Exploring Our Fluid Earth

Importance of Substrates

The composition of the benthic, or ocean bottom, habitat is an important physical factor of the marine environment. Benthic substances, also known as substrates, can include sand, mud, rocks, rubble, or boulders. Substrates are important because they are both a foundation and a product of the environment. Substrates affect the physical and biological processes in an area. Substrates are also a product of the physical and biological processes in an area.

Sand Characteristics

Fig. 5.23. A scientist studies the rock and sand substrate at an intertidal site.

Photo by Kanesa Seraphin

When most people think of the substrate along the edge of the ocean, they think of sand. Scientists study sand to learn about the biological, chemical, and physical processes in an area (Fig. 5.23).

Beach sand can appear fairly uniform, but it is actually a complex mixture of substances with various dimensions. When scientists study sand, some qualities are particularly useful in characterizing the type of sand. These qualities include the colors, texture, and size of the sand grains and their material origins. In general, sand observations can be divided into three broad categories:

1. observations about size,
2. observations about shape, and
3. observations about the probable source of the sand.

From these three characteristics scientists can learn about the physical, chemical, and biological processes at the beach from which the sand came.

Sand Size

The Wentworth scale is one system used to classify sediments, including sand, by grain size. The word sediment is a general term for mineral particles, for example individual sand grains, which have been created by the weathering of rocks and soil and transported by natural processes, like water and wind. In decreasing order of size, sediments include boulders, gravel, sand, and silt. When using the Wentworth scale, the substance the sediment is made of is not part of the classification. For example, the term sand is used for sediment with grain size between 0.25 mm and 2 mm in diameter (Table 5.6), whether it is made of granite or silica. Sediments with smaller grain sizes are classified as silt or mud, and sediments with larger grain sizes are classified as gravel or boulders. Not all sediment on beaches is classified as sand! For example, gravel granules (2–4 mm diameter) are common on sandy beaches, but they are too large to be classified as sand (Table 5.5).

Understanding the distribution of sand grain sizes on a beach can help in understanding the oceanographic processes that shape the coastline in a particular area. For example, high-energy waves, which have longer wavelengths, generally produce beach surfaces with a relatively similar, or homogenous, grain size distribution. Lower energy waves, which have smaller wavelengths, tend to produce beach surfaces with a more mixed, or heterogeneous, grain size distribution. Most of the time, beaches exposed to high-energy waves have larger sediments than those that are exposed to lower-energy waves.

Factors other than wave energy also determine sand grain size at a beach. The size of sand grains is related to the slope of the beach. For example, the steeper the beach, the larger the sand grain size tends to be. This is because larger particles can be cast higher up the beach by the waves on steep beaches. On flatter beaches, however, sand grains tend to be rolled back and forth and broken into smaller pieces.

On some beaches, sand grain size composition varies with distance from the water. A greater proportion of finer, smaller sand grains may be pushed higher up the beach by waves or by wind, whereas larger, coarser grains are deposited closer to the water. However, beaches are complex and highly variable environments, and there are many areas where this distribution is not found because there are many conditions that affect sand size and distribution. Additional factors influencing sand grain size include the nearshore and offshore seafloor features, substrate type, sand source, currents, wind exposure, and coastline shape.

Knowing the grain size distribution of a beach is important, not only for understanding a beach’s ecology, but also for knowing how best to replenish sand on a beach that is eroding. The grain size distribution of a sample of sand can be determined by shaking it through a set of sieves. Sieves are containers with mesh bottoms that can filter and separate sediment grains into size groups (Fig. 5.24). Graduated geology sieves stack; the sieve with the largest mesh openings is on top and the one with the smallest mesh openings is on the bottom. As the set of sieves is shaken, sand falls through the different mesh sizes. The larger particles stay in the levels with the larger mesh, and the smallest particles fall through each mesh size all the way to the bottom of the container (Fig. 5.24). The blue, black, light green, and orange pieces in Fig. 5.24 (A) are fragments of plastic debris.

Image

Fig. 5.24. Beach sand sifted in sieves to show (A) very coarse (2 mm diameter) sized sand grains. The blue, black, light green, and orange pieces are fragments of plastic debris.

Image from Spalding, H.L., K.M. Duncan, and Z.N. Nuu. (2009) Sorting Out Sediment Grain Size and Plastic Pollution in Sand. Oceanography, Vol 22(4):244-250.

Image

Fig. 5.24. Beach sand sifted in sieves to show (B) coarse (1 mm diameter) sized sand grains.

