Well, hello, and welcome. I'm glad you could join us today. We have a good program for you today. We have some unique pictures to show you, and I hope today's program will give you the opportunity to see some of the features of Hawaii's volcanoes that aren't normally accessible.
You know, Hawaii is rich in a variety of volcanic features; in fact, just about everything we see here in Hawaii is the result of volcanic processes acted upon by forces of erosion except maybe a small amount of reef and sediment here and there. We'll study these erosional processes and the reefs and sediments in future programs, but for now we want to turn our attention to volcanic features.
The world's most active volcano, Kilauea, and the world's largest volcano, Mauna Loa, are both here in the Hawaiian Islands, and basaltic volcanism is pretty much similar everywhere. Many of the islands in the Pacific, the island of Iceland, and some of the Atlantic islands as well all have similar features, and because of that studying Hawaiian volcanoes has provided a model for studying volcanoes everywhere. Here in Hawaii we have, usually anyway, relatively gentle eruptions which can be studied up close without too much danger of getting blown apart.
"Andesitic" and "rhyolitic" volcanoes, these are so-called continental types, are much more violent and harder to study, but they produce similar features, similar enough, in fact, that we can learn much about them by studying the basaltic volcanoes here in Hawaii, so in today's program we'll concentrate on the Hawaiian- type volcanoes and the features associated with them.
In today's program we'll take some video tours of several of the volcanic features which we see commonly in the Hawaiian Islands, so this lesson continues Lesson 13 in the study guide, and there is no "Earth Revealed" video that goes specifically with this program. We have put together a good record, I think, of the variety of common features associated with Hawaii's volcanoes. Now, some of this footage we've put together is a little shaky and not maybe the best quality, but many of the features that we're going to be showing you are in places that are accessible only by foot or by helicopter, so on several occasions we took a small video camera into the high desert of Mauna Loa to film some of the stark beauty that hides there. Some of these places have never been seen on television before because no cameras have been there before. I hope these pictures will supplement what we saw in the last "Earth Revealed" video in the last program and add to the somewhat limited descriptons in the text.
You may wish to review Chapter 11 in the text to compare the Hawaiian volcanoes with volcanoes elsewhere in the world and especially study Figure 11.27 on page 256 to see how volcanic rocks are classified.
Well, one of the distinctive features of Hawaiian volcanoes is that as they age they pass through three fairly distinct stages: the shield building stage, the alkalic stage, and a rejuvenated stage. Now, each volcano like each person is really an individual, but like people volcanoes are different from one another, but they share certain features in common, and we can use these common features to try to get an understanding of how the volcanoes work.
It's important to note here that not all of the volcanoes pass through all of the stages; in fact, a certain volcano may die or quit erupting before it reaches a certain stage, or it may skip a stage. For example, there has been no alkalic stage on the Koolau volcano here on Oahu, and there's been no caldera, as far as we can tell, on Mt. Hualalai and on Mauna Kea on the Island of Hawaii, and the Island of Lanai, the volcano which forms that island stopped erupting before a caldera ever was filled.
Okay, but even though all volcanoes don't go through all the stages, at various places in the Hawaiian Islands we can find all of the stages represented on various islands. Now, the actual age in years that's associated with these stages various with the individual volcanoes. It depends on processes deep in the volcano in the magma chamber. It depends on the movement of the volcano over and away from the hot spot mantle plume, and it also, the actual appearance as we'll see in the programs on erosion depends upon the rainfall and climate.
Many of the volcanoes developed fairly large erosional unconformities. Generally, there's one major one, but there also may be shorter minor ones. You may remember an unconformity is a period of geologic time which represents erosion happening at the surface. So let's take a look at these three stages of volcanic activity and see if we can characterize them for each of the three stages.
The first stage is the shield building stage, often referred to as the "youthful stage". This is the main constructive stage of the volcano where the volcano erupts mostly basaltic lavas either tholeiitic basalts or alkalic basalts. The volcanic eruption first starts as what we might call the deep submarine stage. Here the eruption occurs beneath a fairly deep sea water where the water pressure keeps the gases in solution, so vesicles don't have a chance to form (Vesicales are holes in the rock) if the eruption takes place below about 2,500 meters. The pressure of the water is simply too great to allow the gases to expand. Around 1,500 meters depth or so vesicles begin to appear the size of pinheads. On these deep submarine eruptions there's little or no surface expression of the eruption, and the lavas that are formed are generally pillow lavas.
Once the lava reaches a depth, I should say the volcanic mountain reaches a depth, above about 1,500 meters depth, then we start to enter what we might call the "shallow submarine stage". Here the cold water breaks the lava, and sometimes steam explosions form thick layers of ash. There are generally no explosions below a few hundred meters because the pressure of the water is too great to allow the water to explode, but much steam may be produced, and sometimes the explosions produces tephra or pyroclastic material of various kinds. These types of processes haven't been observed directly in Hawaii, but in 1963, an island now called Surtsey arose from the sea just off the south coast of Iceland, so we study quite extensively where jets of ash laden steam and water shoot out. These processes as ash falls back to the water may form a shallow ash cone in shallow water. The best example we have here in the Hawaiian Islands is the little island of Molokini off the southwest coast of Maui. During this stage we often find ash interbedded with lava flows because the volcano may produce lava if the vent is protected from water or if there's a large volume of lava.
