Hawaii Geography and Geology - Hawaii is All Volcanoes

The Geography you have to know:


The Hawaiian Islands from geoatlas.com
The Island Names: (From oldest to youngest)

Tongue-tied? Check out Omniglot's pronunciation guide.


Traditional districts of Hawaii from Kumukahi.org
Traditional districts:



The Island of Hawaiian from Hawaii Guide
The Island of Hawaii is a composite of five volcanoes:

Geology of Hawaii I: Building the Islands

Plate Tectonics: The Context

In the 19th century, people expected that the topography and composition of the ocean's floor should resemble that on land, but as real data started to flow in, it became clear that this was wrong:

Clearly the geology of the oceans was unlike that of the continents. WTF?


Answering that question took about eighty years, but the final answer:



Layers of the upper Earth:

Processes: Mid-ocean ridges and subduction zones largely divide the oceanic crust and uppermost mantle into rigid plates that glide across a deeper layer of ductile and partially melted rock and move relative to one another. Hence - "Plate Tectonics."

Hot spots:

Emissaries from below the realm of plate tectonics.


Hawaii from The New York Times
There are places where volcanoes erupt not in a plate-boundary subduction-zone volcanic arc, but at a single point. Examples are

These are called hot spots. Why are they there?

The asthenosphere extends no deeper than 400 km. The deeper mantle is not involved in the processes of plate tectonics. Instead, the deep mantle is the source of mantle plumes - columns of hot rising rock that are the source of the magma for hot spots.


Hawaii hot-spot schematic from Puna Rideg.0rg
In contrast to the volcanic arcs (lines of active volcanoes) that occur at subduction zones, active hot-spot volcanoes tend to be single or tightly clustered, Nevertheless, they often sit at the end of lone strings of extinct volcanoes. Hawaii contains the youngest volcanoes in the Emperor Seamount chain. These chains form as a plate moves over the mantle plume, continually carrying away the older volcanoes and allowing new ones to form. Similar to the effect of allowing an old vinyls LP to rotate over a bunsen burner flame.




Puu O'o eruption, HI from Kilauea Mitigation & Preparedness

About Volcanoes:

Volcanism is the eruption of magma - molten rock - onto Earth's surface. But Earth's crust and mantle are solid. What causes rock to melt in the first place?


Where does magma come from?

Composition:


Peridotite from University of Pittsburgh
Here's a paradox: Mantle rocks are even denser and more iron and magnesium rich than oceanic crust. The rock peridotite (right) is typically what melts to make magma, but hardly any magma ever solidifies into peridotite. Why not? Four factors drive magmas to differ chemically from their parent rocks and, eventually, from one another, but first, a definition:


Geologists use a convenient vocabulary to describe a continuum of the composition of magmas and the rocks they solidify into:

Felsic (~70% silica (SiO2)) - Intermediate (~60% SiO2) - Mafic (~50%SiO2) - Ultramafic (<=40% SiO2)

As a rule, rocks with felsic chemistry have the lowest melting points than those with ultramafic have the highest.



Basalt

All Hawaiian magmas are mafic and solidify to form basalt, but not all basalts are identical.


How does magma behave?

Mantle rock is solid. When melting does occur, it happens at the peripheries of individual crystal grains (b - right), yielding minute pockets of magma. When these pockets grow to the point that they interconnect (a - right) the magma is able to move.

Being liquid, magma tends to percolate upward by any available means. As this happens, droplets coalesce, eventually forming large magma chambers that can be relatively small (Enchanted Rock, TX) to very large (Sierra Nevada Batholith).



Mauna Loa (background) is young, erupting fluid magmas that form smooth slopes
Mauna Kea (foreground) is old. Its last eruptions formed cinder cones.

The Big Message:

Because Hawaiian volcanoes: Each volcano undergoes a "life-cycle" characterized by slightly different styles of

Not all volcanoes are like those of Hawaii


Mt. Redoubt, AK from Wikipedia. Not like Hawaii.
Depending on their composition, magmas behave differently:


Basalt flow with pahoehoe texture on Fernandina - Galapagos Islands
  • Mafic lava characteristic textures:

    Additional Information.

    Volcano types:


      Mauna Loa, HI. - a shield volcano, from Wikipedia

    • Shield volcanoes:

      Scale - very large - Large, with gentle slopes, formed of copious fluid flows. Large examples on Earth may be 10 km high from base to summit and 120 km in diameter. Generally associated with hot spots. Classic example - Mauna Loa, HI.


      Sunset Crater cinder cone, AZ from Wikipedia
    • Cinder-cone volcanoes:

      Scale - Small, consisting of basaltic cinders in unconsolidated mounds. Forms geologically short-lived cones that erode away quickly when volcanic activity ceases. Sunset Crater, AZ (right) is a large example, and began to form in 1065 CE. Activity lasted for over a century.


      Mt. Fuji, Japan, a composite volcano, from Wikitravel
    • Composite volcano: (also "stratovolcano")

      Scale - medium. Volcano built of alternating lava and pyroclastic deposits. Solidified lava flows protect underlying pyroclasts from erosion. Large but frequently steep sided. What we typically think of when we hear the word "volcano."

    Associated structures:


      The crater of Santa Ana Volcano, El Salvador, from Wikipedia
    • Crater:

      Pit or depression at summit of most volcanoes.


      Active lava tube from USGS
    • Lava tube:

      Tubes formed when an active lava flow skins over then the liquid lava flows out, leaving the hardened outer surface roofing a tube.


      Diamond Head, HI Wikipedia
    • Caldera:

      Large basin that forms when a volcano collapses into its magma chamber as it empties. Characteristic of dormant volcanoes.

      Classic example: Diamond Head (right) on Oahu, HI. (Try not to think of the earworm!)



      Pit crater on Hualalai from Wikipedia
    • Pit crater:

      A pit (without raised walls) that forms from the collapse of ground surface when magma is withdrawn from underneath. Like a caldera on a smaller scale.



    Styles of volcanic eruption:

    Effusive eruptions: These produce low-viscosity (i.e. highly liquid) basaltic lava.


      Puu O'o from USGS
    • Central vent eruptions: Lava emerges from a point source

      Fissure eruption at Puu O'o, HI from YouTube
    • Fissure eruption:

      Copious amounts of basaltic lava erupting through a fissure rather than a point source.


      Halema'uma'u lava lake from Hawaii Magazine
    • Lava lakes:

      Not exactly eruptions. When there is sufficient hydraulic pressure to force magma toward the surface but not quite enough to pump it out, the result can be a lava lake, where molten rock is visible from the surface.


    Volcano - water interactions:


      Fumarole at Kilauea, HI
    • Fumaroles: Places where groundwater is heated to boiling and escapes as steam.




      Mafic ash deposits at Wawamalu Beach State Park, Oahu, HI
      WTF?
    • Phreatic Explosions: Mafic eruptions don't typically result in volcanic ash, except under one condition - When groundwater and magma come into contact underground, causing the water to boil explosively in a phreatic explosion.

      In 1790, roughly 80 warriors of the king of Ka'u were killed in such an explosion just west of Kilauea. A rare example of violence from the typically predictable Kilauea - the world's only drive-in active volcano.