Image from Spalding, H.L., K.M. Duncan, and Z.N. Nuu. (2009) Sorting Out Sediment Grain Size and Plastic Pollution in Sand. Oceanography, Vol 22(4):244-250.

Image

Fig. 5.24. Beach sand sifted in sieves to show (C) medium (0.5 mm diameter) sized sand grains.

Image from Spalding, H.L., K.M. Duncan, and Z.N. Nuu. (2009) Sorting Out Sediment Grain Size and Plastic Pollution in Sand. Oceanography, Vol 22(4):244-250.

Sand Shape

Sand grains are shaped by their composition and their history. For example, minerals form shapes such as cubes or pyramids, and pieces of shells in sand can be identified as part of an organism. However, distinctly shaped minerals or shells in sand can become difficult to identify because over time they are rounded and polished through weathering. Weathering is the breaking down of rocks and minerals by waves, wind, and rain. When wind or waves move particles like sand, the particles rub against each other, wearing down rough edges and smoothing surfaces. Water from waves or rain also changes particles by dissolving soluble substances. Over time, these processes transform large, angular particles into small, rounded sand grains (Table 5.6).

Table 5.6. Magnified sand grains showing classification by shape

Shape Type	Diagram
Very Angular	Image Image copyright and source Image by Byron Inouye
Angular	Image Image copyright and source Image by Byron Inouye
Subangular	Image Image copyright and source Image by Byron Inouye
Subrounded	Image Image copyright and source Image by Byron Inouye
Rounded	Image Image copyright and source Image by Byron Inouye
Well Rounded	Image Image copyright and source Image by Byron Inouye

Sand grains from beaches with high wave action tend to be more rounded than those from beaches with low wave action. On beaches with steep slopes, sand grains are more angular than the particles on flatter beaches. On gently sloping beaches, sand grains tend to be rolled back and forth so, over time, they become more rounded.

Sand Grain Cards

Fig. 5.25. Example of a sand grain card

Image by Fan Yang

Sand grain cards are used in conjunction with sieve sets to determine sand particle size as well as other sand characteristics. While sieves are important tools for quantifying sand grain size distributions, they have drawbacks. Sieves are large and difficult to carry to remote field sites, they require the sand to be dry, and sieving sand takes time. Sand grain cards are used as a quick tool for determining sand grain size, sorting, and shape during field analysis (Fig. 5.25). Sand grain cards allow scientists to easily determine sand size in the field according to the Wentworth scale. Scientists compare the sand at their field site to the pictures (on the left of the card in Fig. 5.26). The sand may match one or more size classes. In the card in Fig. 5.26, size classes are abbreviated by upper case letters: VC stands for very coarse, C for coarse, M for medium, F for fine and VF for very fine. The size classes correspond to size range measurements in microns. Note that 1000 microns (or micrometers, symbol μ or μm) equals 1 millimeter. Thus, C, coarse sand, ranges in size from 500 microns up to 1000 microns (or 1 mm). The sand grain card in Fig. 5.26 also allows scientists to assign sand to a standardized sorting scale (poor, moderate, well, or very well) for describing sand composition and classify sand by shape (angular, subangular, subrounded, rounded, or well-rounded) to characterize site wave action and weathering.

Sand Source

By identifying the components of sand, it is possible to tell what the sand is made of. Sands can be broadly classified by their source into two types, biogenic sand and abiogenic sand. Biogenic (bio = living; genic = produced by) components are the living or once-living components of an environment. Abiogenic (a = not) components are the non-living chemical and physical components of an environment.

Abiogenic, or “lithogenic” (litho = stone), sand grains are formed as rocks break down through weathering and erosion. Erosion is the movement of weathered rocks and minerals from one location to another. Abiogenic sands can be formed from rocks in the continental crust or the oceanic crust of the earth. The continental crust includes most of the major landmasses of the world. Mountains in the continental crust are composed mostly of granite. Mineral sands formed by the breakdown of granite usually contain quartz, feldspar, mica, and magnetite. Minerals are solid, naturally occurring substances composed of a single chemical compound. For example, quartz is a mineral composed of the chemical compound silicon dioxide (SiO₂). For more information on weathering and erosion see the units The Ocean Floor in the physical aquatic science module and Seafloor Chemistry in the chemical aquatic science module.

The sands of most beaches along the coasts of the continental United States, where quartz is the most abundant, resistant component, are quartz sands. In areas that have continental volcanoes, olivine and obsidian (a type of volcanic glass), may also be found.

