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Introduction to Density



Fig. 2.1. Fresh water plume in stream near Alega, American Samoa. The fresh water layer is on top of the seawater layer. The fresh water layer is cloudy with sediment.

Image by Cheryl Squair

Water in the ocean is not uniform in its composition; the properties of ocean water vary in different parts of the ocean. Even within one geographical area of the ocean, there are water masses with different properties. If water masses do not have the same densities, they will form layers of water. The relative density of each water mass determines whether it will float or sink in relation to another water mass. For example, fresh river water floats on top of salty ocean water (Fig. 2.1). The relative floating and sinking of water masses affects vertical ocean circulation.

Ocean Literacy Principles

Principle 1: The earth has one big ocean with many features

Ocean Literacy Fundamental Concept: Throughout the ocean there is one interconnected circulation system powered by wind, tides, the force of the earth’s rotation (Coriolis effect), the sun, and water density differences. The shape of the ocean basins and adjacent landmasses influence the path of circulation. (OLP 1c)

Ocean Literacy Fundamental Concept: Most of Earth’s water (97 percent) is in the ocean. Seawater has unique properties: it is saline, its freezing point is slightly lower than fresh water, its density is slightly higher, its electrical conductivity is much higher, and it is slightly basic. The salt in seawater comes from eroding land, volcanic emissions, reactions at the seafloor, and atmospheric deposition. (OLP 1f)

To build an understanding of the densities of seawater and fresh water and of how water density differences affect ocean circulation, it is important to understand how temperature and salinity differences form water layers of different densities. Density differences between water layers determine whether a layer will float or sink in relation to other water layers.

These concepts will be explored in this unit through the following activities and investigations: 

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.