method is used for unconsolidated
material. Sieving is perhaps the oldest and most traditional of the
analytical techniques for sand and gravel sized material. The coarsest sieve required is placed at the top of the stack in which
the square screen openings become progressively smaller down the stack.
Sieving generally gives the intermediate measure of a particle because
of the way particles orient themselves to pass through the mesh. The mesh size is defined by the number of openings across one linear inch of screen.
This method is used to measure the grain size of sediments in thin sections or polished rock surfaces. A digital image of the sediment is taken, and the grain size is measured using image analysis software.
The pipette method
One inexpensive sizing technique for the mud fraction is the pipette method that relies on the principle that particles in a dilute suspension settle through a column of water at velocities that are dependent on their size.
The sedimentation tube
The second technique based on settling
velocity is the sedimentation tube, which unlike the pipette technique
that is confined to analysis of silts and clays, can be applied to sand
samples as well.
Laser diffraction analysis (LDA)
LDA has become increasingly popular over the past 15 years as a method for the analysis of particle-size distribution of sediments and soils. The instrument measures the particle-size characteristics of a sediment sample using the principles of laser diffraction and relies on the fact that the diffraction angle is directly proportional to the particle size.
Other PSA techniques
Other techniques used in geomorphology include electrical zone sensing (EZS), XRay sedimentation and scanning electron microscopy (SEM).