Oceanic crust, made of volcanic material called basalt, contributes to another type of abiogenic sand. Volcanic islands, lava from volcanic eruptions, and many of the hard substrates covering the seafloor are made of basalt. Basalt is rich in metal-containing minerals, such as iron and manganese, which makes basalt denser and darker in color than granite. Basalt contains no quartz, but it does contain resistant minerals like olivine. Smaller amounts of other less resistant inorganic minerals, such as magnetite or hornblende, are also found in basalt sands. Components of abiogenic sand are listed in Table 5.7.

Table 5.7. Common components of abiogenic sand

Picture	Abiogenic Sand Origin and Description
Image Basaltic rock fragment sand at Kehena Beach, on Hawai‘i’s Big Island, US. Width of view is 10 mm. Image courtesy of Siim Sepp (www.sandatlas.org)	Basalt. Black lava flows are basalt. As they erode, they may form dull black, gray, or brownish red grains of gravel and sand.
Image Image caption: Swedish sand sample composed of feldspar (yellow and pink grains) and quartz (clear grains). Width of view is 20 mm. Image courtesy of Siim Sepp (www.sandatlas.org)	Feldspar. Feldspar has clear, yellow, or pink square crystals with a smooth, glossy, or pearly luster.
Image Pink garnet sand from Emerald Creek, Idaho, US Image courtesy of Siim Sepp (www.sandatlas.org)	Garnet. Garnets are silicon crystals, often amber or brown in color. Some are light pink, red, or orange.
Image Continental sand from Botany Bay, Sydney, Australia Image courtesy of Siim Sepp (www.sandatlas.org)	Granite. Granite grains are usually light-colored to pink, with a salt-and-pepper pattern of mineral crystals all about the same size.
Image Magnetite crystals, width of view is 25 mm. Image courtesy of Siim Sepp (www.sandatlas.org)	Magnetic mineral grains. Magnetic mineral grains may be grains of iron ore, magnetite, or other metals. These grains are dense and tend to accumulate at the bottom of containers. Magnetite crystals resemble a double pyramid. Magnetic mineral grains in sand can be observed by passing a magnet over a sand sample.
Image Mica sand in Finistere, France Image courtesy of Siim Sepp (www.sandatlas.org)	Mica. Mica forms shiny, paper-thin, translucent flexible sheets. It is light-colored or white and may appear iridescent.
Image Olivine sand at Papakōlea Beach on the Big Island of Hawai‘i Island, US Image courtesy of Siim Sepp (www.sandatlas.org)	Olivine. Olivine is a shiny crystal that can be various shades of olive-green to almost brown. It may be transparent or translucent and often contains specks of other crystals. It is found in basalt.
Image Quartz sand from Minnesota, US, the width of the view is 7 mm. Image courtesy of Siim Sepp (www.sandatlas.org)	Quartz. Quartz crystals are clear or transparent, resembling small pieces of broken glass. Quartz comes from granite and sandstone erosion. It is the most abundant mineral found in continental sand.
Image Image A by Joanna Philippoff   Image Obsidian sand at Punalu‘u Beach on Hawai‘i Island, US Image courtesy of Siim Sepp (www.sandatlas.org)	Volcanic glass. Volcanic glass forms when hot lava is rapidly cooled, forming black, shiny, irregular, sharp-edged particles. Continental volcanoes form obsidian.
Image Glass beach on Kaua‘i, Hawai‘i, US. The width of the view is 20 mm. Image courtesy of Siim Sepp (www.sandatlas.org)	Manmade substances. "Beach glass" is formed when shards of manufactured glass are rounded and frosted by wave action. Other manmade substances, especially plastics, may also be found on the beach.

Biogenic sands are also sometimes called calcium sands or limey sands because the chemical composition is mostly calcium carbonate, CaCO₃. Parts of organisms such as coral skeletons, mollusk shells, worm tubes, or sea urchin spines are made primarily of CaCO₃. These organisms remove calcium (Ca²⁺) and carbonate (CO₃^2-) ions from the water and incorporate them into their hard structures as the compound CaCO₃. When the organisms die, the hard structures remain. These hard structures are worn down into sand by the tumbling of waves, grinding of organisms like parrotfish or sea urchins, and other weathering processes.

It is not always possible to identify biogenic sand just by looking at it, because weathering processes can turn organism shells and other structures into unidentifiable, smooth sand grains. One method of identifying biogenic sand is an acid test. If vinegar, which is acetic acid, is dropped onto sand containing calcium carbonate, it will react to produce bubbles of carbon dioxide gas. Sand that does not come from a living source, like quartz sand, does not react with acids like vinegar.

