What Can We Learn about Plate Motion from Hotspot Tracks?

In this week's lab we're gonna see what we can learn about plate motion based on

the ages of volcanic islands and sea mounts produced as an oceanic plate

moves over the hot spot. Probably the best-known hot spot is the Hawaiian

hotspot; here the Pacific plate is moving northward over a stationary hotspot or

magma source in the deep mantle. As the plate moves over that magma source, it

produces a chain of volcanic islands and sea mounts. That chain is age progressive,

meaning there's an orderly relationship between the ages of islands and sea

mounts in the track relative to their distance from the part of the plate

currently over the hotspot. A less well known hotspot is found in the South

Pacific. It's the same geologic setting as what we have in Hawaii, only here the

Pacific plate is passing over another hotspot, the Louisville hotspot... Lesser

known because the Louisville volcanoes are not tall enough to rise above the

surface of the sea as oceans. Instead they form underwater volcanoes called

sea mounts. But because they're formed in the same way that the Hawaiian chain is,

they also have an age progressive relationship, and the distance or

relationship between distance and age along that track indicates changes in

the velocity of the Pacific plate. Here we've plotted... here we've plotted age of

sea mounts in the Louisville chain versus their distance from the youngest

sea mount, which is inferred to be the part of the plate currently over the

hotspot. You're going to do this same exercise, only you're going to be working

with the Hawaiian hotspot. You will have data on the ages of islands and sea

mounts in Hawaiian chain verses there distance from Kilauea, the active volcano

currently over the Hawaiian hotspot. Age on the x-axis, distance on the y-axis. The

relationship between distance and age can be used to infer the velocity of the

plate as it passes over the hotspot remember velocity is simply distance

divided by time. We've plotted distance versus time, so the slope of our line

indicates the velocity of the plate as it passes over the hotspot. That velocity

may not be constant but rather the plate may slow up, may slow down, the plate may

slow down or speed up as it passes over the hotspot. If you're doing this by hand,

you want to get out a ruler and draw your line representing the relationship

between distance and age which gives us the velocity of the plate as it moves

over the hotspot. Where you have a lot of data points as you do for the Hawaiian

hotspot, you're not going to draw, you're not

going to draw a line that connects the points, but rather you're going to draw a

single line that best fits those points. Now there's some trial and error

involved in this. What I'm doing now is I'm placing my ruler over my data points

and I'm adjusting the position of my ruler in such a way that I have an equal

number of points above and below the line that I'm going to draw.

I have placed my best fit line for this portion of the track something like that,

and again the slope of the line--rise over run--represents distance divided by

time. In other words, velocity of the plate as

it passes over the hotspot. Notice that the two lines we've drawn here for the

Louisville hotspot track have somewhat different slopes. Down here in the

younger part of the Louisville hotspot track, the relationship between distance

and age shows a slightly steeper slope, indicating that the plate has been

moving at a faster relative velocity over the Louisville hotspot when it

formed the younger part of the hotspot track. Up here on the older part of the

track our line is a little bit flatter. It's a little bit less steep, indicating

a slower velocity as the Pacific plate moved over the hotspot and produced this

part of the Louisville hotspot track.