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Geological Overview
 

As an “armchair” natural sciences enthusiast (particularly geology and astronomy), I find the history of Hawai’i's creation and evolution very interesting.  I love to study and ponder the “how did it all begin?” and ”where are we all headed?” types of questions, and I’d like to share with you a few facts about the origins of the Hawaiian Islands that you might find interesting.

First, I remember being surprised when I heard how young the Hawaiian Islands are (roughly five million years for Kaua’i, the oldest island, and approximately 500,000 years for the Big Island), and was also surprised to see how much geological change has occurred during their relatively brief existence.

It is important to put the Hawaiian Islands in perspective.  They are not, as one might think, a stand-alone group of islands in the middle of the Pacific Ocean, unrelated to any others, that just happened to emerge as the result of volcanic activity.  Rather, they are simply the most recent of a very long chain of islands that runs approximately 2,400 miles to the northwest, then takes a sharp turn to the north and runs yet another 2,500 miles or so all the way to the Aleutian Islands.  Most of the former islands to the northwest of Midway Island have sunk back into the ocean, but their remains are there just the same.

The source of the lava that makes the volcanoes that make all these islands is a “hot spot” in the earth’s crust that is stationary and up through which the lava is transported and vented to the earth’s exterior.  (Note: until it reaches the surface, the liquid rock is known as “magma,” once it reaches the surface, it is known as “lava.”)

But wait…how can islands form over thousands of miles if the hot spot is fixed in the earth?  The answer is simple and lies in the concept called “plate tectonics.”  The earth’s crust is not one solid, contiguous covering like the peel covering an orange.  Rather, it is made of many (approximately 16) distinct and individual plates, kind of like the plates of an armadillo’s hide or of that of medieval armor.  Some of the plates are quite large and some are small; there is no “standard sized” plate.  And as strange as it may seem, these plates really do “float” on, and move over the material beneath them.  It may seem impossible that a piece of the earth’s crust (a tectonic plate) weighing trillions upon trillions of tons could actually float on top of something, but it’s all a matter of physics.  Any object will float on any other object, if the former is less dense than the latter.  It’s simply a matter of relative densities.  “Lighter stuff floats, heavier stuff sinks.”

These plates move over time and the dynamics of this fascinate me.  Here’s how it works:  we’ve all seen large pots of water boiling on our stoves.  The hot water boils up in the center, spreads out to the side of the pot then goes down the sides to the bottom again.  Welcome to the principles of “plate tectonics” on a very small scale!

There is a tremendous amount of energy in the earth’s interior and it is this energy that drives the “engine” of plate tectonics.  The energy, derived from the radioactive decay of material deep in the earth, rises to the surface where it often manifests itself in volcanic activity.  The Hawaiian hot spot, a source of tremendous volumes of magma, is fixed in the earth, and the Pacific Plate moves across it at a rate of approximately 4 inches per year (so Pearl Harbor Naval Station, the homeport of my first ship, is now about fifteen feet farther to the northwest than it was when I first arrived there in 1971; that’s pretty neat, I think.)

So there you have it.  The hot spot periodically spews lava that builds up into mountains.  It then goes dormant for a while as the Pacific Plate passes over it, then when a new batch of magma has bubbled up from deep within the interior into the hot spot, it erupts yet again and forms a new island.  (It is worth noting here that the Hawaiian Islands are in the middle of the Pacific Plate, not at one of its edges.  You may have heard about all the seismic activity that takes place at the edge of tectonic plates, but that’s a totally different concept (sub-duction of the plates at their edges, as opposed to the action of a hot spot.)

It is also interesting to know that these forces and process apply on a more “local” basis.  Each of the islands, once it has been raised to its highest elevation by its volcanic activity, will begin to sink back into the earth again.  That is because when considered as a whole, the new, taller island is denser than its supporting material.  (A good example of the concept:  when a whale wants to dive, it just doesn’t “nose over” and head for the deep.  Rather, they get as much of their bodies (tails) out of the water as they can; this weight, unsupported by the water, helps drive the whale underwater more quickly.  Same principle.)  The new, large island of Hawai’i is sinking at the rate of 1-2 inches every ten years, while O’ahu has sunk enough into its underlying base that it has reached a balance (called “isostasy.”)

And these principles can be seen to exist on a state-wide scale too. The Hawaiian islands collectively sit in the middle of what scientists call “the Hawaiian Deep,” which is a depression (about 1,500 feet or so deep) made by all the islands as they sink as agroup into their collective supporting material (think of eight children sitting in a straight line on a trampoline; they will create quite a dent in the fabric due to their weight.)  To complete the point, the “Hawaiian Deep” has in turn actually caused the creation of “the Hawaiian Arch,” a ring of raised material that surrounds the Deep.  As you can see, the earth’s crust is a dynamic, flexible and fascinating subject!

