# Geology

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.

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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.

Geology