Case Study: New Land from the Seafloor
Documenting Change over Time
Conditions on Earth are constantly changing. Because we tend to focus on the way things are today, and forget how they were in the past, photographs are valuable in helping us document change over time. Just as a child's growth is more obvious when you look at snapshots taken over their lifetime, changes on Earth can be documented by comparing satellite images taken over time. Sets of images that show the same place at different times are called time-series images.
Time-series satellite images can be used to study several types of change. For example, if you examined yearly images of a growing city, you could identify the portions of the city that were built each year. Highlighting each of these portions with a different color would result in a map that shows where and when the city grew over time.
To study deforestation, you could examine time-series satellite images of forested land, and identify areas that had been cleared between images. By outlining the newly deforested areas in successive images, you could compile a single annotated map that illustrates how the forest changed.
The image to the left is an example of the annotated image described above: it documents an increase in deforested area from 1976 to 1998 in Bolivia. The colors indicate when each area was cleared. Examine the legend to interpret how the annotated image indicates change over time.
For background information and more details about this sample image, see this brief article from NASA's Earth Observatory.
Over thousands of years, the shapes of harbors and coastlines change naturally. Wave erosion can eat away at land along one part of a coast while a river delta forms new land along another part. On a geological time scale, plate tectonic motions also change coastlines. Humans, another part of the Earth system, have developed technology that can change the shape of a coastline in fewer than 10 years. The process is called land reclamation, and it is accomplished through dredging.
Dredging is the process of removing sediments from the bottom (floor) of a body of water. In harbors where a river enters the sea and sediments tend to accumulate, dredging is performed to keep waterways deep enough for ships to move through them. Dredgers have a constant battle against the accumulating sediments to keep the harbor deep enough for ships.
In some areas, sediments that are dredged from the bottom of a harbor are piled up onto low land to fill swampy areas. This makes the land more useful to humans. Dredged sediments might also be pumped into holding pens near the shore, eventually accumulating to a height that the gravel and sand rise above the water level. In this way, dredging the sea floor can create new land in locations that were once covered by water.
A surprising example of new land that was recently created by dredging sediments from the seafloor is Palm Island, a resort in the Persian Gulf. Boston Harbor in Massachusetts is another site where extensive areas of land have been reclaimed from the sea. In Boston's case, the sediments used to create new land were brought in from regional gravel mines rather than dredged off the seafloor, but the end result is similar. This article on Boston Harbor shows how the coastline looked in 1830 compared with 2001.
In this chapter, you'll examine satellite images of an area in China, near Hong Kong, where large land reclamation projects have taken place. You'll analyze the images to produce an annotated map that shows where and when new land was created over time.