Okay, the main shield building stage in this youthful stage is called the "subaerial". "Subaerial" means under air as opposed to under water. The shield building stage may take a million years or so. In order for the island to grow, lava must be added at a rate faster than it erodes. Keep in mind there's always this battle between the constructive forces of mountain building and volcanism and the erosive power of the sea. The thin fluid lava flows spread great distances from the vent.
The caldera, which may form at any stage, may become completely filled during this stage, but near the end of the youthful shield building stage, the volcanic activity becomes less frequent and more violent, and we often find layers of soil in the upper beds. We also find that ash and cinder, pyroclastics of various type, become more common, and the composition of the lava changes to become more alkalic.
We know that the caldera probably forms early in this stage because even Loihi, which is the new island forming off the southeast coast of the Island of Hawaii already has a caldera being formed.
In the so-called "old age stage" of development, sometimes called the "post caldera period" marks the end of the principal period of volcanism of the Hawaiian volcano. During this time the volcano forms a cap of lava and pyroclastics. This cap may hide the caldera and commonly forms a profile that's steeper and less extensive than the shield. Someone once described it as "perching like a limpet" on a larger shell. A limpet is like a little opihi. The best example on the Hawaiian Islands is Mauna Kea.
During this stage, the post caldera stage, the composition of the magma also changes. It may change late in the early shield building stage, or it may change transitionally into this new stage, but basically the lavas become more andesitic in composition. We think this is probably due to the fact that the magma chamber is cut off from its source of magma deep within the hot spot as the island drifts away from the hot spot over the over the moving lithospheric plate. This means that chemical changes take place in the magma chamber, and we'll study those changes in the next program.
At first, these changes are detectable only by chemical analyses, a gradual transition from tholeiitic to alkalic basalt, and it may occur over several hundred meters of the rock column as we examine the lavas later on. The later eruptions tend to become more explosive. The alkalic basalts are still quite fluid, but they're much gasier and much more explosive.
Okay, the eruptions become more like the spattering of thickening oatmeal, higher viscosity and more gas. The flows are shorter and thicker than basalts; for example, on PuuWaaWaa on the Island of Hawaii on the Kona side, one flow is more than 300 meters thick. The lava at this stage may also be so thick that it forms a thick pasty dome. There are several examples of this on Kohala on the Big Island and several examples on West Maui. Generally, following the post caldera stage, the volcano appears to die, and there's a long period of quiescence or erosion, where no volcanic eruptions take place. In many cases, this may be a million years or more. During this time deep valleys are cut into the rock, benches are formed at sea level, and so forth, and if the conditions are right, reefs may grow. On volcanoes this principal period may mark the end of eruptions. Some volcanoes never show the post caldera stage at all; others are rejuvenated after this long erosional period. On the older islands of Kauai, Oahu, Molokai, and Maui, we find a very well developed series of eruptions that formed after this post erosional period.
Okay, here the lavas show another change in character and composition. They form a type of rock called "nephelinite" and "basanite", which all we have to know about that for our purposes is that the silica content of these lavas is extremely low, under 40 percent in many cases, and the origin of these magmas is somewhat of a mystery. Many of these eruptions are quite explosive. They tend to be rather fluid, but they have a rather high gas content as well. They have a very high temperature, and they have few phenocrysts because of the high temperature.
On Oahu, especially, we find what are called "phreato magmatic".This is a fancyword that means contact of magma with steam. Many of these secondary cones or post erosional cones on Oahu, such as Diamond Head, Hanauma Bay, have large chunks of coral imbedded in the ash, and many of them sit on sand dunes.
As opposed to this, places like Tantalus, Round Top, and Sugar Loaf, all familiar features on the Honolulu skyline, consist of large amounts of black sandy ash, which are spread over the city and revealed in excavations, which give us a time table. Many of these form lava flows, which float into valleys and form thick pools that remained liquid so long that there are virtually no vesicles. One good example is the Moiliili flow, which formed the lower part of Manoa Valley from which the quarry at the University of Hawaii is formed. This flow was 40 feet thick and changed the course of Manoa stream. These flows are not continuous, meaning that there are often long periods of quiet between eruptions, and soil layers develop as well as valleys hundreds of meters deep.
The Koolaus, in other words, looked pretty much like they do today when these post erosional Honolulu series eruptions began about 800,000 years ago.
On Kauai, Niihau, and Oahu, these eruptions lasted tens of thousands of years. I want to note here that the typical time between eruptions on Oahu is about 250,000 years. It's only been 10,000 years since the last of these Honolulu series of eruptions took place. That means that we don't know for sure that these post erosional eruptions are done on Oahu, but they do form most of the recognizable volcanic features on Oahu, places like Diamond Head, Punchbowl, Hanauma Bay, Koko Head, and Koko Crater to name a few, but we know the Hawaiian Islands are younger to the southeast, and this is because the Pacific plate moves over the hot spot.
