Basalt: The Rock of the Hawaiian Islands

Rocks are made of minerals

Most rocks are made of minerals (crystals). But some volcanic rocks are different: they are made of minerals and glass. Glass is not a crystal - it is a frozen liquid, that forms when the liquid part of the lava cools too quickly for any crystals to form, just like the pane of glass in your window. Volcanic glass is extremely useful for studying volcanoes, because it tells us what the melt was like when it erupted.

Sometimes crystals trap small bubbles of liquid inside them as they grow; these are called melt inclusions. By the time we see them in solid rock, they have cooled and formed a pocket of tiny crystals or glass, or both, inside the larger crystal. When it erupts, the melt + crystals + gas bubbles are together called lava. Before it erupts, when it is still inside the Earth, it is called magma.

The kind of lava that erupts on Kilauea is basalt. Shown to the left is a piece of basalt that was dredged from the top of the Puna Ridge at a water depth of 2850m. 

Most of the crystals that we find in Kilauea’s lavas are olivine (a translucent green mineral made up of iron, magnesium, and silica) and the rest are plagioclase feldspar (a mineral made of calcium, aluminum, and silica) and clinopyroxene (a mineral made of calcium, iron,  magnesium, and silica). The lavas also contain vesicles, which are holes left by gas bubbles when the lava cooled.

To the right is a photomicrograph of a thin section of basalt showing large crystals of the 3 most common kinds of minerals in Puna Ridge lavas: olivine (blue and pink crystal), plagioclase (crystal that is dark brown on one half and light on the other with a white stripe), and clinopyroxene (patchy brown crystal). The roundish black patches inside the crystals are melt inclusions. The black background is glass, and the tiny color patches are small crystals of the same minerals.

What can rocks tell us about Kilauea and the Puna Ridge?

What the rocks are made of gives us information about:

1. The composition of the solid rock deep in the Earth that melted in the first place, how deep it was and how hot.
2. What happened to the magma on the way to the surface. It can pick up material along the way, or lose crystals that form and drop out of the melt, or mix with other magmas.
3. The depth in the ocean where the lava erupted - from measuring the gases that are dissolved in the glass. The deeper the water depth, the higher the pressure and the more gas that will be dissolved.

This information can be put together to get a better understanding of the Hawaiian hot spot, and how magma is fed to Kilauea and its rift zones.

Dredging

Dredging is the most common way of sampling rocks on the seafloor.  Most dredges have a  metal collar and a chain sack attached to it.The dredge is lowered to the seafloor and dragged slowly along it.  As the dredge bounces along the seafloor, pieces of rocks are scooped into it.  The picture to the left shows the results of a successful dredge haul that has come back to the surface with rocks in the chain sack.

The dredge is then pulled back up to the ship, and the rocks are emptied onto the deck, sorted, labeled and stored for later analysis in our laboratories.  One dredge haul usually takes 5-6 hours.  The benefit of dredging is that large pieces of the rock are obtained and these can be used to examine the crystals in the rock.

Rock coring

The rock coring technique takes advantage of the fact that when lava erupts underwater, the surface of the lava cools very fast and a glassy layer forms. The basic idea is to slam the end of a steel pipe into the lava surface, break the glass and pull up some of it. We use a meter-long pipe designed for coring sediment (thus the name), and attach weights to get a better impact. There are core tips, or short sections of hollow pipe, fastened on the end and filled with wax. A steel collar with more core tips is bolted around the weights. The corer is fastened onto the end of a steel cable and dropped at high speed through the water. A good speed is 120-150 meters per minute, the faster the better. When the core tips hit the lava surface on the seafloor, chips of broken glass stick in the wax. The corer falls over onto its side, and the core tips on the collar pick up more glass. Those glass chips are what we analyze.

The photo shows Gary Burkhart, Master Machinist at Monterey Bay Aquarium Research Institute, standing next to the wax corer.  Burkhart refined the design and built the corer for us.

The classic wax used on the wax corer  is surfer’s wax, because it is firm but malleable, and its stiffness does not change much from the warm temperature on the deck of the ship down to the near-freezing temperature of deep seawater and back up. So the glass chips stick in the wax and don’t fall out. Wax coring is obviously low-tech. This can be a great advantage, because there is almost nothing that can break down -- the only moving part is the ship’s winch pulling the wire up and down. There is nothing complicated about the method -- the hardest part is to put the ship directly over the target on the seafloor that you want to hit, and that is much easier now that we use GPS navigation.

The photograph to the right shows glass and sediment embedded in the wax on the tip of a  wax corer used on a cruise in the Atlantic ocean.

Rock coring is very fast -- it takes only 1-2 hours to get a sample. This means that a large number of samples can be collected. The disadvantages are that it only recovers a small sample, and because most of the sample is glass you don’t learn much about the crystals in the lava. But for obtaining  lava compositions and studying how they are related to eruption patterns, wax coring is a wonderful tool.

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