Judy's Online Journal

<< Previous

Next >>

Nov. 2, 2008 by Judy

Lava Luv. A primer, with loads of thanks to USGS.

As I'm sure you well know, the planet is a hot ball of boiling iron with a crunchy thin crust on the outside. The crust is made of plates that float around on the surface of the mantle, which is kind of a semi-gooey layer between the hot boiling iron and the thin crust. Most of the world's volcanoes form as 'pressure vents' around the edges of the plates.

The Pacific Plate is one huge plate, volcanoes are all around it, and we are in the middle of it. We are also volcanoes, but we are different. Instead of being pressure vents, and very explosive and volatile, we are made by that superheated boiling iron, way underneath the crust, boiling up and through in a big plume, right through the mantle layer, and glopping out onto the top of the plate/crust. This happens at a depth of 20K ft. It takes about a million years for enough hot, thin layers of lava to pile up enough to emerge from the sea.

The plate is scooting over the hot spot (top of the boiling plume) about 10 cm. a year and it's headed for Asia, in a NW direction. When it meets the Eurasian plate it is forced under and melts. Lots of volcanoes dot that meeting edge line.

We spend a couple of million years building, then are carried off the hot spot plume enough to go extinct. The plume, still not very well understood, periodically takes a break. For that reason we have clumps of volcanoes-

The Big Island has 5 above water and one young one building below

We have 7 in the Maui Nui clump

Oahu is made of 2 - Ko'olau and Wai'anae

And Kauai/Niihau had at least 2

- instead of a long ridge line.

Nobody's sure how the flow of lava upward works, how many channels there are, and how they connect into the bodies of the volcanoes as they build. Geologists used to think that Kilauea was a side vent of Mauna Loa, but now know she's her own volcano and just merging onto Mauna Loa's flanks.

Haleakala started oozing onto the ocean floor about 2 million years ago.

About a million years ago, her head came above water.

Erosion is estimated to have reduced her original height by about 4000 ft. - that big dent in the top is an erosional valley, not a 'crater.'

These big wide volcanoes - called shield volcanoes because of their long, low shape - are freaking huge. What you see of Maui is 13% of her mass, the rest is underwater. She, like all of them, makes a dent in the crust as she grows and forces the crust to flex down. When she erodes enough, the crust tries to flex back up.

However, that process creates huge stress fractures in the volcano, and she develops rift zones. Rift zones also develop when the volcano is young, from the force of magma trying to get out - imagine anything baking.

Because our lavas come from deep in the planet and are fairly homogenous, they are superhot and runny. They melt their way through the volcano fairly easily and tend to just spray and ooze out of a rift zone somewhere. When first expelled from a volcanic vent, lava is a liquid at temperatures from 700 °C to 1,200 °C (1,300 °F to 2,200 °F).

When they do that, they create cinder cones. Cinder cones are made when lava sprays up very high - some of Kilauea's fountains were 2000ft - and turns to little cinders as it falls and cools, and creates a cat litter pile around the base of the spray. Usually, lava flows out of the base of the cone as well, and the end result can be a split-open cone structure.

Haleakala has two massive rift zones-one running up from Hana to the summit and one running up from the Ahihi-La Perouse area to the summit. A third one runs through the Ha'iku area - Giggle Hill/Haiku Hill is a cinder cone.

The SW rift  has been the most recently active, taking over from the E./Hana rift, which was plenty active for several hundred thousand years. The east rift zone continues under the ocean beyond the east coast of Maui as Haleakala Ridge, making the combined rift zones one of the longest in the Hawaiian Islands chain.

There have been at least ten eruptions in the past 1,000 years, and numerous eruptions have occurred there in the past 10,000 years. Thus, East Maui's long eruptive history and recent activity indicate that the volcano will erupt in the future.

The most recent eruption site, the a'a flow emerged from Ka Lua o Lapa cone/vent, is the most recent eruption and one of the reasons the Natural Area Reserve was formed. It is Last Lava To Flow.

Until recently, East Maui Volcano was thought to have last erupted around 1790, based largely on comparisons of maps made during the voyages of La Perouse (1786) and George Vancouver (1792). Recent advanced dating tests, however, have shown that the last eruption was more likely to have been in the 1600s. These last flows from the southwest rift zone of Haleakala make up the large lava deposits of the Ahihi Kina`u/La Perouse Bay area of South Maui.

There are two kinds of lava above sea level and one kind below. The below kind is called pillow lava because it forms in pillow-shaped blobs under the insane stress of insane water pressure

The other two kinds are a'a and pahoehoe.

Pahoehoe, meaning "smooth, unbroken lava", is basaltic lava that has a smooth, billowy, undulating, or ropy surface. These surface features are due to the movement of very fluid lava under a congealing surface crust.

A pahoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust. It also forms lava tubes where the minimal heat loss maintains low viscosity. The surface texture of pahoehoe flows varies widely, displaying all kinds of bizarre shapes often referred to as lava sculpture. With increasing distance from the source, pahoehoe flows may change into a'a flows in response to heat loss and consequent increase in viscosity. Pahoehoe lavas typically have a temperature of 1100 to 1200 °C.

A'a is basaltic lava characterized by a rough or rubbly surface composed of broken lava blocks called clinker. The clinkery surface actually covers a massive dense core, which is the most active part of the flow. As pasty lava in the core travels downslope, the clinkers are carried along at the surface. At the leading edge of an a'a flow, however, these cooled fragments tumble down the steep front and are buried by the advancing flow. This produces a layer of lava fragments both at the bottom and top of an a'a flow. Accretionary lava balls as large as 3 m (10 ft) are common on a'a flows. a'a is usually of higher viscosity than pahoehoe. Pahoehoe can turn into a'a if it becomes turbulent from meeting impediments or steep slopes.

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.

A'a lavas typically erupt at temperatures of 1000 C to 1100 °C

A kipuka is an area of land completely surrounded by one or more younger lava flows. A kipuka forms when lava flows on either side of a hill, ridge, or older lava dome as it moves downslope or spreads from its source. Older and more weathered than their surroundings, kipukas often appear to be like islands within a sea of lava flows. They are often covered with soil and late ecological successional vegetation that provide visual contrast as well as habitat for animals in an otherwise inhospitable environment.

Kipuka, along with a'a and pahoehoe, are Hawaiian words related to volcanology that have entered the lexicon of geology.

Here's an update from the USGS site about the much-debated corrected date of Haleakala's last eruption:

http://hvo.wr.usgs.gov/volcanowatch/1999/99_09_09.html

September 9, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Youngest lava flows on East Maui probably older than A.D. 1790

In our efforts to refine the geologic map of Haleakala, we recently obtained radiocarbon ages from the youngest lava flows, those at La Perouse Bay. The ages indicate these flows were emplaced sometime between A.D. 1480 and 1600. This finding shakes the long-held assumption that the flows are vintage A.D. 1790. The charcoal that produced the ages was sought to test the 1790 hypothesis, and therein lies an unfinished story of scientific investigation.

The lava flows in question lie 2.5 miles south of Makena, south of the resorts that line Ma`alaea Bay from Kihei to Wailea. Erupted from a prominent spatter cone, Kalua o Lapa, the flows spread outward and built the promontory of Cape Kina`u and, to the east, La Perouse Bay. The La Perouse flows, as they became known informally, were thought to have been emplaced in the time between the voyages of La Perouse (1786) and Vancouver (1793) because of the subtly different charts produced by geographers from those journeys. But neither of these charts is accurate enough for definitive comparison.

Our initial interest in these lava flows was to use them to calibrate our knowledge of the Earth's magnetic pole. The magnetic north pole changes through time, so that a compass needle at a stationary site will point in a different direction today compared with 10 or 1,000 years ago. Lava flows contain minerals that record the magnetic orientation existing at the time they form. By determining the magnetic orientation of precisely dated flows, we can learn of the exact path followed during magnetic polar variation.

The surprise came when we compared the magnetic record of the La Perouse flows with Big Island flows emplaced in 1802; although thought to be similar in age, the pole positions were substantially different. This raised serious doubt that the La Perouse flows were erupted in 1790. Indeed, the La Perouse magnetic poles more closely match those from Big Island flows whose ages range from 350 to 460 radiocarbon years before present.

This information led us to look for charcoal beneath a La Perouse flow and beneath spatter at the Kalua o Lapa vent, which we found at both sites. The laboratory ages are 390 and 460 radiocarbon years before present (before A.D. 1950); analytical uncertainty is plus or minus 50 years. Unlike many isotopic dating clocks, the radiocarbon clock must be calibrated to account for changes in the atmospheric abundance of the carbon-14 isotope, which varies as a consequence of cosmic-ray bombardment. When calibrated, these ages correspond to dates ranging respectively from A.D. 1428 to 1640 and from 1402 to 1609. Thus the two charcoal ages are roughly coincident, within analytical error. They indicate an age substantially older than the previously assumed age of A.D. 1790. Our effort to resolve the discrepancy between different scientific findings, however, is unfinished. People questioned in 1841 about the age of the flow stated that their grandparents saw it. Their reports indicate a lava-flow age of about A.D. 1750. Oral history is important and not to be overlooked among the numbers from laboratories. Perhaps some of our Maui-born readers have additional knowledge to share about the La Perouse flows that could resolve all pieces of the puzzle.

------------------------------------------------

So, study up, kids. The final is worth 25% of your total grade!

J*


Leave a Comment