October 2, 2003
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Continent dwellers take colorful rocks for granite
A recent Hollywood disaster movie depicts a scenario in which the earth's Geologists on the Big Island know that they are among the privileged few in their profession to work around active lava flows. Yet after a few years in Hawai`i, many are overcome by a secret longing for the more varied rocks of the mainland. Memories of pink and white granite flecked with shiny mica, fossil-laden limestone, or pinnacles of red sandstone evoke nostalgia for continental rocks that cannot be eased by a hike over dark lava flows.
How come the rocks of Hawaii seem so devoid of color compared with those on the continents? Let's start with some basic geology. The earth's rocks can be divided into three main categories: igneous rocks, which crystallize from magma; sedimentary rocks, which form when fragments of eroded rocks are cemented together to form a new rock, and metamorphic rocks, which form when rocks recrystallize in response to temperatures and/or pressures greater than those under which they first formed. Sedimentary and metamorphic rocks form only a small proportion of the earth's crust relative to igneous rocks, but they add a great deal of variety to continental rocks and landforms that we are missing out here in the middle of the ocean.
In Hawai`i, we see almost nothing but igneous rocks. Most of those are black or gray when young or brown-to-red when old and weathered. The lava in Hawai`i owes its dark colors to a high iron and magnesium content. As it ages, oxidation of the iron creates the orange and red hues.
The only white rocks we're likely to encounter on the island (aside from the white crust that sometimes forms on new lava flows due to chemical alteration by hot gas) are pieces of coral--the skeletons of reef-building organisms. Our white sand beaches are also composed of coral fragments eroded from the reef.
In contrast, on the mainland, as on all continents, the earth's crust has a higher concentration of the elements that form quartz and alkali feldspar, minerals that are in short supply here in Hawai`i. These minerals account for the bulk of white and pink rocks in the world, including the igneous rock, granite.
Granite forms the backbone of the Rocky Mountains and the Sierra Nevada. In fact, the average composition of the upper continental crust is granite. On the continents, nearly all the white sand beaches derive their color from quartz grains, most of which are eroded from granitic rock.
The reason for the discrepancy between oceanic and continental rock types goes back to the earth's beginnings, when the chemical elements that compose our planet became layered according to density. The result was a dense core of iron and nickel, surrounded by a less dense mantle. The lighter chemical elements rose to become concentrated in the crust, the outermost and thinnest layer of the earth.
The ocean basins formed as convective movement deep within the earth split the crust and initiated plate tectonics. Oceanic crust forms at plate boundaries called spreading ridges, where magma from the mantle rises to fill the gap as plates pull away from each other. Because the oceanic crust is formed from mantle material, it is denser and thinner than continental crust and consists mainly of basaltic rock.
Oceanic crust may not be as pretty, but it's far more dynamic than its continental cousin. The average age of oceanic crust is about 55 million years, compared to an average age of 2.3 billion years for continental crust. This is because oceanic crust is constantly being recycled at convergent plate boundaries, where an oceanic plate meets a continent. Since the oceanic plate is the denser of the two, it sinks, or subducts, beneath the continental plate. Because of its lower density, little of the continental crust is recycled. The stable interior of the continents is very old, indeed, with rocks dating back 3.96 billion years.
Of course our youngest rocks, the ones that are still flowing, are very colorful, and many visitors would argue that they beat anything the continent has to offer. But variety is the spice of any rock collection, so next time you're on the mainland, keep your eyes on the ground. You just might find a piece of continental crust worth lugging home to the middle of an oceanic plate.
Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Although no breakouts can be found in the coastal flats, surface activity is visible on Pulama pali in both the Kohola and the August 9 segments of the Mother's Day flow. The Kohola arm of the Mother's Day flow, on the western flow field, has some incandescent patches high on Pulama pali, as well as just above Holei pali. On the east side of the flow field, the August 9 breakout continues to develop crust and so is less incandescent than it has been. Patches of incandescence track the lava from the top of Pulama pali to the gentle slope below. No lava is entering the ocean.
No earthquakes were felt on the island during the past 7 days. Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low, with only one earthquake located in the summit area during the last seven days. Visit our website (hvo.wr.usgs.gov) for daily volcano updates and nearly real-time earthquake information.
Updated: October 17, 2003 (pnf)