March 11, 2017 by Rokman61
The Geologic setting of Hawaii
The Hawaiian Island chain comprises a series of volcanoes, which are essentially stupendous piles of volcanic rock. Hawaiian volcanoes are very unlike, and unrelated to, the ones surrounding the Pacific Rim. Those on the Islands are very broad, gently-sloped dome-like mountains referred to as shield volcanoes. By contrast, the Pacific Rim composite volcanoes are steeper-sided cones like the classic Mount Fuji and Mount Baker (which we can see from our home yard). The latter are not just steeper, they behave more erratically and can be downright nasty.
The source of the magmas that build the Hawaiian volcanoes is very deep in the Earth’s mantle – 100s of kilometres down. The Islands are carried on the floor of the Pacific Ocean, which is part of the tectonic Pacific Plate, which is slowly moving northwest with respect to the relatively fixed position of the magma source deep in the mantle below. Consequently, the volcanic activity is progressively younger towards the southeast, so that Hawaii itself is the youngest island in the chain. Mauna Loa comprises much of the island and has been active in historic times. Kilauea is a separate volcano on the south flank of Mauna Loa, and it is the one that is presently erupting. In fact it has been erupting continuously now for nearly 35 years – the longest sustained volcanic activity of modern times. And THAT was one of the main things we came to see. There are other areas of active or recent volcanism (Galapagos, Aleutians, Southeast Asia), but none are so accessible as Kilauea.
I first heard about Hawaiian volcanoes in an introductory geology course when I went to university in 1957. In subsequent years I learned much more about them, and eventually taught students about them myself, but had never actually seen one until I visited briefly – 40 years after that first course. My visit (August 1997) was to attend a teaching conference. We were taken by bus into the field to view the volcanic features; it was a most exciting event but also a frustrating tease in that we were only able to spend so little time there. In 1997 there was no activity at the summit, and the erupting P’u O’o vent was too remote to visit. Flows from that vent have been pouring into the sea continuosly since the 1980s, except for a few months in ’97 during our visit! So, nearly 60 years after that intro geology course, I still had not seen a real live lava. Obviously, a return to the scene was desired.
First a bit of lexicon to prep the way, and then we can proceed.
* magma is molten rock containing dissolved volatiles, mostly water and carbon dioxide;
* lava is magma that is extruded at the surface, where it quickly loses volatiles;
* lava flow refers to magma moving over land surface ;
* basalt is the dark volcanic rock formed when lava crystallizes to a solid;
* so a basalt flow is a solidified mass of dark lava;
* pahoehoe describes the smooth ropy surface seen on many basalt flows.
Lava meets the sea
The source of the lava comes from P’u O’o, a small cone on the flank of Kilauea, which is fairly remote and not near any road access. Most often, this lava quickly disappears into a tunnel or lava tube, formed when the surface of a flow hardens and roofs over, and the interior continues to move through. Eventually it pours out into the ocean, with dramatic effect. The lava builds up a large delta-like deposit of basalt rock which ultimately becomes unstable and collapses into the sea. At the end of 2016 a major collapse resulted in a vertical cliff along the shore, with the opening of the lava tube exposed to view. To observe the lava one must walk to the viewing site (about 8 miles or 13 km return), or rent a bicycle from concessions at the ‘official’ end of the road. Fortunately we found a way to cut the walking to about 4 km.
The ‘trail’ is a rough roadbed cut to provide emergency access from the area. This road was built because the P’u O’o lavas occasionally spill out in new directions, and in 2014 a flow nearly cut off a major highway on the east side of the area. Numerous estates and housing tracts are accessed only by this highway, and invading flows could completely block access for those residents.
The roadway passes over flows from the 1980s – the famous ones that ate the town of Kalapana. Walking along the road is like traversing an endless expanse of bare rock – that’s all that one can see in all directions.
The details of shapes and textures in the crusted lava are a wonder to behold. I just couldn’t stop marvelling over them.
The surface of flowing lava quickly forms a thin skin which is solid but still very supple and easily deformed. The fluid lava underneath keeps moving, causing the skin to wrinkle into the ropy forms seen here. The Hawaiians refer to lavas with these ropy shapes as ‘pahoehoe’.
If you are wondering: no, these are not black-and-white photos; they are normal full-colour images.