Examining beach sand can tell us something about the local biology. Most biogenic sands are composed of fragments of coral skeletons, coralline algae, and mollusks. This type of sand is described by its most abundant component. For example, sand composed mostly of coral skeletons is called coral sand.

Some of the components of biogenic sand are small fragments of larger organisms, like pieces of coral and shells. Other biogenic sand components are the skeletal remains of entire organisms, such as very small mollusks or single-celled foraminifera. Biogenic sands can also include resistant biological fragments of organisms, such as sponge spicules, or fossil remains of teeth and parts of jawbones. Some biogenic sand components are listed in Table 5.8.

Table 5.8. Common components of biogenic sand

Picture	Biogenic Sand Origin and Description
Image Barnacle base plate on upper left, side and top plates to lower right. Image courtesy of Maine Geological Survey	Barnacle fragments. Pieces of the calcareous plates that form the carapace of a barnacle, may be white, yellow, pink, orange, lavender, or purple. Occasionally they have a striped or notched pattern. The rest of the barnacle is made of chitin, which is not resistant and thus will disintegrate over time rather than form sand.
Image Sediment made from a variety of crushed bivalve shells, Catawba Island in Port Clinton, Ohio Image courtesy of kreezzalee, from Flickr. CC by 2.0	Bivalve mollusks. Bivalve shells or pieces of clam, oyster, or mussel shells may appear white, gray, blue, or brown. These are usually not shiny, and are slow to dissolve in acid.
Image Sand grains made from coral fragments and gastropod snail shells, Tankah, Mexico. Width of view is 25 mm. Image courtesy of Siim Sepp (www.sandatlas.org)	Gastropod mollusks. Snail shells, or fragments of shells, vary widely in color, shape, and pattern. Juvenile shells are more fragile than their adult forms and may differ in appearance. Eroded fragments may reveal internal spiral growth patterns. "Cat's eyes," white disks, round on one side and flat on the other, are intact operculums, trapdoor-like structures used to close the outer opening when the foot is withdrawn into the shell. "Puka" shells are the tops of eroded cone shells that appear as light-colored disks with a hole in the center. The word puka is Hawaiian for hole. Their slightly concave undersides sometimes show concentric rings.
Image Flake-shaped sediment produced by Halimeda green algae, Palmyra Atoll, central Pacific ocean basin Image courtesy of Ingrid Knapp	Calcium-depositing algae. Calcareous algae are green or brown algae, like Halimeda, that secrete small amounts of calcium carbonate to form delicate skeletons. Coralline algae are marine algae that secrete large amounts of calcium carbonate to form robust skeletons. Encrusting coralline algae appears rose or lavender when alive and white when dried.
Image Coral fragment gravel from Jeju-do Island, South Korea Image courtesy of Siim Sepp (www.sandatlas.org)	Coral. Fragments of dull-white coral rubble are common in tropical sand. Larger, intact pieces from the outer layer of coral skeletons may be identified by their many small holes (cups) where individual coral polyps once lived.
Image Individual foraminferan skeletons. The rounded discs are wave-worn Amphistegina foraminiferans, common in Hawai‘i. Width of view is 10 mm. Images courtesy of Siim Sepp (www.sandatlas.org)	Foraminifera. Foraminifera are the skeletons of protozoans, one-celled animals. They may be white, dull or shiny, or covered with tiny sand grains. They look like tiny shells except that their openings are small and look like slits or pores. These opening are where the living animal extended its false feet to trap food.
Image Sea urchin spines and test fragment. Width of view is 10 mm. Image courtesy of Siim Sepp (www.sandatlas.org)	Sea urchin fragments. Sea urchin spines may be white, purple, black, beige, or green. Viewed under a microscope, some have crystalline matrices that look like ornate corn-on-the-cob structures from the side or concentric growth rings from the top. Tests are the inner skeletons of sea urchins. Test fragments have tiny holes and raised knoblike structures arranged in regular sequences; they appear dull white or lavender.
Image Sponge spicules Image courtesy of the National Oceanic and Atmospheric Administration (NOAA). Public domain.	Sponge spicules. Spicules are usually clear and transparent or whitish. Large triaxon sponge spicules may resemble the three-pointed logo of the Mercedes-Benz automobile. They make up the internal skeletal support structure of some sponges.
Image Assorted biogenic sand grains from Majorca, Spain. Width of view is 10 mm (1-3 Sea urchin spines; 4, 8, 19 Gastropod; 5 Bivalve; 6, 10, 14, 15, 17, 21, 23 Foraminiferan; 7, 18, 20 Bryozoan; 9 Ostracoda; 11 Scaphopod; 16 Mollusk; 22 Sponge spicule). Image courtesy of Siim Sepp (www.sandatlas.org)	Other animal or plant parts. Biogenic sand may contain other animal parts such as calcareous marine worm tubes, pieces of crab or shrimp skeletons, or the colonial animals known as bryozoans (numbers 7, 18, and 20 in image).