Even in Hawaiian legend, the fire goddess, Pele, was driven successively from Kauai to Oahu, on down to where she now resides on the Island of Hawaii. She was driven away by the goddess of the sea, Namaka O' Kaha'e, so even in Hawaii legend, they recognize that the islands get older as you go to the northwest. The older islands are deeply eroded, and the volcanoes on each island are individuals even when they're on the same island, such as Mauna Kea and Moana Loa on the Island of Hawaii, both of which rise more than 13,000 feet above sea level, but even though the volcanoes are individuals, all volcanoes share similar characteristics and features.
Here, for example, is Mauna Loa as seen from Kilauea. Note the long broad profile. Volcanoes in this shield building stage normally form a caldera at the summit, and we'll examine this caldera in some detail later. During this phase the activity is more or less continuous with occasional quiet periods.
Kilauea, the world's most active volcano, and Mauna Loa, the world's largest volcano, are both in the shield building stage.
In the alkalic stage, the volcano produces a more viscous magma and a steeper slope like Mauna Kea here as seen from the Kona Coast of the Island of Hawaii. Note the steep bulge. This is the andesite cap, and on Mauna Kea it's about 6,000 feet thick. In this stage, the volcano also produces more cinder cones, which give Mauna Kea this sawtooth appearance, as we see here taken from Mauna Loa at sunset, and here at a closer view shrouded in clouds.
In this stage the magma becomes more andesitic in composition.
We'll discuss this transformation in the next program. The magma may even approach rhyolitic composition like this lava dome on West Maua, which is called "Puukawae". The post erosional eruptions have occurred mainly on the older islands and formed many of the prominent features, such as Diamond Head, here on Oahu. Here you see Diamond Head in the foreground with a small shield, called the "Kaimuki Dome" on the landward side. Koko Crater and Koko Head, two other cinder cones of this Honolulu volcanic series, are visible in the distance. Hanauma Bay on Oahu was formed when a wave action eroded away the outer portion of an almost circular cone as these arching layers of ash on the seaward side show us.
This eruption, like many of the post erosional eruptions, contacted coral reef and sea water as shown by the pieces of coral imbedded in the ash. Some of these volcanic bombs were quite large, and, in fact, some of them mashed the loose layers of ash beneath them to form a feature called a "bomb sag", and on the Island of Molokai, a small eruption offshore of the cliffs, produced a small nearly perfect shield that now forms the Kalapapa Peninsula, and on the Island of Maui within a large erosional valley cut into the center of Haleakala, this post erosional phase overlaps the andesite phase and has nearly filled this large central valley with lava, cinder cones, and spatter cones.
Lava flowed out of this large central valley in several series of eruptions through Koolau gap on the northwest side and through Kaupo gap on the southeast side, so try to look for some of these features and learn to recognize the various stages of development of different parts of the different Hawaiian Islands as you drive around or fly around the Islands.
You know, Hawaiian volcanoes erupt lava, which has been stored in a large magma chamber, which is a few kilometers below the body of the shield. The magma chamber itself usually invades the body of the volcano and is a source for magmas or lavas that are erupted at the surface, but most of the lava flows which build the shields are erupted from radial rifts rather from the summit vents themselves, and in most cases these rift zones or rifts intersect at the summit of the volcano, usually intersecting a caldera if that particular volcano has one.
Hawaiian volcanoes usually have three rifts, and two of these are usually better developed and more active than the third. Most of the volcanoes the rifts are northeast rift, a southeast rift, and a northwest rift. The rift itself results from stresses as magma forces its way upward, and, for example, on Mauna Loa and Kilauea gravity anomalies radiate outward from the summit, indicating the presence of massive material underground, probably the magma chamber, so you can think of the rift zone in many ways as an extension of the magma chamber that just happens to follow this "y" shaped appearance.
Also, gravity anomalies roughly coincide with the locations of swarms of small earthquakes, which we call "harmonic tremors", which indicate the movement of magma underground and also with the surface expressions of the rift. Now, the rift zone itself contains many cinder cones and spatter cones but also many open cracks, which were the sites of eruptions and also parallel stress graphs, and stress cracks, and pit craters. Eruptions along the rift built it up giving an elongated shape to the shield as we see here looking at Mauna Loa from Mauna Kea, and in this air photo, we see a line of spatter cones and several lava flows along the southwest rift of Mauna Loa. Here we see a line of spatter cones along the east or puna rift of we see two large cinder cones along the rift. The rift zone may also be marked by parallel cracks, as we see in these two views of Kilauea's southwest rift. On Mauna Loa because of its size the eruptions involve much larger amounts of lava. Here the northeast rift is covered with cones and lava flows of various ages, textures and colors. Where there's no vegetation to obscure the features, you can see the many different episodes of eruptions which build the shield along the rift zone. The northeast rift of Mauna Loa is quite extensive in size and extends all the way to the summit where it interesects the summit caldera.
On Maui, the southwest rift of Haleakala is very well developed, also with many cinder cones. This rift is the source of the only eruption on the island in historic times, which occurred in about 1790. The newer black lava is a part of 1790 flows as seen here from La Perouse bay, which was formed by these flows when lava from that eruption entered the sea. Most eruptions of Hawaiian volcanoes along the rift begin as a curtain of fire when lava fountains out of a line of fissures. The eruption usually centers on a few sites along the rift as time passes. The rift itself may be oxidized by hot gasses, and minerals may be deposited as the steam rich gases cool. The edges of the rift may be glassy since the lava strikes the ground near the vent while it's still hot and liquid and cools quickly. The rift itself may extend for tens or even hundreds of meters and maybe 20 or more feet deep.