When lavas inundated the town of Kalapana, private lots were converted into a sea of stone as illustrated in the preceding photos. Those lots are still private property, and when the emergency access route was established, a few of the owners have attempted to reclaim at least some of their ‘life on the land’. Some are little more than modest shacks. The red ‘soil’ is basaltic cinder trucked in from another location. Yard maintenance would appear to require minimal effort.
The neighbouring lot is a little less spartan – gotta love those palm trees!
As one looks toward the area where the lava is moving to the sea, the track of the subterranean lava tube is visible looming in the distance.
Rainwater heated by the hot lava below gives rise to steam which condenses to mist.
Eventually we arrived at the viewing point.
At first there were just billowing clouds of mist. Quite an impressive view it was, but no magma was visible. Occasional explosions erupted from the water, sending bursts of rocky debris to heights well above the top of the cliffs. Informational signs told observers not to wander over to the cliff edges.
After a short wait the show really got going. The lava poured from the tube on the cliff face like water from a fire hydrant. Most of the time the billowing steam and clouds of rocky shrapnel concealed the lava. Here is my best peekaboo shot of the molten stream.
I tried showing some variation with this montage. As the flow ebbs (left to right), bits of lava separates from the stream which gradually breaks up as the volume drops off. (Click for larger view).
Better still: although the wind was blowing, the rain threatening, and without a tripod, David managed to get some video footage. CLICK HERE to watch.
(WARNING! contains scenes of uncontrolled violence).
The Hawaiian volcanoes get so massive that they can become unstable because of their own weight, and hence prone to partial collapses on their flanks. This can happen as a catastrophic landslide or as a very slow-moving slump, an example of which can be observed on the lower southeast slopes of Kilauea. The process is marked by several long steep escarpments running approximately parallel to the coast (the Chain of Craters Road drops over one of these slumps). These scarps are known to be slipping a few centimetres per year. If the slump were to ‘let go’ and become a fast-moving landslide, the displacement of ocean water would result in a very destructive tsunami, examples of which are well known from the Hawaiian geologic record.
In this view, recent lavas from the Pu’u O’o vent can be seen draped over the steep face of the escarpment. That must have been quite something to see!
Black sand beaches
These beautiful beaches are small, local, and geologically ephemeral. I got to wondering why, and after walking over all that pahoehoe I think I got it figured out.
Sands on Hawaii can only be derived from two sources, namely coral (which would be very pale) and volcanic rock. The basalt rocks are dark, but rarely are they actually black, as shown in this photo of an assortment of beach cobbles.
The reason for the less-than-black colour is because the crystallized basalt is composed of about equal amounts of pyroxene (a blackish iron-magnesium silicate mineral) and plagioclase feldspar (a sodium-calcium-aluminum silicate mineral which can range from very light to dark in colour). The light-coloured felspars are readily visible under high magnification. This should explain why basalt itself does not produce really black sand.
But the back sand beaches are truly ‘jet black’, so how is that explained?
Note the shiny surface of some of the lava lobes in this photo. These are trickles of lava that leaked out from the interior of a cooling flow. The skin on these lobes congealed so quickly that the minerals did not get time to crystallize, and the result is a basalt glass. Such glass appears truly black in colour; it looks very light in these photos only because it is so reflective.
Unlike the tough nature of crystalline basalt, the glass is very weak and friable; it can easily be rubbed off the surface of a flow. In this photo you see how thin glassy skin is virtually flaking off the surfaces of the lobes. (Copious amounts of basalt glass are also produced when the lava flows directly into sea water as shown previously).
The sands in the blackest beaches appear to be almost entirely composed of basalt glass. They are noticeably lighter in weight, and because the grains have angular (instead of rounded) edges the sand has a distinctive feel. Such neat stuff!
This macro photo shows the angular shapes of the particles.
Now we can understand why the black sand beaches are relatively uncommon and not long-lasting. Every beach requires a constant supply of new sand. A local source of glass sand from the basalts is likely to be quickly removed by erosion, and once on the beach the mechanically fragile sand is ground up and consumed by wave action.
It is a classic shield volcano, and is the largest single pile of volcanic rocks on Earth. Here is what it looks like as viewed from the north side. Note the extremely gentle profile.