Sediment Availability

Sediment availability is also a critical factor in determining beach characteristics. Beaches are often made from materials that are in the area, like coral, quartz, or basalt. However, beach sediments can also represent past conditions that are out of sync with current wave conditions. For example, in Hawai‘i, much of the sand on beaches today was deposited by waves thousands of years ago. In addition, beaches are often dramatically altered by human activity. Many beaches have sand that has been brought from other locations, such as inland deserts, other beaches, or offshore sand bars. This movement of sand makes it hard to use the sand as a predictor of beach characteristics. Thus, it is important to understand the history of a beach when studying its sand.

Sand Transport, Coastal Erosion, and Human Impact on Beaches

Fig. 5.27. Diagram of sandbar, spit, and barrier island

Image by Byron Inouye

The size, shape, and source of sand at a beach are influenced by local sand transport patterns. Sand transport is the movement of sand, and it is primarily achieved by waves and currents. This movement sorts sand by size and density. Lighter, less-dense sand grains are more easily transported by waves and currents, whereas larger, more-dense grains are left behind.

As sand is transported along coastlines, it often forms characteristic beach formations such as sandbars, spits, and barrier beaches (see the topic Wave-Coast Interactions in this unit). Sandbars (bars) are hills of sand that are usually submerged or only partially exposed. A spit is a curved sandbar connected to the beach at one end. A barrier island is a ridge of sand that is above water at high tide. Barrier islands are parallel to the shore and separated from the beach by a lagoon. If a spit or barrier island is stable, vegetation will start to grow on it. Barrier islands are found along approximately 15 percent of the world’s coastlines.

Fig. 5.28. Shoreline erosion along Kailua Beach, Hawai‘i

Image by Alyssa Gundersen

Sand on a beach may erode—be lost (Fig. 5.28), or accrete—be built up. For example, in some areas, beaches may accumulate sand in the summer that is eroded in the winter due to seasonal weather and wave patterns. Although erosion and accretion are natural processes, they can be accelerated by human activity. Sea-level rise due to global climate change is eroding beaches. The construction of harbors and other structures can enhance sand accretion and necessitate dredging to maintain boating channels.

There is concern about beach erosion because it causes a loss of property for those living along coastlines. In an effort to prevent erosion, humans attempt to harden the shoreline and make it more stable, often as a way to protect property in the immediate area (see examples in Table 5.12). Unfortunately, this protection is often short-lived and often comes at the expense of beach health. Hardened structures can cause erosion by preventing waves from accessing sandy reservoirs and by changing nearshore wave patterns. For example, since 1949 approximately 25% of the sandy beach in Hawai‘i has been narrowed or lost due to beach hardening.

Table 5.12. Examples of structures humans have build to prevent erosion along shorelines.

Type	Definition	Diagram
Seawall	A rigid wall structure made of cement or other building materials placed parallel to a shoreline	Image Image courtesy of Geert Schotanus, Flickr
Rip Rap	Loose collections of large rock or cement blocks placed along a shoreline	Image Image courtesy of Greensheep, Wikimedia Commons
Groin (or groyne)	A rigid structure often built perpendicular to the shoreline that interrupts water flow and movement of sediment	Image Image courtesy of Pkuczynski, Wikimedia Commons
Breakwater	Offshore structures made of large boulders or cement blocks used to protect anchorages or harbor entrances from wave energy	Image Image courtesy of Claus Ableiter, Wikimedia Commons
Jetty	A rigid structure built in pairs perpendicular to the shoreline to stabilize inlet channels	Image Image courtesy of WPPilot, Wikimedia Commons

Fig. 5.29. An aerial view of Kailua Beach on O‘ahu, Hawai‘i. Based on current erosion rates, the two properties on the right side of the image are sufficiently set back from the coastline. In contrast, the two properties on the left side of the image have insufficient setback distance.

Image courtesy of Google Earth

Beaches play an important role in protecting the coast, enhancing tourism, and serving as a place to relax and refresh. The loss of beaches negatively impacts human activities and property as well as the environment. For example, beach loss can cause the smothering of local marine life with eroded sediment. In order to maintain healthy beaches, scientists recommend replenishing sand, keeping coastal areas free of hardened structures, and requiring large setbacks for new property development (Fig. 5.29).