Part III: Plate Motions Using Hot Spots:

The Hawai'i-Emperor chain of seamounts (volcanoes resting on the ocean floor) stretches from its active end at the Big Island of Hawai'i west and north across the Pacific Ocean floor to the Aleutian trench near the Kamchatka Peninsula (first figure). There are about 110 individual volcanoes in the Hawai'i-Emperor chain (see the data table), which is about 6000 km (3800 miles) long altogether. The Hawai'i- Emperor chain is divided into two segments, the WNW-trending Hawai'ian chain and the N-trending Emperor chain. The two chains meet at a prominent bend, around the underwater seamounts Daikakuji and Yuryaku.

The active end (youngest end) of the Hawai'i-Emperor chain is at the Big Island of Hawai'i and the offshore, still underwater volcano Loihi. Kilauea volcano on the Big Island is active today, and other centers on the Big Island and on Maui have erupted recently. As one progresses towards the west-northwest, the volcanoes of the Hawai'ian Islands get progressively older (see data table). Once active volcano building through eruptions of lava ceases, the erosional forces of tropical weathering, landslides, river erosion, and wave action overcome the island, and erodes it down to sea level (second figure). The extinct volcano evolves to a flattopped mesa ringed by coral reefs, and then to an atoll with nothing but the circular reef showing. Finally the volcano sinks beneath the waves, and becomes an underwater seamount.

The Hawai'i-Emperor chain is a classic example of a hot spot track. The standard explanation begins with a hot spot whose source of magma is rooted deep in the Earth's mantle. The hot spot magma source is thought to be fixed in the deeper mantle, with a slab of ocean crust and uppermost mantle (called a plate) moving laterally above the hot spot. As the Pacific Plate moves over the Hawai'ian hot spot, magma punches up through the Pacific Plate, creating an active volcano. Plate motion carries the active volcano away from the magma source, the volcano goes extinct, and a new volcano grows over the hot spot. As the extinct volcano is carried farther and farther from the hot spot source, the volcano sinks beneath the waves mostly due to aging and cooling of the ocean crust underneath the extinct volcano; this cooling causes subsidence of the ocean floor.

Hot spot tracks are very important geologic features for determining both the direction and speed of the plate upon which the seamounts rest. The direction of plate motion is given by the orientation of the chain of seamounts and volcanoes. Using your "hands of science", you can quickly determine that the plate "moves towards the oldest volcano." As can be seen in the first figure, the Pacific Plate moved almost due north during "Emperor time" (from 75 to 42 million years ago), and then changed direction about 42 million years ago (the age of the volcanoes at the bend in the chain), to move west-northwest during "Hawai'i time" (from 42 Ma to the present). Note that the azimuths cited here assume no rotation of the Pacific Plate during the last 75 Ma.

Hot spot tracks also give the speed of plate motion, if the length of the chains of volcanoes and seamounts, and the ages of the volcanoes and seamounts are known (data table). Plates typically move about 1-10 cm/year, which is equivalent to 10- 100 km/Ma (kilometers per million years). These speeds are about the rates at which fingernails grow, and may seem rather slow on the human time scale, but are very fast on the geological time scale. The Earth is 4.55 billion years old; one million years is a brief moment in Earth time. The speed of the North American plate (for example) is fairly typical for plates, about 6 cm/year, whereas the Marianas plate is one of the fastest (today), moving about 13 cm/year.

Distances from the active Kilauea volcanic center (measured parallel to the Hawai'i-Emperor chain) and ages of each volcano and seamount are given in the data table (Clague and Dalrymple 1989). These data have been compiled from a wide variety of sources and researchers, which can introduce uncertainties. For example, different geochronologic laboratories determined the ages of the volcanic rocks from these seamounts, and different labs often use different machines, different standards, and different analytical techniques. Even with the highest quality of work, the ages have uncertainties that vary from sample to sample. Furthermore, volcanoes do not have a single age; a typical Hawai'ian volcano builds up over half a million years or more. Who is to say that the volcanic rocks dredged up from the underwater seamount Jingu (for example) are representative of Jingu's eruptive history? It is very difficult to sample Jingu's older rocks; they are covered by the young lavas. The data in the table are not without problems, and data should not simply be accepted at face value.

Part III Questions:

★Q3.1: First plot age (x-axis) versus distance from Kilauea volcanic center (y-axis).

Data given in following table. Using Excel or some other graphing program, fit an unconstrained linear regression to the entire Hawai'i-Emperor chain and find the "typical" rate of Pacific Plate motion over the last 65 Ma.

a. What are the units of both the rate and the Y-intercept?

b. How would you write the equations using centimeters and years for units?

★Q3.2: For the second part of their exercise, fit unconstrained regressions (do not force a linear fit through zero) to both the Hawai'i chain and the Emperor chain separately.

a. Has the Pacific Plate sped up or slowed down over the last 65 Ma?

b. A change in speed from the older part of the chain to the younger part of the chain seems reasonable from the data. Must a change in speed accompany a change in direction?

c. Approximately what direction did the Pacific plate move older than 42 million years ago? Younger than 42 million years ago?

Yellowstone Hot Spot:

Hot spots may occur on continental lithosphere as well as oceanic lithosphere. For example, Yellowstone National Park is a huge volcanic caldera (collapsed summit of a volcanic cone) that we believe had a culminating eruption some 600,000 years ago. This is only the latest in a series of major caldera-forming eruptions that have traveled occurred during the last 16 million years. In fact, we can track the movement of this still-active volcanic hot spot as it has shifted from Oregon through Idaho (creating its Snake River Plain Volcanic Province) into Wyoming. See map below. Because the hot spot is stationary, we can apply the same method as before in order to calculate this rate.

★Q.3.3:

a. From the figure above (and the scale give or Google Earth), estimate the direction and speed of the North American plate over the Yellowstone hot spot (plot your data and fit the data using an unconstrained linear fit like in the Hawaiian example – show your figure).

b. How does your estimate of speed compare with the Pacific Plate? Why do you think they are different?

Data Link