Something really incredible about being down here, the idea that these solid rocks were once liquid and even still bear the marks of their liquidity and their fire that formed them. The rocks on the side peels off layer after foamy layer. The mountain's dotted with these linear rifts, source of curtain and fire eruptions. It's the way the mountain's built. Layer by thin layer, each feature slowly covered by the next eruption till millions of millions of lava flows have built up a mountain nearly 14,000 feet high. It really is quite an experience to be inside of a rift that now looks so placid and docile, but only a few years ago was erupting lava up to 1,500 degrees centigrade. It's just awesome. It's hard to describe.
Well, let's turn our attention to "tephra"."Tephra" is a general name for pyroclastic material. You may remember "pyroclastic" means broken by fire. Pyroclastic material is material that's broken apart by the force of the eruption. The individual particles may vary quite a bit in both size and shape, all the way from microscopic size particles of ash, all the way up to rather large pieces of cinder or blocks or bombs. The distinction, by the way, between blocks and bombs is that if the material hits the ground while it's still liquid, it's called a "bomb". If, on the other hand, the material is solid before it hits the ground, it's called a "block". Not a particularly important distinction, but it's one that you may come across in reading about this. The material itself, the tephra, may be from the molten lava that's broken apart by the force of the explosion, or it may be from pieces of the lava conduit, the sides of the crater, or deep within the plumbing of the volcano.
Either one of these still are considered to be "tephra". Well, I have some examples of some "tephra". The first ones are simply pieces of cinder. "Cinder" is a general catch-all name for types of material which is basically solid before it hits the ground. These particular pieces are not only irregular in shape and have a rather dull luster, but they're also red in color. The red color here comes from oxidation of the iron, and you may remember that basaltic lavas are very rich in iron and magnesium, so the rusting here comes form the iron, and some of these pieces, here, for example, you can see that the center actually has holes in it called "vesicles", which come from the expansion of the gases after the material is erupted.
See, here we have a material that's a little bit too dense to be called "pumice" but still has that kind of spongy appearance. Okay, there are several other types of material that is ejected from the volcano.
Another one of these is called "Pele's tears". Now, Pele's tears are basically droplets of lava, which have solidified on the way down, and they take on this streamlined appearance like this nice piece here on my finger, much in the same way that raindrops take on the streamlined appearance in falling through the air. Most of the time Pele's tears are solid by the time they hit the ground, so they don't get deformed, but occasionally you find one which is flattened on the bottom, which indicates that it probably hit the ground and deformed somewhat at the time it hit. Pele's tears are fairly common in Hawaiian eruptions, but because they're glassy and a little bit frothy and pumicy inside, they don't tend to stick around too long after an eruption.
Another type of pyroclastic material is what we call here in Hawaii "black sand". Sand is actually any type of material which inhabits the beach as we'll see in later lessons, but this black sand is actually tiny glassy shards of lava, which were broken on contacting the cold water, much in the same way that you can break a glass baking dish by heating it in the oven and then suddenly plunging it into cold water. These fragments, as you can see, are various sizes and shapes, but generally they're glassy and irregular, so a black sand like this may form actually in the water and sink to the bottom and be washed up onto the beach to form the actual black sand beach in many case covering pre- existing features.
Okay, the next example of pyroclastic material is a rather specialized type of material called "tuff". Some people pronounce it "toof" I say specialized material because this is actually somewhat of a cross between a volcanic rock and a sedimentary rock. It's volcanic in origin, but "tuff" is a general name for a consolidated material, a consolidated volcanic material. By "consolidated" I mean that either the ash has stuck together after it falls because of its stickiness, or as in the case of this Diamond Head tuff has been cemented later on by minerals precipitated from ground water which percolates through the ash, so in this case, this example is particularly interesting because you can see here that each of these fragments like this fragment, for example, is a fragment of a pre-existing rock, but if you look at this very closely, you'll see that this itself is made up of pieces of ash, so what you've got here is pieces of former ash imbedded in pieces of new ash.
As you rotate the thing around here, as I rotate it around, I think you can see that there are pieces of darker material as opposed to pieces of lighter material. Each one of these represents different layers of ash that formed part of the episodes that built the volcanic edifice that we now call "Diamond Head". Well, the Diamond Head tuff shows quite a number of different types of ash all imbedded together, so it's really a very complicated mixture, then, of various types of material, in this case representing the material from several different volcanic eruptions. Okay, the pyroclastic material, the tephra, is very sticky, for one thing, but it may cover very large areas after a major volcanic eruption.
The Hawaiian-type eruptions that we have here in the Hawaiian Islands are much less violent than continental types, so the tephra doesn't spread quite as far as these continental-type eruptions like Mt. St. Helens, which are quite a bit more violent; in fact, ash from the 1980 eruption of Mt. St. Helens was found hundreds of miles away and formed a fairly thick coating on the streets of Portland, which was nearly a hundred miles away from the volcano. In many cases, this volcanic dust and ash may be blown way up into the stratosphere, which begins 40,000 feet or so into the atmosphere and may circulate around the planet several times, and you may recall that Mt. Pinatubo, which erupted in June of 1991, the ash from that eruption circulated through the atmosphere caused brilliant sunsets and sunrises, which we're still feeling the effects of.
