Geos1040 pr02 2019 answers PDF

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Solutions to practical 2 for GEOS1040...

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GEOS1040 Earth’s Dynamic Systems PRACTICAL Nº 2 - Seafloor Spreading & Ocean Island Chains

INTRODUCTION Most geoscientists believe that throughout geologic time, rifting of continents has taken place. The best example of this occurred fairly recently when Africa drifted away from South America, creating the Atlantic Ocean. Responsible for the rifting, is hot mantle rock (asthenosphere) which lifts the overlying continental crust along a linear region. With time, the continents move further apart, leaving a space filled by crystallised basaltic magma derived from the asthenosphere below. This magma extrudes at the rift created when the continents subsided along normal faults, subsequent to doming. As the continents drift away from each other, basaltic magma continues to extrude at the Mid Ocean Ridge, and is then conveyed away, according to the theory of Seafloor Spreading (Figure 1).

Evidence for this theory arose once geoscientists began to explore the ocean floor. They noted magnetic anomalies symmetrically arranged in bands parallel to the Mid Ocean Ridges, which, in later investigations, were shown to increase in age away from the ridges. Further, sediment thickness increased away from these ridges, with the oldest sediments occurring next to the continents.

Objectives

When you have finished this practical, you should be able to:

1.

Calculate spreading rates, given data on the location of drill-sites, age of basalt beneath sediment at these sites, or polarity reversals.

2.

Construct graphs which show the relationship between sediment thickness and distance to a midocean ridge.

3.

Calculate migration rates of plates over fixed positions such as hot spots.

1

Ocean Island Chain Subduction Zone

re

Mesosphere (lower mantle)

Sp re

ad i ng

Oc ea Lit

ho

re he sp

2

Co nt in e nt al

e ph

Mid Ocean Ridge

c ni

Lit ho s

Volcanic Arc

H ot Spo t p lume

Asthenosphere

Subducted seafloor

Core

e ntl a M

Figure 1. Schematic diagram showing interpreted mantle structureand convection , including the cycling of seafloor fromm id -o ceanr idges through to subduction zones and into the mantle. Note that hotspots forming Ocean island Chains are stationary with respect to the geoid.

Part A: Seafloor Spreading The data in Table 1 come from a transect across the East Pacific Rise (EPR) carried out by the drilling ship Glomar Challenger. The transect is depicted on the map in Figure 2. At each site, drilling was carried out to determine the depth of sediment overlying the basalt (layer 2 of oceanic crust), and the age of the basalts directly underneath the sediments. Table 1

Drill Site Number & Location

Distance to middle of East Pacific Rise

Sediment thickness above basalt

Basalt age at base of sediment (Ma)

77 (W of East Pacific Rise)

3,359 km

481 m

36

79 ( " )

2,086 km

414 m

21A5

81 ( " )

1,280 km

409 m

14A5

82 ( " )

549 km

214 m

9A5

Ridge axis

0

0m

0

83 (E of East Pacific Rise)

797 km

241 m

10A5

Q1. (a)

Calculate the average spreading rate (in cm/year) from the data given in Table 1. To do this you will need to construct a graph of age vs. distance from the EPR for each site, and draw a line of best fit through the data (note that for this calculation it is not important which side of the EPR that data are derived from, since the spreading rate should be similar on both sides). From the slope of this line you can calculate a spreading rate (note: be careful that you convert to the correct units). Spreading rate: ~9.3

cm/year

Do you think this spreading rate has varied with time? (give reasons) The spreading rate has slowed with time as can be seen from the graph, the rate is only ~5.75 cm/yr in the last 10 Ma. (b)

Plot distance to the middle of the East Pacific Rise against sediment thickness (table 1). Since you are essentially drawing a profile this time, it is crucial to take into account whether samples are from east of the EPR or west of the EPR (draw the profile as if you are looking north - so east should be on the right-hand-side of your graph). See Plot Does this plot support the seafloor spreading theory? (give reasons) The plot does support the theory of seafloor spreading, since the sediment thickness increases away from the ridge crest, as there has been more time for the sediment to accumulate.

3

3500

Q1(a)

Average spreading rate 9.33 cm/year

Distance from East Pacific Rise (km)

3000

2500

2000

1500

Pacific Basin 1000

500

0 0

5

10

15

20

25

30

35

40

Age (Ma) The spreading rate has slowed with time as can be seen from the graph, the rate is only ~5.75 cm/yr in the last 10 Ma.

Sediment thickness (m)

Q1(b)

400

300 200 100

-3500

WEST

-3000

-2500

-2000

-1500

-1000

-500

Distance from East Pacific Rise (km)

0

500

1000

EAST

The above plot does support the theory of seafloor spreading as the sediment thickness increases away from the ridge crest, since there has been more time for the sediment to accumulate.

