NWHI: For Teachers: Life of an Island

Focus Question: Why are the Northwestern Hawaiian islands so much smaller than the main islands of the Hawaiian chain?

Lesson at a Glance: Students will learn about the evolution of a volcanic island from origin to erosion. They will be able to determine the relative ages of the Northwestern Hawaiian Islands given their position in the archipelago.

Key Concepts: Volcanic islands form over a hot spot on the ocean floor. Islands form and erode in eight stages

(1) deep submarine stage
(2) shallow submarine stage
(3) subaerial shield-building stage
(4) post-caldera stage
(5) erosional stage
(6) stage of reef growth
(7) stage of post-erosional eruptions, and
(8) atoll stage. Refer to the website for an animation of this process.

The relative age of an island or atoll can be determined based on its state of growth or erosion. The Hawaiian archipelago rides on the Pacific tectonic plate that is moving in a northwesterly direction. The oldest islands in the archipelago are located northwest of the main islands, and the youngest island (Lo'ihi) is forming under the ocean off the eastern coast of the island of Hawai'i.

Objectives: Students will be able to:
1. Identify the eight life cycle stages of an island.
2. Identify the stage of an island based on given criteria.
3. Demonstrate their understanding of the life cycle of an island by illustrating the process and creating their own names for the eight stages.
4. Identify the stage of an island based on photographs.

Materials: NWHI website, photographs of selected islands in the Hawaiian archipelago.

Preparation and Procedure: Teachers should explain the concept of plate tectonics and hot spots to show how volcanoes are formed over a single spot in the ocean floor, become islands, and drifted toward the northwest. A map that shows the submerged ridge that connects the islands and atolls in the archipelago can be used to illustrate this concept. The cross-section of the Hawaiian islands in the NWHI map can also be used to stress this point. Aerial photographs of the NWHI can be used to illustrate the different stages of growth and erosion of an island. These photographs can be downloaded from the website. Teachers should begin by showing photographs of eruptions on the island of Hawai'i as an example of an island in the shield-building stage.

Talk About It!

The formation of archipelagos

Asked by jake on Mar 25, 2004.
how are archipelago islands formed

Answered by Paulo from UH on Mar 30, 2004.
An archipelago is a group of islands that normally share a common geological history. Most of them involve a volcanic origin, such as the Hawaiian Islands. When magma (underground lava) flows out of the sea floor, it slowly grows upwards, and sometimes after millions of years it can reach the surface forming an island. The place where magma flows out on the sea floor is known as a hot spot, and its location relative to the center of the Earth tends to be fairly stable. But the crust that makes the actual sea and land surface, known as a tectonic plate, moves over this hot spot. This movement is very slow, but over millions of years the hot spot position over a tectonic plate changes. The flow of magma can be sporadic, leading to periods of intense activity and periods of rest. This can often lead to the formation of many islands (periods of activity) separated by ocean (marking the periods of rest), eventually forming an archipelago.