Emplacement Temperatures of Boiling-Over Pyroclastic Density Currents from Tungurahua and Cotopaxi Volcanoes, EcuadorGP23A-1039

Figure 2. Tungurahua volcano, 5023 m, is one of Ecuador’s most active stratovolcanoes. Sequences of lava flows, PDCs and ash fall are followed by sector collapse over a span of ~15,000 years. The most recent activity began in 1999 and continues today with explosions, ash plumes, lava flows and boiling-over PDCs. Unexplained as-pects of these flows include odd socriacious flows, bread-crust bombs up to 2m in length, and evidence of cool em-placement temperatures (Te), like unscorched wood.

1. University of Idaho 2. University of Texas Dallas 3. Georgia Institute of Technology

South America

Ecuador

1000 Km

0o Equator

2o South

4o South

QuitoCotopaxi Volcano

Tungurahua Volcano

Figure 1. Ecuador is located in the Northern Andes. Tungurahua and Cotopaxi are located in the eastern Cordillera of Ecuador. Both volcanoes have similarly interesting deposits from pyroclastic density currents (PDCs).

Erika L. Rader1; Dennis Geist1; John W. Geissman2; Karen S. Harpp1; Josef Dufek3

Figure 6. One lithic and one juvenile sample were col-lected at each site labeled on the image above. While most of the lithic clasts give flow temperatures of 90oC, there were two outliers. Sample Tu-01 was emplaced at ~210oC and Tu-02 was emplaced above 590oC.

Tu-02

1 Mile

Tu-08Tu-07

Tu-10

Tu-05Tu-04Tu-03Tu-01

Tu-06

N

Figure 7. Samples Co-03 and Co-06 are examples of lahar deposits where both juvenile and lithic clasts give a Te of < 90oC. This illustrates the importance of sampling both juvenile and lithic fragments on volca-noes that produce boiling over pyroclastic flows as well as lahars.

Co-06

Co-09

Co-08

Co-07

Co-02

Co-01

Co-03

Co-04Co-05

1 Mile

N

Figure 3. Cotopaxi volcano is 5,897 meters high, with a circular glacial cap. Previous eruptions melted the glacier and resulted in numerous voluminous lahars which have traveled up to 326 km. The most recent eruptive activity occurred in 1877 with a large number of boiling-over PDCs, lava flows, and lahars. Some 1877 PDCs mixed with rain or melt water morphing into lahars, making deposits difficult to interpret. Modern activity includes seismicity and gas emission.

Juveniles with Te > 590oC cannot be lahar deposits

Figure 4. Deposits were sampled longitudinally with an oriented juvenile and lithic clast. Cores were taken, thermally demagnetized and analyzed at U of New Mexico. Thermoremnant paleomagnetic directions of most juve-nile clasts give a Te of >570oC. This is consistent with the observations listed in Fig. 11.

Figure 5. Data from the lithic clasts contrasts the pattern of the juvenile clasts, with a wide scatter of paleomagnetic directions. This indicates no heating above 90oC after they were incorporated into the PDC. The few clasts with coherent demagnetization patterns indicate that some were heated to 210-590oC (Fig. 9, 13).

Juvenile Clasts

Far from star = Cold

B. TungurahuaA. Cotopaxi

95 - 7.1Dec: 10.0Inc: 21.1N: 18

95 - 10.0Dec: 11.0Inc: 18.9N: 19

Lahar

B. TungurahuaA. Cotopaxi

95 - 17.4Dec: 10.4Inc: 22.5N: 4

Lithic ClastsClustered around star = Hot

Expected geomagnetic field at time of emplacement = or

Boiling-over pyroclastic flows typically are cool, between 90-210oC Drooping Juvenile

Figure 11. The majority, 16 out of 18, of the sampled flow deposits contain drooping, cauliflower-textured juvenile clasts (right) and have paleomagnetic properties that record a Te above 590oC. In contrast, lithic clasts (above) were not remagnetized at the time of emplacement. These deposits must have been emplaced below 90oC.

1

0

0.8

0.6

0.4

0.2

100 200 300 400 500 600 7000Temperature oC

M/Mmax

N

W

S

EMmax = 17.0e-3 A/m

Co-10-09 Lithic

575oC

Te

Juvenile ClastLithic Clast

1000oC 500oC 100oC

Matrix

Cooling During Transport

Lithic clasts from other 16 deposits indicate a Te of <90oC

Figure 12. On the basis of the observations reported above, we propose that the boiling-over mechanism produces an ashy flow that cools during transport, owing to the incorporation of cold air. The temperature distribution is highly heterogeneous: juvenile clasts remain at nearly magmatic temperatures, where-as the bulk flow (including ashy matrix and lithic clasts) is relatively cool.

12

3

Rim

Core

Figure 9. The magnetic intensity and paleomagnetic direction of each sub-sample was measured after each subsequent heating step, which ranged be-tween 50-80oC, starting at 25oC and ending at 610oC. This clast shows a change in the paleomagenetic data at 210oC, which when modeled, translates into a deposition temperature of 230oC (see Fig. 10).

1

0

0.8

0.6

0.4

0.2

100 200 300 400 500 600 7000Temperature oC

M/Mmax210oC

Mmax = 22.8e-3 A/m

Tu-10-01 LithicN

W

S

E490oC

Re-heated lithic clast yields a Te of 210oC for one deposit.

Figure 8. Clasts were sampled in a core perpendicular to the surface of the rock to assess the thermal history of the entire clast.

0 5 15140

180

200

240

Time (hours)

Temp

(o C)

8 cm5 cm1 cm Core10

Rim

Run at 230oC

220

160

Figure 10. Numerical modeling of the conduc-tion of heat from the matrix of the flow to a lithic clast gives the temperature of the flow required to achieve the temperature recorded by the paleomagnetism.

1

0

0.8

0.6

0.4

0.2

100 200 300 400 500 600 7000Temperature oC

M/Mmax

N

W

S

E

Mmax = 37.5e-3 A/m

Tu-10-02 Lithic

575oC

1

0

0.8

0.6

0.4

0.2

100 200 300 400 500 600 7000Temperature oC

M/Mmax

N

W

S

EMmax = 30.3e-3 A/m

Tu-10-06 Juvanile

610oC

10 cmFigure 13. The cool Te of flows at both vol-canoes points towards similar emplacement mechanisms. We believe that the common mechanism is related to boiling-over foun-tains, the abundant loose material on the flanks of the volcanoes, and the steep slopes over which the currents travel.

Eighteen lithic and juvenile clasts from Cotopaxi and Tungurahua vol-canoes were thermally demagnetized to obtain emplacement temperatures of boiling-over pyroclastic density currents (PDC). We found that:

1. Most lithic samples have cold (<90oC) emplacement temperatures.

2. Most juvenile clasts have high (>590oC) emplacement temperatures.

3. This dichotomy suggests strongly heterogeneous temperatures at the time of deposition: the bulk of the flow is cool, but some fragments are very hot. Models of flows that average the temperature throughout the flow might be missing some of their most crucial properties.

4. Boiling-over PDC deposits can be distinguished from other types of PDC deposits by low emplacement temperatures of the lithic clasts, but can be distinguished from lahar de-posits by juvenile clast emplacement above 590oC.

Summary

10

0

8

6

4

2

100 200 300 400 500 600 7000Temperature oC

12

14

16

18

Num

ber of D

epos

its S

ampled

TungurahuaCotopaxi