Pacific Plate

Cocos Plate 79

77

83

82

81

Ri

se

Galapagos Islands

Nazca Plate

km

e

Tr en c

h

East

1000

h il -C

Pa ci

f ic

ru Pe

0

Figure 2. Sites of Glomar Challenger deep sea samples in the eastern Pacific 180°

160°E

160°W

Meiji Seamount A L E UT I A N

I SL

AN

Ale

140°W

120°W

DS

n u ti a

T r en

60°N

NORTH AMERICA

ch

EMPER

50°N

AMO OR SE

Suiko

P

A

C

I

F

I

C 40°N

UNTS

Koko

O

Kanmu

C

E

A

N 30°N

Midway HA

Hawaiian - Emperor Bend

WA

IIA

N

RID

Hawaii

20°N

GE

0

1000

2000

kilom etres

Figure 3. Map of the northern Pacific showing the Hawaiian Island and Emperor Seamount chains. 4

10°N

Part B: Ocean Island (Hotspot) Chains You are given a map (Figure 3) of the North Pacific Ocean with the location shown for the Hawaiian Chain of islands and seamounts, and the Emperor Chain of seamounts. You are provided with a table of the age of either island basalt, basalt dredged from the top of the seamount, or the oldest sediments found directly above basalt. You are also given the distance from Kilauea, the active volcano on the big (most south-eastern) island of Hawaii. Note that the average age of the oceanic crust around the HawaiianEmperor Chain is 100 Ma. Both the Hawaiian and Emperor Chains are oriented approximately perpendicular to the magnetic anomaly patterns being generated at the East Pacific Rise but are at a slight angle to the anomaly patterns in their region.

Q2. For the Hawaiian Chain: (a)

Plot the seamount/island average distance versus age (table 2) from the hot spot under Kilauea. Use the Kilauea value as 0,0. Plot age as the X-axis and distance as the Y-axis.

(b)

Construct a line of best fit for age versus distance, and from this line calculate the average rate of migration (in cm/yr) away from the present location of Kilauea. Calculation from line of best fit: ~10 cm/yr

(c)

In which direction was this migration? Measure this with a protractor or estimate by eye. Use a line of best fit through the Hawaiian Chain on your map and express your answer as a compass bearing (e.g. 270E). Migration direction is 290 E

(d)

What is the age of the Hawaiian-Emperor Bend? (Note that you have already calculated the rate, so if you measure the distance you can calculate the age as distance/rate) The calculation shows that the age is ~35 Ma

(e)

Approximately how long did it take to make a giant shield volcano like one of the Hawaiian islands? (Hint: Divide the total age of the chain [from (d) above] by the total number of islands/seamounts in the chain, as counted from the map in Figure 3). 18 shields in 35 Ma so ~ 2 Ma for each shield.

General 3. (a) What causes chains of seamounts? Oceanic crustal plates moving over stationary hotspots. (b) What can you conclude from the change in migration direction and rate at the HawaiianEmperor bend? When did this change occur? Prior to ~35 Ma, the Pacific spreading centre must have been in an orientation of ~080E and been situated to the south of the of the bend itself.

4.

(a) Does the migration rate of the hotspot match the rate of spreading for the Pacific Rise Plate away from the EPR as calculated in Q1a? Over the time represented on the plot, the spreading rate of the EPR is about 1 cm/yr lower than the migration rate of the hotspot, although the two may be within error. (b) What set of circumstances would allow the spreading rate to be different to the migration rate of the hotspot? The hot spot migration rate measures the absolute motion of the plate, so if the ridge itself was moving, this would be added to or subtracted from the spreading rate. This may be the case particularly in the last 10 Ma where the slower spreading rate is apparently slower, but the migration rate over the hot spot remains the same.

5

Table 2. Summary of K-Ar age data for the shield building phase of volcanoes of the Hawaiian Islands. Volcano

Distance from Kilauea (km)

Average age of shield building (Ma)

Kilauea

0

0

Kohala

90

0A38

Haleakala

180

0A84

West Maui

220

1A28

Lanai

235

1A30

East Molokai

255

1.48

West Molokai

280

1.84

Koolau (Oahu)

360

2A25

Waianae (Oahu)

375

3A24

Rauai

530

4A43

Nihoa

800

7A0

Necker

1080

10A0

French Frigate Shoals

1240

11A7

Pearl & Hermes Reef

2260

20A1

Midway

2430

27A0

Unnamed seamount

2585

27A3

Unnamed seamount

2820

26A7

6

Q2 (a)

Distance from Kilauea (km)

4000

3000

2000

1000

0 0

10

20

30

Average age of volcanic centre (Ma)

(b) Average migration rate: 100.56 km/Ma

10.06 cm/yr

(c) Direction of Migration 290° (d) Age of Hawaiian-Emperor bend 34.6 Ma (e) Average age of shield building 1.5 - 2.0 Ma (depending on how many “shields” you count).

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