The Earth -- Introduction

# Class Notes - Shorelines

## Introduction

Shorelines are the areas between low tide and the highest level on land affected by storm waves. Approximately 67% of the world's population is concentrated in narrow bands adjacent to shorelines. In addition, coastlines often provide a convenient storage place for sediment derived from a continental mass. The Atchafalaya Delta, LA, is a good example of a river dominated (bird's foot) deltaic complex (Landsat image). Read about Louisiana's Barrier Islands: A Vanishing Resource ... in the link given in the Chapter Heading.

## Wave Dynamics

Most waves are produced by the interaction between the wind and the surface of a body of water. Waves can also be produced when water is displaced; for example, the release of energy accompanying an earthquake. The greater the distance the wind blows over a stretch of water (the fetch), the greater the size of the waves. The water molecules primarily move up and down (unless the wave moves into very shallow water), so it is the wave form that is traveling. The speed of the form (the celerity) is given by:

C = wave length/period

The wave length is the distance from peak to peak or trough to trough and the period is the time it takes for two crests to pass a given point.

Wave base is one half of the wave length. When the depth of the water is less than the wave base the wave begins to interact with the floor of the ocean or lake. The wave length decreases and the velocity increases. Water molecules are displaced towards the shore and breakers form.

Shorelines tend to be irregular in shape and the waves generally approach the shore at some angle other than 90 degrees. Part of the wave may be influenced by wave base and begin to break, whereas the rest of the wave may not be modified. The net effect is for the waves to bend (refraction) so that the wave hits the beach nearly parallel to the shoreline. Even a slight angle of approach can cause water between the breakers and the beach to flow parallel to the coastline, producing a longshore current. These currents are extremely influential on the sand budget of the beach as they transport large quantities of sediment along the coastline. Rip currents are created when water piled up on the beach and transported laterally flows back through the breaker zone.

## Shoreline Deposition

1. Beaches - unconsolidated sediment extending from low tide to the dunes or other topographic high. When high energy storms strike a coastline sand is moved offshore and the beaches are depleted. Normal waves will replenish the sand from the offshore source.

2. Spits - a continuation of a beach forming a point that extends into a body of water. Currents can cause the spit to curve (see Cape Cod).

3. Barrier Islands - elongated sand bodies parallel to the shoreline. Usually developed along a passive continental margin. Barrier islands migrate as a function of sea level. If sea level rises the islands migrate landward. Along the Texas Gulf Coast this presents a real conflict between the developers and the property owners.

## Shoreline Erosion

Along passive margins (coastlines that do not coincide with a plate boundary) the coastline tends to be an area in which deposition of sediment is the primary process. Along active margins, however, erosional processes dominant. Such shorelines are characterized by cliffs and isolated country rock - stacks.

## Tides

Both the moon and the sun have strong influence (the moon dominates) on the shape of the Earth - including the oceans. Interactions between the moon, sun and Earth result in the tides.

## Salt in the Oceans

If we evaporated 100 pounds of sea water we would get about 3.5 pounds of solids. That is, about 3.5% of sea water is material carried as part of the chemical load of the sea water itself. For fresh water the amount of dissolved material is much less than 1% by weight.

The average composition of the total dissolved solids in sea water is:

ElementWeight % Total Solids
Chlorine 55%
Sodium 30%
Sulfate 8%
Magnesium 4%

The oceans consist of about _______% Sodium (Na).

30
10
1.05

The surface area of a sphere is 4*pi*r2. If the radius of the Earth is 6,360 km, what is the its surface area (in cm2: 1km = 105 cm) (use 3.1416 as pi).

5 X 1013km
5 X 1018cm2
5 X 1018 cm

The density of sea water is 1.03 g/cm3, 72% of the Earth's total surface area is sea water and the average ocean depth is 3 km. Surface area times depth gives us an estimate of the volume of sea water.

1.1 x 1024 cm3
1.1 x 1024 cm
1.1 x 1018 cm 3

Knowing the volume and the density we can compute the mass : Density = Mass/Volume. The mass of sea water is:

1.15 x 1024 cm
1.15 x 1024 grams
11.5 x 1024 grams

The mass of sodium in the world's oceans is therefore (in tons - 106 grams = 1.1 tons)

.12x 1016 tons
1.2 x 1016 tons
12. x 1016 tons

Which of the following minerals is a likely source of the sodium in sea water?

quartz
feldspars
calcite
olivines

An early attempt to date the Earth began by arguing that there was no sodium in the original oceans and that weathering released sodium. These estimates were much less than the "known" age of the Earth.

Sodium present in the oceans is equilivant to what would be produced by the chemical weathering of about 1 km of continental rock. _________________________________________________________________________________________________

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Copyright by John C. Butler, July 29, 1995