October 9, 2003
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Earth's spinning core provides magnetic protection and disaster movie material
A recent Hollywood disaster movie depicts a scenario in which the earth's magnetic core has stopped spinning, causing the planet's electromagnetic field to rapidly deteriorate. A series of magnetic anomalies disrupts life and threatens to destroy the earth. Groups of scientists watching this movie, have been observed chortling with amusement at the entertaining portrayal of scientist personalities, speedy development of implausible equipment, and fanciful depictions of the earth's interior. The movie, however, also presents interesting nuggets of information regarding our planet.
For instance, the earth's core really does spin. If you cut the earth in half, it would look like a hard-boiled egg with a very large yolk. The shell represents the earth's 35-km (22 mile)- thick (or less) crust on which we walk and live. The egg white represents the 2,850- km (1770 mile)- thick mantle, the source of heat for Hawaiian and other hot spot volcanoes, and the yolk represents the 3,500-km (2172 mile) -thick core.
The core is made of two sections - a liquid outer core and a solid inner core. Both sections are made predominantly of iron and nickel. The inner core is under such great pressure that it cannot melt, even though temperatures are estimated at 3,700-6,000?C (6,700-10,800?F). This is about the temperature of the surface of the sun, while a sizzling molten lava flow from Kilauea is a cool 1,100?C (2,000?F).
In the late 1990s, enterprising scientists discovered that the solid inner core is spinning independently of the rest of the Earth. So, while the earth spins on its axis one complete revolution each day, the inner core spins in the same direction but slightly farther. Some scientists believe that over the past century, the extra speed has gained the core a quarter-turn on the planet as a whole, while others think that it would take more than 1,000 years for the inner core to completely "lap" the planet.
Since no one (other than terranauts in sci-fi movies) has been able to observe the earth's inner core directly, indirect methods are being used to reveal the inner workings of the planet. Seismologists have found that earthquake waves pass through the solid core at different speeds depending on their direction of travel. When the waves travel in the direction of the neatly aligned iron crystals in the core (like traveling with the grain in wood), the earthquake waves travel more quickly. The direction of quickest travel varies over time since the core (and its crystalline grain) rotates slightly with respect to the rest of the earth. Computer simulations of the earth's interior dynamics also support the concept of an independently rotating inner core. The fact that something deep within the earth may be changing on the scale of human lifetimes provides potent disaster movie material.
Another interesting topic in the movie involves the disruption of the earth's magnetic field. This field influences the migratory paths of birds, the needle of the modern compass, and also shields the earth from damaging solar radiation.
The earth's magnetic field is generated mainly at the core, as the highly conductive metallic fluid of the outer core rotates with respect to the solid inner core, generating electric currents and an accompanying magnetic field. This field is dynamic, having flipped over completely from time-to-time. As little as 100,000 years ago, a compass could have pointed south instead of north.
A history of the earth's changing magnetic field is trapped in solidified lava flows. Magnetic particles in the molten lava orient themselves toward the magnetic pole just like a compass, and when the lava solidifies, a record is preserved of the magnetic field existing at that time. When lava flows are dated, the magnetic-field history is revealed.
In both the Hollywood and real-life versions, the earth's magnetic field is vital to survival. If for some reason the core did stop producing a magnetic field, life as we know it might change dramatically. So, in addition to our warm-hearted planet providing us with good movie material, it also protects us from dangerous cosmic rays, and continues to surprise us with its wonderful complexity.
Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Sparse surface activity is visible on Pulama pali in both the Kohola and the August 9 segments of the Mother's Day flow. On the west side of the Mother's Day flow field, the largest breakout is still at about the 1000-foot elevation, well above Holei Pali. Other speckles of incandescence occur higher and a little farther east. In addition, one new area can be seen above Pulama pali, at about the 1950-foot elevation, where lava is encroaching into forest. On the east side of the Mother's Day, two closely spaced patches of surface lava dot Pulama pali at about the 1500-foot elevation. A pinprick of light occurs higher, at the very top of the pali. The coastal flat below Paliuli is devoid of any surface lava activity. No lava is entering the ocean.
Two earthquakes were reported felt in the past week ending on October 9. A resident of Leilani Estates felt the earth move at 45 minutes after midnight on Friday, October 3. The tiny magnitude-1.3 earthquake was located 5 km (3 mi) southwest of Pu`ulena Crater at a depth of 0.8 km (0.5 mi). On Tuesday, October 7 at 5:30 a.m., a magnitude-3.5 earthquake shook residents of Kurtistown, Glenwood, and Mauna Loa Estates. The temblor was located 7 km (4.2 mi) north of Ka`ena Point at a depth of 10 km (6 mi).
Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low, with no 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)