Not only that, but this ash obscured the sky through the July, 1991 eclipse, which was so popular here in the Hawaiian Islands. This ash circulating in the atmosphere may also block sunlight and may actually cause measurable cooling worldwide for several years afterwards. The other thing about the ash is that if it's very widespread, it can also help us to establish relative dates over fairly large areas sometimes even worldwide if these ashes are recognized as layers in ancient rock deposits. You may want to refer back to Chapter 8 on "Geologic Time" to get a sense of how we correlate these things. Sometimes the tephra may form what's called a "nuee' ardente", which is a French word that basically means "glowing cloud" or "glowing avalanche".
A particular violent explosion may involve so much energy that it pulverizes all the lava into very fine grained material, very fine grained ash, and this hot ash, then, as it settles back on the surface of the volcano may flow downslope, but it flows aided by expanding gases. You may remember that the continental violent type volcanoes have a much higher amount of gas than the basaltic volcanoes so imagine these layers of ash with gases being released expanding forming this lubricating layer very much like the air holds up the pucks on an air hockey table. The result of this is that the hot ash flows downslope with this expanding gas acting as a lubricant sort of like an emulsion of solid and gas. These clouds are at very high temperatures as much as 800 or 900 degrees Celsius and may reach high speeds; in fact, at Mt. Pele on the Island of Martinique in the Caribbean in 1902, an eruption reached 160 kilometers per hour. This particular eruption destroyed a village killing all but two of the 30,000 residents of the Town of St. Pierre. When these types of nuee' ardentes or glowing clouds stop rolling, they leave a particular type of deposit called "welded tuff".
Again, the word "tuff", but in this case, it's "welded" by the fact that the ash is still hot enough to stick together once it comes to rest on its own, and there are many examples on the north island of New Zealand, for example, of this welded tuff which is also sometimes called "ignimbrite".
Okay, another effect of the volcanic eruptions involving tephra are mudflows. Volcanic mudflows are very common in wet regions and in snowy regions. The water from melting snow, ice, rivers, and lakes, and so on can mix with the tephra forming a very thick slurry of mud, which can cause much destruction; in fact, much of the damage from the Mt. St. Helen's eruption in 1980 was, in fact, due to the mud flows and not due to the force of the explosion itself. We have several ancient examples of mud flows found here in the Hawaiian Islands.
So tefra cones or tefra features specifically cinder and spatter cones are common features along the rift zone of Hawaiian volcanoes. Both cinder and spatter cones are formed by similar processes depending basically on how high the lava is thrown into the air by the force of the eruption and whether the material is liquid or solid, or more specifically, how liquid or solid it is upon hitting the ground. The cinder cones themselves may be large or they may be small. Some, such as Puuo'o, which is the current site of the Kiluaea eruption, is more than 800 feet in height. Cinder cones of this size are common in Hawaii as are smaller cinder cones. Cinder is irregular spongy fragments of lava, which is thrown out by fountainy eruptions like this one. The fragments are mostly solid before striking the ground, but lava which flows close to the vent and is welded together is called "spatter" if it's still liquid and pasty when it strikes the ground. Cinder cones of various sizes like these in Haleakala are common features everywhere in Hawaii, and if the wind is blowing during the eruption, the resulting cone may have horns like a sand dune, which indicate the wind direction. Older cones may be covered or partially covered by later lava flows like these along the northeast rift of Mauna Loa. Note the newer blacker lava in contrast to the older or oxidized cones.
"Pumice" is an especially frothy or spongly lava which is quite fragile. It breaks apart quite easily in the hands and also weathers away quickly in humid climates like we have in most parts of Hawaii. Because pumice is so light, it may be blown great distances by the wind covering large areas and may survive for a long time where the climate is dry, such as here between 11,000 and 12,000 feet on Mauna Loa. The pumice in this pumice field was erupted in the middle of the Nineteenth Century nearly 150 years ago. Spatter cones and ramparts may build up slowly along the eruption rift and like cinder cones may be the source of lava flows like this one which issued from the base of a small spatter cone. Most of the Hawaiian volcanoes develop a caldera early in the shield building stage. A "caldera" is a large feature, usually more than a kilometer in diameter, which forms by slow successive collapse of the top of the volcano when magma is withdrawn from the magma chamber below. Actually when magma is withdrawn faster than it can be replenished during periods of high volcanic activity.
By contrast, a caldera formation in continental-type volcanoes may be rapid and very explosive; for example, in Crater Lake in Oregon is the top of an ancient mountain called "Mt. Mazama", which is actually the caldera in Mt. Mezama, which was formed by the explosion of the top of the volcano similar to the way that mount Krakatoa in Indonesia exploded in 1883. By contrast, a crater is a smaller feature, usually found within the caldera, or along the rift zone, or the top of a cinder cone, and, of course, many craters actually mark the site of volcanic eruptions. Halemaumau Crater on Kilauea, for example, is the result of a crust of lava which subsides when the eruption has ceased, and Halemaumau sometimes contains a molten lava lake. Pit craters, on the other hand, are formed along the rift zone due to the collapse of overlying material much in the same way that a caldera is formed except that it lies along the rift zone, is at the summit, and, in fact, is smaller in size.
