Instructions: First print copies of Exercise 3, Lag-time questions, and Real-time questions. Read the text and examine the images. Write answers to the questions on your copies. When you are satisfied with your answers, type them into the computer and submit. Keep the copies to study aids.
We have learned in class that volcanoes occur at plate boundaries, particularly at divergent and convergent types. However, if we look at the Hawaiian volcanic island chain, we see that it is not located at the edge of a plate.
Figure 1. This map shows the north-central part of the Pacific Ocean sea floor. The boundaries of the Pacific plate are near the edges of the Pacific Ocean. The volcanically active Hawaiian islands, however, are located in the middle of the Pacific plate. They are part of a long chain of seamounts that extends westward and northward to the Aleutian trench. The seamounts of the Hawaiian Ridge and Emperor Seamount Chain are volcanoes that are no longer active and that have sunk below sea level. The dates on Figure 1 (<1 my to 70 my) are the ages of the volcanoes; they indicate when lava extruded from the sea floor to form the labeled island or seamount (my = million of years).
Figure 2. This diagram explains the progression of ages observed in the Hawaiian / Emperor Seamount chain. Hot spots are places in the asthenosphere that are hotter than usual and that produce large volumes of melted rock called magma. (When magma erupts on the sea floor or on land, it is called lava.) Hot spots are features that are fixed in the asthenosphere. Magma produced at a hot spot punctures the overlying lithospheric plate and forms a volcano on the sea floor. The lithosphere, however, is not fixed and as it moves over the asthenosphere, away from the hot spot, the volcano stops erupting and a new one is formed in its place. With time, the extinct volcanoes keep moving northwestward and new active volcanoes form over the hot spot.
As the extinct volcanoes age, the crust upon which they sit cools and subsides. This, combined with erosion once active volcanism stops, leads to a shrinking of the islands with age and their eventual submergence below the ocean surface. When the U.S. Geological Survey completed bathymetric side-scan sonar surveys of Hawaii'a EEZ in the late 80's and early 90's, they discovered huge landslides that extended 100's of km on the seafloor away from the islands. These submarine slides are similar to slides that occur on land along the California coast, only their scale is much larger.
Figure 3. This map shows the islands of the Hawaiian chain (labeled in blue) and the volcanoes that make up the islands (labeled in orange). Loihi is a volcano that is building up toward the sea surface and that will be the next island in the Hawaiian volcanic chain.
Figure 4. HUGO, the Hawaii Undersea Geo-Observatory, is a submarine observatory located on the summit of Loihi (see Figure 3 for location), an active undersea volcano that is building up toward the sea surface. HUGO is connected to the shore via fiber optic cable. Scientists monitor activity such as earthquakes, volcanic eruptions, and hydrothermal venting by collecting data from instruments on HUGO. An undersea camera photographed the pillow lavas shown in Figure 4 near the summit of Loihi. When lava flows onto the sea floor at hot spots or along the mid-oceanic ridge, it encounters cold ocean water and pillowed shapes form.
Click here to take a virtual tour of Loihi, the new volcano that is building up on the seafloor southeast of Hawaii. In particular, look at the bathymetric map of Loihi on the top part of this page.
Features such as the Hawaiian / Emperor Seamount chain are useful for calculating the speed of lithospheric plates that move about on the Earth's surface. Because the eastern edge of the Pacific plate is our San Andreas fault, understanding the rate of motion of the Pacific plate helps us to calculate the frequency of earthquakes in our own state. Before our in-class discussion of plate rates, let's gather data on rates from a practical example and use the observations to calculate rates in the real- and lag-time questions.
Figure 5. On Friday, September 4, 1998, the San Francisco Chronicle's lead article was entitled "Mayor Walks, Muni Runs". This picture of the mayor and two reporters illustrated the front page. The mayor walked the distance from the Civic Center station to the Embarcadero Station (1.4 miles) and the reporters rode the Muni street car between the same two stations.
START: Civic Center
7:40 a.m.: Mayor leaves Civic Center station entrance, heading east.
7:40 a.m.: Reporters enter Civic Center station to wait for streetcar.
7:42 a.m.: Mayor passes Seventh Street, yells "We're on a death march, want to join us?"
7:42 a.m.: Streetcar arrives at Powell station (Fifth Street).
7:45 a.m.: Streetcar arrives at Montgomery station. (Third Street.)
7:46 a.m.: Mayor passes Fifth St. "The train better win," he says.
7:48 a.m.: Streetcar arrives at Embarcadero station. "I'm shocked," says passenger.
7:48 a.m.: Mayor passes Third Street.
7:50 a.m.: Reporters who rode streetcar arrive at finish line at Market and Front streets.
8:00 a.m.: Mayor arrives at finish line, Market and Front streets. Sees he finished second. "Lord, you've answered my prayers," he says. Drinks $4 coffee.