A very large depression marks the summit, visible on the skyline just left of centre. This is a cauldera, formed when the top collapses after large volumes of lava have been extruded. Some young lava flows show as dark streaks on the flanks. And yes, that white stuff is snow – at elevations around 4000 m (13000′) that happens, even in the tropics. Access to the higher levels on Mauna Loa is by primitive 4×4 roads. Most who go there are government agents, researchers, or outdoor adventurers. We didn’t qualify as any of those.
The most recent flows on Mauna Loa occurred about 35 years ago, and the volcano is currently resting and likely near the end of its shield-building phase. All the recent action is happening at Kilaea, a younger volcano on the south slope of Mauna Loa.
Topographically, Kilauea is barely noticeable as a slight bump on the side of Mauna Loa, but it has its own separate magmatic plumbing system and is considerably more active at present.
In this Google Earth view of the summit area you see the very large summit caldera – the large ovoid patch in the centre. Two smaller ‘pit craters’ are also visible: In the southwest corner of the floor of the summit caldera is the currently erupting Halemaumau Crater – (more on this one later) and the small oval to the right of the main caldera is Kilauea Iki Crater – the subject of our next adventure.
In 1959 a spectacular series of eruptions partly filled the Iki crater with molten lava, resulting in a ‘lake’ about 120 m (400′) feet deep. After each eruption a solid crust formed on the surface of the lake, and eventually the mass of lava had completely solidified – which took about 30 years. When I visited in 1997, I noticed that people walking on the floor of the crater . . . they were walking across a lava lake! That was something I wanted to do, and my opportunity finally came nearly 20 years later.
A Walk on a Lava Lake
This photo is looking down about 120 m (400′) onto the lava lake from the rim of Iki crater. The view is westward, with rising mist from the active Halema’uma’u Crater visible in middle background. Our route went from right to left along the visible ‘beaten path’.
Looking eastward towards the other end of the trail. From above it appears almost as smooth as a billiard table.
The trail winds its way down through pristine native forest before reaching the barren lava lake bed. The most conspicuous large trees are the Ohi’o, as mentioned in Part 1.
Several varieties of ferns are also abundant here. These giant ferns provide a somewhat ‘Jurassic Park’ feeling to the scene.
Note the ‘fiddleheads’ emerging from the base of this tree-size fern. The edible fiddleheads you may be familiar with would be about the size of your thumbnail. These would more than cover your entire hand.
A fiddlehead of a very different fern.
As the trail enters the floor of the crater, it passes through some rather jumbled and chaotic rock, likely because it is close to the source of the lavas that filled the lake (in right edge of image and second photo below).
This mound, with the tongue-twister name Pu’u u Pua’, marks the source of the 1959 irruptions. Intermittent episodes of fountaining, explosive cinder ejection, and lava flows built up large cones like the very complex structure visible here. As the cones built up they became unstable and repeatedly collapsed into the lava lake, accounting for the mangled lava lake crust in the photo above. The huge volume of lavas that filled the lake did not come in a single pulse; instead the lake would partially fill, drain back, and refill a number of times.
The view below shows the relatively smoother eastern ‘shore’ of the lake: note the remnant of crust formed along the margin when the lava was at a higher level (analogous to a ‘raised beach’ on a ‘lake’ lake).
When 400 feet of lava crystallizes and cools, there has to be some shrinkage. One result is the rather impressive foundering of the top surface.
The rocks below contain pockets that are still hot. Rainwater percolating down through fractures gets converted to steam which condense to mist on contact with the air. This was the biggest such vent we observed. Note the colours in the rocks, indicating chemical alteration by the hot vapours.
The white patches mark bleaching around the vapour vents.
Somewhere about in the middle background of this view, the US Geological Survey drilled holes into the lava crust as it formed, to observe and study how the solidification proceeded.
One of the fascinating things to observe was how the plants struggle to get started in the barren lava.
As expected, they start along cracks where they can gain some foothold.
The Ohi’o tree is one of the first colonizers.
The native Hawaiians held this plant in highest esteem; it provided fine hard wood, and the flowers were prized for making their famous leis.
This picturesque ‘Amah’u fern was also able to get a nice start.
This one was a surprise. It’s Vaccinium reticulatum, which is very closely related to our blueberry (or huckleberry if you prefer). Such a pretty little shrub, but admittedly it is probably the least tasty vaccinium I have ever sampled.
