365 Days of Climate Awareness 211 – Coastal Geomorphology 2: Sediment

Sediment is any natural solid which has been produced volcanically or broken down by weathering and can be transported by wind, water, ice or gravity. (You can whimsically think of snow and dried leaves as sediment too, but they don’t quite qualify.) Generally we’re talking about rock fragments, ranging from microscopic to small house-sized. Right now we’ll focus on sand.

Sediment clasts, as they are called (from the Greek adjective “klastos”, broken), are classified by their diameter. The British geologist Chester Wentworth devised a scale for this, called the phi scale, based on powers of 2. The scale is centered on sand, which ranges from 4 (0.0625 mm diameter) to -1 (2 mm diameter) phi.

Sphericity vs. roundedness.

Sand occurs on nearly one-third of the world’s beaches, and can be either terrigenous (from rocks on land), biogenic (from living animals, such as shell and skeleton fragments) or volcanic. Terrigenous sand is mostly quartz, silicon dioxide Si0₂, an extremely hard and tough mineral, and to a lesser extent other rocks such as feldspar (which is typically more colorful but softer).

A huge range of sizes, shapes and minerals make up sand.

The shape can be described in two ways, first in terms of its sphericity: high, medium and low; and then by its roundedness, or how weathered and smooth the surface is. Roundedness can range from highly angular (not much weathered) to well rounded (highly weathered). These factors, along with the sand’s size and mineral composition (which includes hardness and density), determine its behavior.

Sediment grains.

Sediment grains require an outside force to move, and larger grains need more force. Large boulders generally move only under the influence of gravity, water or ice (glaciers can carry sediment of any size). Smaller grains can be pushed or carried by progressively weaker flows or water or, in the cases of sand and finer sediment, wind.

Sediment deposits are described in terms of their “sorting”, acknowledging that deposits of a certain range of sediment grain size are reflective of a certain energy environment. Rapidly flowing mountain rivers tend to have cobbles and gravel lining their beds. Larger, slower rivers have silty, sandy bottoms. Beaches are generally made of sand, which waves can pick up and transport. Lagoons and lakes tend to be silty, and the deep ocean generally features a deep clay bed, because only microscopic clay particles can float through the water for great distances.

Angle of Repose. 

An important aspect of sediment behavior is the angle of repose, the natural slope a pile of sediment will assume, balancing the cohesiveness of sediment grains against the pull of gravity. Angle of repose is a function of many factors: sediment grain size (and size range if there’s a wide mix), mineralogy, shape and roughness. Also important is whether the sediment is surrounded by air or water. Generally speaking, coarser sediment has a higher angle of repose: that is, the grains are able to cohere (hold together) at a greater angle than smaller, smoother grains. 

Tomorrow: intro to coastal geomorphology 3: beaches.

Be brave, and be well.