Geology

The
Hawaiian Hot Spot

63 points total

Introduction:
Review the text on hot spot volcanism and recall that hotspots produce a string
of dormant volcanoes behind an active volcano. Because we know the age of the
volcanoes and their distance from the hot spot we can use the dormant volcanoes
produced by a hot spot to determine the speed and direction that a tectonic
plate is moving. This exercise will guide you through that process.

There are a couple
of different ways to do this. One would be to recognize that if a dormant
volcano is 5 million years old and is sitting 450 km from a hot spot then it
has moved 450 km in 5 million years. If we divide 450 by 5 we get 90
km/Ma.  That unit is kilometers per
million years (Ma is an abbreviation for millions of years). This is not a
particularly useful unit. A million years is a very long time so it’s difficult
to really understand how fast a speed given in km/Ma really is. For most of
what we do we measure speeds in miles per hour. You know how long and hour is
and you know how far a mile is so it’s a useful unit. For Plate tectonic
velocities it’s best to measure the speed in centimeters per year (cm/yr).
Doing this gives a number usually between 5 and 15 or so which is a very useful
and manageable unit. Since there are 100,000 centimeters in a kilometer
converting from km/Ma to cm/yr is relatively easy: divide my 10. So 90 km/Ma is
9.0 cm/yr.

Use the map below
to figure out how fast the Pacific plate has been moving since Oahu formed over
the hot spot. The questions on the next page will guide you through the
process.

  1. How
    old are the lava flows on Oahu? 
    ___________ Ma (3 points)
  • Use
    the Map scale to determine how far Oahu is from the hot spot (which is the
    brand new underwater volcano Loihi) 
    ___________ km (3 points)
  • Divide
    the distance (#2) by the time (#1) to get a speed for the Pacific plate

             ___________ km/Ma (5 points)

  • Now
    divide by 10 to convert to cm/yr ___________ cm/yr (5 points)
  • What
    direction did Oahu move as it moved off of the hotspot. This is the direction
    that the Pacific plate is moving. _____________ 
    (5 points)

While this
technique is useful it’s limited in that it doesn’t take advantage of all the
data we have. We have age and distance data for the entire Emperor Seamount
Chain as well as the Hawaiian islands. The following exercise will guide you
through the process of using all the available data to learn about the speed
and direction that the Pacific plate has been moving.

First the data.

# Name Age (Ma) Distance
from the hotspot (km)
1 Kilauea 0.20 0
3 Mauna Kea 0.38 54
5 Kohala 0.43 100
6 East Maui 0.75 182
7 Kahoolawe 1.03 185
8 West Maui 1.32 221
9 Lanai 1.28 226
10 East Molokai 1.76 256
11 West Molokai 1.90 280
12 Koolau 2.60 339
13 Waianae 3.70 374
14 Kauai 5.10 519
15 Niihau 4.89 565
17 Nihoa 7.20 780
20 unnamed 1 9.60 913
23 Necker 10.30 1058
26 La Perouse 12.00 1209
27 Brooks Bank 13.00 1256
30 Gardner 12.30 1435
36 Laysan 19.90 1818
37 Northampton 26.60 1841
50 Pearl &
Hermes
20.60 2291
52 Midway 27.70 2432
57 unnamed 2 28.00 2600
63 unnamed 3 27.40 2825
65 Colahan 38.60 3128
65a Abbott 38.70 3280
67 Daikakuji 42.40 3493
69 Yuryaku 43.40 3520
72 Kimmei 39.90 3668
74 Koko 48.10 3758
81 Ojin 55.20 4102
83 Jingu 55.40 4175
86 Nintoku 56.20 4452
90 Suiko 1 59.60 4794
91 Suiko 2 64.70 4860
  • One of
    the most effective and easiest ways to analyze data is to graph them, so the
    first step in our analysis will be to graph the data. You’ve been provided with
    graph paper. Graph the age on the X axis (the one on the bottom) and the
    distance from the hot spot on the Y axis. (10 points)
  • Once
    you’ve graphed your points draw one straight line that goes through your
    ‘cloud’ of points. Don’t try to ‘connect the dots’ draw one straight line with
    about half your points above and about half your points below the line. It
    doesn’t need to be perfect just one straight line that approximates your data.
    (5 points)
  • The
    slope of this line is the average speed that the Pacific plate has been moving
    over the past 65 million years or so. So let’s calculate the slope of the line.
    The slope of a line equals the change in y divided by the change in x for two
    points. Even though your line might not go through them it’s easiest to use the
    first and last points to do this so look at the data chart and fill in the
    appropriate numbers and subtract.

Volcano 91 Suiko 2
age (x) ____________Ma, Distance (y) ____________ km (4 points)

Volcano 1 Kilauea
age (x) ____________ Ma, Distance (y) ____________ km (4 points)

Difference in the x
values ____________ Ma. Difference in the y values ____________ km (4 points)

(Subtract the two numbers above the blanks)

Now divide the
difference in y by the difference in x:

 ____________km / ____________Ma=____________
km/Ma (4 points)

convert km/Ma to
cm/yr (like you did in question # 4)

Speed of the
Pacific tectonic plate ____________ cm/yr (2 points)

Now that we’ve done
speed, let’s do direction. Look at the map below.

Note that there is
a bend in the seamount chain (labeled bend). The Daikakuji seamount is located
right at the bend.

  • How
    long ago did the bend happen? _________ million years ago (hint: you have a
    data set that includes Daikakuji) (3 points)
  1. Keeping in mind how plates move over hot
    spots, what direction was the Pacific plate

moving
between the formation of Meiji and 
Daikakuji? ________________ (3 points)

  1. What direction has the Pacific plate been
    moving since the formation of Daikakuji?

              ______________ (3 points)

So there you are,
you just used real geoscience data to do what real geoscientists do, you
calculated the speed and direction of a tectonic plate.

Turn the word file
with your answers into the drop box. Photograph or scan the graph name it with
your name and turn it in to the drop box as well. 

 

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