Another distinction we have to make here between crater and volcano. The crater is not the volcano. We often refer to Diamond Head Crater when what we really mean is Diamond Head Volcano. On Mauna Loa, the summit caldera is called "Mokuaweoweo". Here we see it in a high altitude air photo, and it lies at the intersection of three rift zones at the summit of Mauna Loa. It's more than three miles long and more than a mile across and contains several cinder cones and numerous or uncountable even lava flows. It is bounded on two sides by cliffs up to 600 feet tall and which were exposed many layers of lava flows, which represent the shield building lavas. The caldera may fill with lava and reform many times during the life of a volcano. In fact, some of the Hawaiian volcanoes, such as Hualalai on Mauna Kea on the Big Island of Hawaii show no evidence of a caldera at all, and it's not clear whether one ever formed or whether it was filled by later eruptions. Across the caldera is the summit of Mauna Loa more than 13,600 feet. As we stand here on the edge of the caldera, we're about 400 feet lower. Cliffs of the caldera on the other side are about feet tall; actually more like 600 feet tall. And the cliffs on this side are about 250 feet.
The calderas of Hawaiian volcanoes did not form in one large catastrophic event but rather in a series of small movements like a graben. A series of cracks parallel to the caldera wall represent faults along which the downward movement takes place. The crack that runs along the edge of the caldera is one way that the caldera is formed. When material is drawn out from below as the magma chamber of the volcano becomes depleted, the caldera collapses, but as it collapses it also slides inward, and as it slides inward it creates stress cracks back from the wall. You can see these cracks. One here. One here. And so on parallel to the caldera wall all along. These cracks are really fault traces. They're formed when chunks of the caldera edge move downward as the magma is withdrawn from the magma chamber deep within the volcano.
Huge blocks of the mountain top stand poised on the edge of the caldera waiting for an earthquake or a volcanic eruption to shake them loose. Here's a large chunk lying nearly intact on the caldera floor shaken loose at some time in the indecipherable past. What an event it must have been as this broke loose and tumbled down the cliffs. Kilauea, Mauna Loa's younger sister, has a well developed sequence of step faults which bound the caldera.
The Halemaumau Crater, which is the site of the most of the summit eruptions of Kilauea, lies within the caldera as seen in this photograph. In this air photo of the northeastern part of Makuoweoweo, a series of cracks are clearly visible just above and to the right of the circular crater known as Luakoholo. Luakoholo represents another feature associated with rift zones, which are called pit craters. Luakoholo is more than 200 meters in diameter and more than 100 meters deep. Pit craters like this one are formed like the caldera by collapse of the rock overlying the rift zone, but pit craters differ from calderas in that they're smaller and do not necessarily occur at the summit of the volcano. Pit craters are especially common along the east rift of Kilauea.
This photograph shows Makaopuhe Crater in the foreground with several small craters in the distance. Napua Crater also on Kilauea's east rift is transected by faults and fissures of the rift zone, including a small graben seen here in the upper right of the photograph. Several smaller pit craters, linear rifts, and cinder and spatter cones are also visible.
Kilauea Iki is a small pit crater, which was partially filled by the 1959 eruption of Kiluaea Iki, which featured lava fountains up to 400 meters high. Today the lava which fills the crater is still cooling and is being extensively studied to learn what happens to it as it cools. You might want to note in that sequence that the air is very cold and very thin up at the top of Mauna Loa nearly 14,000 feet above sea level even here in Hawaii, so the very slow pace is largely due to the fact that there's not much oxygen up there to breathe. Well, let's turn our attention now to volcanic rocks. Volcanic rocks, as opposed to plutonic rocks, which we'll study in the next lesson, form from lava.
Plutonic rocks form from magma which cools underground and recall the distinction between lava and magma. Molten rock is called "magma" before it erupts; it's called "lava" once it breaks the surface and leaves the ground. So the term "lava" refers both to the liquid that comes out of the volcano, as well as the solid rock, which forms when it cools. Volcanic rocks formed from lava are fine grained meaning that the lava cooled too quickly for large crystals to form.
There are basically three types of textures found in volcanic rocks, which represent the cooling history of the lava. I have some examples here. The basic fine-grained texture is call "aphanitic" or "fine grained". That's this one. "Fine grained" means, in this case, that you can tell that there's a graininess to the rock, but the individual crystals are much too small to make out exactly what they are. As I move this around, you may occasionally catch a flash from a small piece of feldspar, but you couldn't tell it was feldspar if you didn't already know. On the other hand, if the rock cools very quickly, it may take on a glassy texture like this. This is actually a piece of "obsideonian", which is the glassy variety of volcanic rocks, which cools so quickly that it doesn't have time at all for any grains at all to form. The third type is what we call "porphyritic" texture. some of this may be a review, but the porphyritic texture is a rock which contains two distinct sizes of grains. In this case, see the white elongated grains of feldspar imbedded in the finer grained dark matrix of basalt.
