Plate Tectonic Workshop: North Western Hawaiian Islands, Exploration Islands, Reefs and a Hotspot

http://www.soest.hawaii.edu/pibhmc/pibhmc_nwhi.htm

Introduction

The islands of the Hawaiian archipelago were formed by a series of volcanic eruptions that began more than 80 million years ago. Volcanoes are often associated with movement of the tectonic plates that make up the Earth’s crust. The outer shell of the Earth (called the lithosphere) consists of about a dozen large plates of rock (called tectonic plates) that move several centimetres per year relative to each other. These plates consist of a crust about 5 km thick, and the upper 60 - 75 km of the Earth’s mantle.

The plates that make up the lithosphere move on a hot flowing mantle layer called the asthenosphere, which is several hundred kilometres thick. Heat within the asthenosphere creates convection currents (similar to the currents that can be seen if food colouring is added to a heated container of water). These convection currents cause the tectonic plates to move. Plates may slide horizontally past each other at transform plate boundaries. The motion of the plates rubbing against each other sets up huge stresses that can cause portions of the rock to break, resulting in earthquakes. Places where these breaks occur are called faults. A well-known example of a transform plate boundary is the San Andreas Fault in California.

 Where tectonic plates move apart (for example, along the mid-ocean ridge in the middle of the Atlantic Ocean) a rift is formed, which allows magma (molten rock) to escape from deep within the Earth and harden into solid rock known as basalt. Where tectonic plates come together, one plate may descend beneath the other in a process called subduction, which generates high temperatures and pressures that can lead to explosive volcanic eruptions (such as the Mount St. Helens eruption which resulted from subduction of the Juan de Fuca tectonic plate beneath the North American tectonic plate).

Volcanoes can also be formed at hotspots, which are thought to be natural pipelines to reservoirs of magma in the upper portion of the Earth’s mantle. The Hawaiian Islands are the result of volcanic activity associated with a hotspot that appears to deeply penetrate the mantle to the boundary between the mantle and the Earth’s metallic core. The Hawaiian hotspot is presently located beneath the Big Island of Hawaii at the south-eastern end of the archipelago.

The Pacific tectonic plate is presently moving over the asthenosphere toward the northwest at a rate of 5 to 10 cm per year. As the plate moves over the Hawaiian hotspot, magma periodically erupts to form volcanoes that become islands. The oldest island is Kure at the north-western end of the archipelago. The youngest is the Big Island of Hawaii at the south-eastern end. Loihi, east of the Big Island, is the newest volcano in the chain and may eventually form another island. As the Pacific plate moves to the northwest, islands are carried farther away from the hot spot, and the crust cools and subsides. At the same time, erosion gradually shrinks the islands, and unless there is further volcanic activity (or a drop in sea level) the island eventually submerges below the ocean surface. To the northwest of Kure, the Emperor Seamounts are the submerged remains of former islands that are even older than Kure.

Scientists recognize eight stages of growth and erosion in the islands of the Hawaiian archipelago:

1. The deep submarine stage begins with submarine eruptions, which eventually reach the ocean surface (Loihi is in this stage).

2. The shallow submarine stage features an above-water crater, which spouts lava from rifts on the side of the cone.

3. The subaerial shield-building stage begins with collapse of the highest point (summit) on the volcanic cone to form a caldera. The volcano continues to emit lava from the summit and from rifts in the side of the cone (Mauna Loa and Kilauea are in this stage).

4. The post-caldera stage, in which lava fills and overflows the caldera to form a rounded summit. While overall volcanic activity may slow down, significant lava flow still continues (the Kohala Mountains, Mauna Kea, and Hualalai are in this stage; Haleakala is also in this stage, even though the caldera is not filled and still has a crater shape).

5. The erosional stage, in which lava is no longer being added, and the volcanic cone is attacked by erosion from the ocean and rainfall. A sea bluff, deep valleys and sharp ridges are characteristic features of this stage (Kauai, Oahu, and portions of all the major Hawaiian Islands are in this stage).

6. The stage of reef growth occurs when volcanic mountains are eroded to the point that they are only rocks that barely break the ocean’s surface. The volcanic island is slowly sinking at this stage, but it is often possible for a coral growth to keep pace with the sinking so that reefs can form (French Frigate Shoals is in this stage).

7. The stage of post-erosional eruptions is marked by minor renewal of volcanism through which a few small cones or lava flows may be formed (portions of West Maui are in this stage).

8. The atoll stage occurs when lava rock has been eroded below sea level, and only the coral reef remains at the surface (Pearl and Hermes Reef and Kure are in this stage).

This activity focuses on the role of plate tectonic movement and hotspot activity on the formation of island in the Hawaiian archipelago, and on the stages in island development that can be seen throughout the island chain.

Tasks

On your copy of the “Central Pacific Map Grid” plot the location of each island on the Map Grid, and label each island with its name and age.

 Deduce the direction of motion of the Pacific plate, and calculate its approximate velocity.

 Note: Velocity = Distance ÷ Time

Label each island with its velocity vector.

Consider the diagram of the Hawaiian Archipelago:

(http://www.soest.hawaii.edu/pibhmc/images/StudySites-CRED-HI_B.jpg)

Discuss why the older islands (to the left of the diagram) have different profiles from those on the right side of the profile.

 Draw a diagram or cartoon to describe each of the 8 stages of volcanic island formation.

Find an example of each stage among the islands of the Hawaiian archipelago utilising the following web resources:

http://www.papahanaumokuakea.gov/imagery/graphicsmaps.html

http://www.papahanaumokuakea.gov/imagery/satimages.html

Further Resources

http://oceanexplorer.noaa.gov  – The NOAA North Western Hawaiian Islands Expedition.

http://www.soest.hawaii.edu/GG/HCV/haw_formation.html – Hawaii Centre for Volcanology website about the formation of the Hawaiian Islands.

http://coris.noaa.gov/portals/nwhi.html – Information about the North Western Hawaiian Islands region

Reference

Ocean Explorer, NOAA North Western Hawaiian Islands Exploration, Islands, Reefs and a Hotspot

http://oceanexplorer.noaa.gov/explorations/02hawaii/background/education/media/nwhi_hot.pdf

Data

Location and Age of Some Islands in the Hawaii Archipelago

(from http://www.soest.hawaii.edu/GGHCV/haw_formation.html)

DPG October 2011 Version 2.0