Low Tide on Lord Howe Island

September 20, 2003


Provided and copyright by: Roger Scott, Pine Rivers State High School
Summary authors & editors: Roger Scott

The photograph above shows Lord Howe Island lagoon at low tide. It was taken in early July from the eastern edge of the lagoon, looking west southwest. Lord Howe Isalnd is located about 500 miles or 800 km off the east coast of Australia. Basalt peaks dominate the topography of Lord Howe Island, but none of the 350 or so island residents live on the basalt bedrock. Instead they've built on deposits of calcarenite, medium grained sediments composed of calcium carbonate. Over the last 6.5 million years, erosion has dramatically reduced the area of basalt above sea level. The highest points on the island are basaltic peaks, which are linked by deposits of calcarenite. The calcareous material is chiefly composed of shell and coral fragments, but foraminiferal material is also present. During the last ice age, sea levels were lower, and persistent winds produced dunes, which link the basalt peaks. Sections of the calcarenite have subsequently been well cemented. These cemented calcarenite beds are more evident in the lagoon at low tide. The wind-produced bedding here has a dip of about 30 degrees.

Waves can just be seen on the horizon -- they're breaking on the fringing reef. The lagoon, behind the reef (approximately 500 meters wide at this point), is very quiet in the photograph.

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