Go On To Day 30Return To Day 28Day 29. Basalt on Board

By Edwin Schiele

October 24, 1998. Throughout most of the trip, we’ve had to watch where we put our coffee down, because maps cover all of the tables. Now new obstacles fill the main lab. There are rocks on the tables, rocks on the floor, rocks in buckets, and rocks in bags.

J. Mercier, M. Bulmer, and F. Trusdell pick rocks from dredge(L. Dolby)
Jennifer Mercer, Mark Bulmer, and Frank Trusdell remove basalt from the dredge. (L. Dolby)

Point to one of the rocks dredged from Puna Ridge and ask one of the geologists what it is, and he or she will answer "basalt." This is a primitive attempt at humor (geologists are a witty bunch). Basalt equals lava. All of Puna Ridge (and Hawaii for that matter) is made of lava. There are no other types of rocks. It’s like pointing to zebra and asking a biologist what it is, and the biologist answers "an animal." Biologists’ jokes, however, are usually much funnier.

Crane lifts dredge on board (L.Dolby)
The crane lifts the dredge out of the water and onto the deck. (L. Dolby)

Once the crane lifts the dredge on board, those of us on watch and other helpful bystanders lift the rocks out of the chain net, wash them with fresh water, put them in buckets (if they will fit), and carry them inside to dry. Then we begin sorting them.

At first glance, there doesn’t appear to be much to sort. All of the rocks are a dull dark gray and have a rough texture. Some of the rocks have orange stains and others have white stains, so that’s a start. The red stains form when iron in the rocks oxidizes. The white stains are composed of clay. The clay is formed when water seeps into cracks in the rock and reacts with the chemicals in the basalt.

Black basalt with green olivine crystals (L. Dolby)

Click here to enlarge

To find something really interesting, you have to look closer. When molten lava and cold water meet, the top layer of the lava cools instantly so crystals don’t have a chance to form. The result is a layer of black, shiny glass that coats the outside of the rock. The blacker and shinier the glass, the younger the lava flow. Beneath the outer layer, the lava cools more slowly, so the minerals in the lava have a chance to crystallize. The slower the lava or magma cools, the larger the crystals. (See Daily Flash 9 for a discussion of how these minerals form and what the presence of these minerals tells us about the lava). The most abundant mineral found in Kilauea’s magma is olivine. Many of the rocks we are bringing up are studded with dark green olivine crystals. As magma ages, new minerals precipitate out. Plagioclase feldspar (the hipper geologists refer to it simply as "plag") is the next most abundant mineral. It forms white, rod-shaped crystals. Occasionally, we find mostly white aggregates of crystals called gabbro. Gabbro forms when magma cools very slowly in a magma chamber. Subsequent eruptions may then carry the gabbro to the surface.

We sort the rocks into categories based on general appearance (color), the type of glass we see, the abundance of different crystals, and other characteristics. The idea is to group all of the rocks that we think came from the same source. We chip off glass samples from each type and bag them separately. The glass provides scientists with a quick and dirty way to determine the melt composition of the lava. If the scientists find interesting trends, they will go back and further scrutinize the original rocks.

Finally we describe the characteristics of each category in detail. After the cruise, Kevin Johnson and Jennifer Reynolds will analyze the chemical make-up of the rocks. From this information, they hope to learn the ages and sources of different lava flows.

Ship Tracks October 22 through October 24

Puna.jpg (55312 bytes)
Blue = Days 22 and 23
Red = Day 24
Go back to day 28. Go to day 30