Bottom line: this little hike was like no other I have ever experienced! Highly recommended!
Another lava lake
In 1997 we were driven all the way around the Kilauea summit caldera and there was nothing volcanic happening other than a few sulphurous vents along the walls of Halemaumau, a pit crater on the caldera floor. In 2008 new eruptions began to pour lavas into the pit crater, creating a new lava lake similar to the one at Iki. The activity continues to present, so that too was something we had to see.
Most people go to view it at night. We were lucky in that the activity had picked up a few days before we arrived, and was putting on an impressive show even in daylight. Here is how it looked just before dusk.
David got out my tripod and shot some video . . . CLICK HERE to view.
I don’t often show people in my blog, especially myself. However, this one was too good to pass up. We were poking around in the Jaggar Volcano Museum at the Halemaumau viewing site. The charming woman in the photo here noticed that David was wearing a T-shirt extolling the virtues of geology, and she made an approving comment. I asked the lady how and why this caught her attention. It turned out that she was a graduate geologist, and she had attended the same school I had, namely the University of Washington. Moreover, she had studied under the same professors that I remembered, all long since gone. How is that for a coincidence!
To the top!
Mauna Kea, at 10,000 m (33,000 ft) height, is said to be the tallest mountain in the world – when measured from its base on the ocean floor to it’s summit at 4,207 m (13,802 ft) elevation.
Hawaiian volcanoes pass through several evolutionary stages in their history. Most of the activity involves construction of a massive shield through a long succession of basaltic flows. Near the end of it’s ‘life’ the volcano changes its behaviour, producing smaller eruptions of slightly different composition and considerably more explosive in nature. Isolated cones result where scattered vents throw out considerable amounts of solid particles (collectively referred to as tephra) in addition to lava flows. Kea has evolved through this stage, and in this Google view of the summit area you see the multiple cones which appear as pimples on the landscape.
This was something else we decided we should check out.
The view below shows the start of the road up the volcano, as seen from the Saddle Road. The dark areas are young basalt flows from Mauna Loa, which overlap and bury the southern flank of Kea.
Part way up the mountain there is a visitors’ centre, and here we encountered a potential impediment. It seems that the Park Service does not encourage people to drive to the top. There is a section of unpaved road, and we were explicitly told that ordinary passenger cars could not proceed further. It was however also mentioned that it is not illegal to drive up the mountain.
Beyond the ‘no-go’ zone we encountered some roadcut exposures of layered tephra deposits. This is particulate material blown out of vents that accumulate in layers like ‘normal’ sedimentary rocks. You can seen that the layers here are gently bent or ‘folded’; I presume this has resulted from a bit of slumping of the layers down the slope.
In another tephra exposure you can see that the top left edges of the layers are truncated or cut off and overlain by a thin layer of tan-coloured, coarser tephra. This is what geologists call an unconformity; it indicates that a period of erosion occurred after the lower layers were deposited, and before the top layer was added.
Features like the ones we observe in the tephra show that volcanic activity is anything but simple, and usually involves a very complex history.
Views from the mountain are awesome. In this panorama you can see the visitor centre in the centre, several reddish satellite cones on the right, a bit of the gravel roadbed on the left, and Mauna Loa in the distance.
Another view looking back to the south, over the flank of Mauna Loa, with mist from the Pu’u O’o vent visible near the middle of the photo. Snow covers the sheltered north slope in the foreground.
As one approaches the top there are excellent views of more of the prominent cones.
It would be difficult to imagine a more barren landscape. The almost total lack of vegetation is due to high elevation and loose porous ‘soil’ that doesn’t hold water. Maybe wind is a factor too – even the signs require modification to deal with that.
At the summit, astronomical observatories come into view, about a dozen of them, including some of the world’s largest. They are here because the combination of high altitude, near-equatorial latitude, accessibility, and a friendly host country provides for advantageous viewing conditions.
Some of the observatories are picturesquely nestled amongst the scattered summit cones. There is no large caldera on top of Kea, as any evidence of one has long been eroded away.
Researchers from different nations are involved here, even some Canadians. This unit is the Canada-France-Hawaii 3.6m Telescope. That’s our ride up the hill parked in front.
One more sunset over a beach.
Having conquered the volcanoes, we had one last night in Hawaii. One might say “Here today, gone to Maui” . . . . (Part 3).