The reason for this, of course, is that the faster the cooling, the less time the crystals have to form, and we will study this in the next lesson on intrusive igneous rocks. Okay, many volcanic rocks contain vesicles. "Vesicles" are simply holes which form due to the expansion of gas bubbles as the lava cools. I have an example here, I think, that illustrates this very well. Here's a piece of dense basalt, dense meaning that it's very fine grained; it's not quite fine grained enough to be glassy, but "dense" meaning that it has very few open spaces or holes in it. The sparkles that you may see here are little pieces of olivine phenocrysts but notice that the rock is incredibly dense. On the other hand, here's a piece of vesicular basalt. The vesicles are the pukas or the holes; in this case, they're quite rounded. You can see that the rock sort of has the texture of a sponge.
The presence of the absence of these vesicles depends upon several factors. Number 1, it depends upon the depth at which the magma or the lava cools. It depends on whether it erupts under water or under air, and it also depends on the amount of gas that's still in the lava at the time this particular rock cooled. Okay, we'll come back and look at vesicles again in a couple of minutes.
Just to refresh your memory, lava basically comes in three main compositions:
Okay, I won't go into the details of this because we'll study the classifications of the igenous rocks in some detail in the next lesson, but basically basaltic lava is rich in ferromagnesian minerals, poor in silica. It contains mostly pyroxene and calcium rich feldspars for the minerals.
The andesitic or intermediate lava is intermediate in silica and intermediate in ferromagnesian minerals. Here, again, the sodium rich feldspars represent the majority of minerals with a small amount of ferromagnesian.
The rhyolitic magmas are very rich in silica and poor in ferromagnesiums. Here the main minerals are potassium, feldspar and quartz. In these ferromagnesian minerals such as biotite, and amphibole, and so forth are quite rare. We will study the classification of these types in the next program along with the plutonic rocks.
Most of the Hawaiian lavas are basaltic in composition, specifically a tholeiite basalt although some are more alkalic meaning they contain higher amounts of sodium and potassium. The differences between the tholeiite and the alkalic basalt aren't present in the hand specimen but show up clearly when chemical analyses are compared and shown on a graph.
The lava itself, at least the surfaces of basaltic lava itself, commonly have one of two contrasting forms. These names are really broad classifications, and there are many variations of surface textures within these classifications, but it's easy to recognize most of the time. Hawaiian names for these two basic textures are used all over the world.
The first type of these is called "pahoehoe". "Pahoehoe" is a smooth or ropey surface lava. Here you see a ropey or billowy surface. Notice that it's relatively smooth; if you look at it from the side, you can see some of the vesicles inside with a smooth surface. Another piece here shows glassy on the outside, and if you look at it in cross-section, you see that the glassiness extends a little bit into the sample, but near the center it's much less glassy, and, in fact, takes on an appearance of a more vesicular or fine-grained rock. Okay, one other example of pahoehoe. Here's a somewhat smoother surface. Notice again you see the glassy surface on the outside. Okay, pahoehoe is the more primitive of the two forms of lava. By "primitive" I mean that usually pahoehoe is erupted from the vent, and if any change occurs, pahoehoe changes to the other type of lava called "aa" rather than the other way around; that is, pahoehoe may change to aa, but we never see aa changing to pahoehoe.
Aa is often defined as "rough", "spiny", "rubbly" or "clinkery". Here we see a piece of aa and notice the rough, twisted surface as I rotate this around. These clinkers form when the lava at the surface is twisted and sheared during the flow, and we'll see in some of the sequences coming up some of the examples of the flow of the aa. Notice that the top part of it is a little bit smoother, but it still has much different appearance than the "pahoehoe".
Now, it's sometimes difficult to classify a particular piece of lava as either one or the other. As the pahoehoe changes to aa, it may take on the characteristics of some, but the essential difference between pahoehoe and aa is the viscosity and gas content of the lava. Aa may erupt from the vent, in some cases, if there's a lot of stirring of the lava in the vent, so it allows the gases to escape and become cooler, and pahoehoe has been known to change to aa as it falls over a slope or over a cliff. The reverse change, as I mentioned earlier, never occurs. There appears to be no consistent difference in composition; the difference appears to be caused simply by the viscosity. The vesicle shape can identify the difference between pahoehoe and aa most of the time, and I show you some examples here.
Okay, here, for example, is the rock I showed you earlier with the vesicles. Notice how rounded the vesicles are if we can get in a little close on this one. The vesicles are somewhat rounded; they're rounded because the lava is not flowing or is flowing very slowly as it cools. On the other hand, here's a piece of aa. Notice here that not only is the piece itself much more spiny and clinkery looking, but the holes in it are not round. It looks as if you've taken round holes and squeezed them and misshapen them. This happens because of the way the aa flows, and as this piece of clinker was dragged along the surface and bent and twisted by the shearing forces. Okay, it's interesting to note here that andesite flows are most often aa because the lava is cooler and more viscous; whereas, basaltic flows all tend to be more often pahoehoe. Rhyolite, the lighter form of the lava, generally is neither because it's usually deposited in ignimbrite; that is, eroded tuff due to the nuee' ardentes or in lava domes where the lava simply squeezes out of the ground like toothpaste being squeezed out of a tube. The fluid pahoehoe lava may travel long distances; in fact, it may flow through lava tubes to emerge must further down slope. In open channels, it may flow as much as 55 kilometers per hour although the rates are typically slower.
