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WEST MAUI VOLCANO

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WEST MAUI VOLCANO

SPECTACULAR

While the scenic and still-active Haleakala volcano on the eastern half of the island of Maui is quite spectacular, I personally think that its older western counterpart is equally fascinating.

Across what is simply known as the West Maui volcano, you can find some of the deepest and most stunning canyons in all of Hawaii, which are up to 4,400 feet deep.

You can also see the remains of an intact body of solidified magma, which fed one of its eruptions, exposed high on a cliff, along with the remains of something you would not initially expect at a Hawaiian volcano: lava domes.

So, how exactly did this volcano form, and how did it at one point grow to become the tallest volcano in all of Hawaii, rising to 13,000 feet (3,962 meters) in height?

The Hawaiian Hotspot

The answer, like all other volcanoes in the Hawaiian Island Chain, is the Hawaiian Hotspot in the underlying mantle.

It is called a hotspot because it represents an area of the mantle which is about 200 degrees Celsius hotter than the surrounding mantle material.

Since an increase in temperature decreases its density, this hotter material rises into the oceanic crust, creating numerous magmatic intrusions and eventually volcanoes and volcanic eruptions.

While the Hawaiian Hotspot itself largely does not move, the overlying Pacific Plate has been moving to the west-northwest, creating a chain of distinct volcanoes.

The Submarine Stage

In the case of the West Maui volcano, it became the most active Hawaiian volcano beginning 2 million years ago when it began erupting as a submarine volcano on the ocean floor.

Layer after layer of pillow basalt was emplaced—so-named due to its appearance of many rounded pillows—causing the volcano to rise from the ocean floor.

• Watch an underwater volcano! 2 minutes

Volcanic eruptions became progressively more common over time, building a vast underwater shield volcano that approached the ocean surface.

Upon reaching shallow depths, a series of shallow explosive eruptions occurred, building a plateau out of ash and lava that the subsequent island would expand upon.

The Shield Stage

Due to a zone of weakness in the crust running south-southeast to north-northwest, magma dikes preferentially trended into it, creating two distinct rift zones.

These rift zones acted as zones of weakness where numerous flank eruptions would occur, causing large volumes of low-viscosity basaltic lava to run down from two ridges and reach the ocean, forming an oblong volcano.

A series of large-volume flank eruptions, which removed multiple cubic kilometers of its magma chamber, caused subsidence to occur at West Maui's summit, causing it to collapse into a caldera.

This caldera measured 1.95 miles long and 1.95 miles wide and was initially 400 feet deep.

Exposed dikes which remain today fed three of these rift zones, with many eruptions forming both cinder cones and spatter cones in mildly explosive to purely effusive eruptions.

Eventually, this volcano reached a height of approximately 13,000 feet due to its multiple eruptions every century across hundreds of thousands of years, reaching this height around 1.3 million years ago.

The Post-Shield Stage

Afterward, the volcano entered a different phase of activity, specifically its post-shield stage.

Look at Stage 3 in the illustration below.

Eruptions became less frequent but more explosive, creating more viscous flows of lava made of both hawaiite and trachyte.

• Hawaiite is a volcanic rock that typically erupts during the late stages of volcanic activity3, a pattern that is common not only in Hawaii but also on other volcanic islands formed by hot spots.

• Trachyte is volcanic rock formed by the rapid cooling of lava (or shallow intrusions) enriched with silica.

Trachyte formed incredibly viscous lava domes which rose as much as 580 feet above the surrounding landscape.

Due to gas being more easily trapped under such highly viscous lava, explosive eruptions were able to occur along with even some pyroclastic flows.

This phase lasted for another 100,000 years until eruptive activity paused.

Erosion and Rejuvenation

Across the next 490,000 years, rainfall expanded existing low points in the terrain, forming deep canyons as large sections of the island eroded away.

Even the summit eroded away.

Then, rejuvenated stage volcanism began because so much of the West Maui volcano had eroded away that it actually caused some minor uplift to occur.

This allowed for very rare but occasional magmatic intrusions to form cinder cones in four distinct eruptions along its shoreline 610,000 years ago, 580,000 years ago, and during two eruptions around 390,000 years ago.

• Basalt (n.): A dark, fine-grained volcanic rock.

• Caldera (n.): A large bowl-shaped crater formed by the collapse of a volcano's mouth.

• Counterpart (n.): Something that has the same function or position as something else.

• Density (n.): How closely packed the material inside an object is.

• Dike (n.): A long, thin body of hardened magma that cuts across other rock layers.

• Effusive (adj.): A type of volcanic eruption where lava flows steadily out onto the ground.

• Erosion (n.): The process where wind, water, or ice wear away rocks and soil.

• Intrusion (n.): The process of magma forcing its way into existing rock layers.

• Mantle (n.): The thick layer of hot, solid rock located between the Earth's crust and its core.

• Oblong (adj.): An object that is longer than it is wide.

• Plateau (n.): A large, flat area of land that is higher than the surrounding land.

• Pyroclastic (adj.): Relating to a fast-moving cloud of hot gas and rock fragments from a volcano.

• Rejuvenated (adj.): Having been given new energy or appearing fresh again (in geology, a volcano starting to erupt again after a long pause).

• Rift (n.): A long crack or opening in the Earth's surface.

• Shield volcano (n.): A broad volcano with gently sloping sides, shaped like a warrior's shield.

• Submarine (adj.): Existing or occurring under the surface of the sea.

• Subsidence (n.): The gradual caving in or sinking of an area of land.

• Trachyte (n.): A type of light-colored volcanic rock that comes from very thick lava.

• Viscous (adj.): Having a thick, sticky consistency between a solid and a liquid.

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