A word also here about lava, that most critical substance without which there would be no Hawai’i.  There are two main types of lava found in Hawai’i's development and a couple of lesser types:

Pahoehoe lava

Pahoehoe lava is one of the most interesting forms of lava. Since it tends to flow more as a thick liquid it can pour uphill as well as downhill and can create a huge variety of interesting shapes.

If pahoehoe lava flows over a fairly flat ground it will coat the ground much like a parking lot – with a thick, smooth, flat coating of lava. However, when pahoehoe encounters barriers in the way, such as rocks, hills, fissures and bumps, it flows over and around them and produces different shapes.
Pahoehoe Lava is one of the most interesting forms of lava. Since it tends to flow more as a thick liquid it can pour uphill as well as downhill and can create a huge variety of interesting shapes.

A’a lava

A’a’ lava is the most commonly encountered lave in Hawai’i, lava flows that cool down forming fragmented, rough, sometimes spiny, or blocky surfaces.  A’a’ lava forms when the viscosity of the lava (e.g. because of high gas bubbles content and relatively low temperatures) and/or the strain rate of the flow (related mainly to eruption rate and steepness of the ground) are high.

Whether a given lava flow is pahoehoe or A’a is driven by the density of the matieral, itself usually determined by how much gas there is in the lava.  Therefore, as pahoehoe lava cools and the gas it contains comes out of the lava, it can turn into A’a lava (changing from a less dense to a more dense material.)  Pahoeohoe lava can morph in to A’a lava, but never the reverse.

Less often seen types of lava

Pele’s Hair

Another form of lava is officially named Pele’s Hair. This is an interesting form of lava, and most unexpected. It is also one of the few lava forms that you can create yourself (though nature produces some of the best examples).

Pele’s hair looks like long strands of greenish-gold hair. In bright sunlight it is a shimmering gold color, perfectly straight and as thick as human hair. The lengths can be anywhere from particle sized to several feet long, though most Pele’s hair that you will encounter (unless it is brand spanking new) will be in the 2 to 6 inch length.

Pele’s hair is created when molten lava is ejected into the air – as happens when lava fountains or enters the ocean and explodes. If the airborne molten lava is small enough, and the wind is strong enough, the wind will pull the lava droplet and turn it into a hair-sized piece of rock. It is as if the wind extruded “rock wire” from the liquid mass, much as wire is created in a factory.

You can find Pele’s hair all over the place, but mostly near vents, skylights and the ocean entry. Crevices in the ground or areas where rocks form a corner are locations where the blown Pele’s hair can collect. Be careful though, Pele’s hair while only the size of human hair it is like glass fiber and is very sharp. It is easy to get a small piece stuck in your hand while you are examining it, if you are not careful. (Gloves can be useful in this situation.)

Pele’s Tears

Pele’s tears are another form of lava related loosely to Pele’s hair. Just as with Pele’s hair, Pele’s tears start out being smallmolten bits of lava sent hurtling into the air due to an explosion or, more often, lava fountain. However, the difference is that the wind is not strong enough to extrude the lava into strands – but instead, the bits tumble back to earth and form round, oval and more importantly, tear shaped bits about 1/4 to 1/2 inch in size. By the time the tears hit the earth they have fully cooled and retain their oval or tear shape.

There are many places within the Hawai’i Volcanoes National Park where there are literally acres of Pele’s tears. Quite a bit is right along the roadway and it is fun to park and go look for Pele’s tears. Because the tears are the result of fountaining fissures, anywhere there was a fissure that fountained you can find the tears downwind. Good places to look include the area on both sides of the road between Devastation Trail and Kilauea Crater on Crater Rim Drive, as well as the first 1/4 to 1/2 mile on either side of the road on Chain of Craters Drive.

Lava Bombs

Lava bombs can be thought of as Pele’s tears taken to an extremely level. Formed the same way; lava bombs are HUGE blobs of molten lava ejected high into the air. As these blobs fall to earth they rotate and gravity converts them into round and oval rocks that hit the earth already cooled. These are huge rocks, ranging from several feet in diameter to the size of cars, and while you might be able to escape a fountain of Pele’s tears, you would never escape a fountain of raining lava bombs.

Lava bombs can be found all over the Big Island. There are many fine examples within the Hawai’i Volcanoes National Park but some of the best examples can be found on the road up to the Mauna Loa Weather Observatory off Saddle Road, as well as the upper road around Mauna Kea (4-wheel only). On Mauna Kea there are huge fields (hundreds of acres) where there are perfect lava bombs every 3 to 5 feet – spaced as if planted – an amazing site to behold.




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