In the 1950 flow of Mauna Loa, for example, the pahoehoe was clocked at about nine kilometers per hour. The flow advances more slowly than this as a whole. The flow front expands outward to cover a mile or more, and this front spreads commonly tens to hundreds of meters per hour. This is compared to continental-type flows of more viscous lava which flow meters or tens of meters per day. Tons of lava may spread outward at the front of the flow and may flow into valleys and older material or over existing vegetation causing bursts of flame as hot gases from the vegetation ignite when exposed to oxygen in the air, or they may flow over black sand formed when earlier lava contacted water to form a black sand beach from glassy tephra. Here we see lava dripping out of a small lava tube into the sea. Floating on the water are small fragments of frothy pumice and ash, which were broken by contact with the cold water. This floating debris may later wash onto the shore to form a new black sand beach.
At the front of the flow, the lava forces its way through the thin, cool crust to form toes of lava. These may move quite slowly as blobs of lava are extruded from a newly formed crust. Sometimes the lava channel divides to pass hills and reunites on the downhill side to form a feature called a kipuka. The surface of a kipuka may actually be lower than the solidified lava after the lava cools. Well, after the lava cools, the flow structures are preserved, and the method of flow can be clearly seen.
In the center is a lava channel, which represents the main part of the active flow and the fastest flow rate. Outside the main channel the lava is twisted with ropey and billowy textures from the different rates of flow between the center and the edges. At the edges of the channel there may be a spatter rampart formed as the degassing lava bubbles and throws liquids into the edges where it cools. This is similar to the way a river builds a natural levy which is higher than its surrounding flood plain. A ropey surface of pahoehoe is formed by dragging and wrinkling the partly solid crust. The ropes may be curved to indicate the direction of flow because it flows fastest in the center. Although this may be only a local direction of flow and doesn't necessarily reflect the overall direction the flow is moving. Ripples of various sizes may also form as the thin crust is dragged along with the flow. All of these structures are preserved when the lava cools as we see here in this old pahoehoe flow along the northeast rift of Mauna Loa at about 8,000 feet elevation. Billows may also be formed as the lava flows down a small incline, and like water flowing in a stream, the lava may flow around objects in its path like this older lava bomb. The glove in this picture is for scale.
If the surface cools rapidly, perhaps in a heavy rain, the surface may be quite glassy as we see in this small flow issuing from a spatter cone on Mauna Loa's northeast rift. In some cases, there may be large bubble blisters near the vent. These are called tumuli. They're large dome-shaped hillocks. They usually form in the crater from buckling like a wrinkled tablecloth that may be aided by the hydrostatic pressure or the liquid pressure of the magma. These tumuli are often cracked, and there may be dribbles from cracks, and sometimes the tumuli themselves grade into pressure ridges. As the lava cools and degases it becomes more viscous and gradually transforms into the pasty clinkery variety known as aa. Note the blocky surface on this flow, which is transitional from pahoehoe to aa.
Aa like pahoehoe is fed by open rivers near the center of the flow which spread both laterally and downhill, but the flow is generally more downhill than sideways. The aa flows much more slowly than pahoehoe due to the higher viscosity and pasty texture; in fact, the flow of aa is very much like a tractor tread, and accretionary lava balls, very much like snow balls, are common. This three-layer structure is visible in road cuts through old flows. The clinkery surface on the top and the bottom encloses a massive plastic center. The clinker surface itself provides an important channel for ground water movement when the lava solidifies. This rough, clinkery surface of aa is easily recognizable when it solidifes. It's difficult to walk on, and the flows may be up to 40 feet thick. This flow is about 15 feet thick and is typical of aa flows in the Hawaiian Islands. The surface texture of aa although loose and broken may remain for long periods after the lava solidifies. Since the surface clinker is very porous and very rough, rainwater tends to percolate through the rock rather than flowing over its surface, thus hindering the development of streams as the volcano ages.
I hope these sequences we put together for this program have helped you to understand how the volcanoes are built, and also I hope it helps you to recognize some of these volcanic features that you'll see on your trips, not only around the Hawaiian Islands, but also around the world because volcanoes around the world do share these features in common. We worked pretty hard to put these sequences together for you.
I recognize that some of them are not the best photography, but, you know, it's hard to carry those cameras up there to the top of Mauna Loa; there aren't any highways to drive your car up there.
Well, next time we'll be studying Chapter 10, which is intrusive activity and the origin of igneous rocks. Yes, Chapter 10 next week is going to be a little bit out of sequence here because that's the way the "Earth Revealed" video decided to do it, so in Chapter 10
It provides a good model to help us understand the cooling of igneous rocks and the origin of magma. Well, I hope this program has given you the chance to see and appreciate the variety and beauty of these various volcanic features, and when you see something whether it's here in Hawaii or anywhere else, flying over and driving by it, look at it, try to picture what we've studied in volcanoes and ask yourself,
"How does that feature form? What stage of volcanic development is this particular feature in?
So keep that in mind. Keep your eyes open, and I'll see you next time.