GEORG Geothermal Workshop 2016 Presentation title: A conceptual model of the Krafla volcanic geothermal system

1. Suggested conceptual model
for Krafla
GGW2016 workshop, November 25., 2016
Knútur Árnason
ISOR

2. Krafla

3. Geological map

4. Faults and fissures (blue)
Caldera,110000 yr. BP,
(black hedged lines)
Eruptive fissures and craters
(yellow)
Geothermal surface mani-
festations (red)

5. S-wave shadows
Observed during
during Krafla
Fires (1975-1984)
between 3 and
7 km depth
(green lines)

6. Faults mowed
in Krafla fires
Boundary faults in Krafla Fires
Bimodal tectonic
setting
Hvannstóð branch
Active 8-3 ky BP
Eastern branch
Active before 8 ky BP
and again since 3 ky BP

7. Buried inner
caldera (80 ky BP)
Buried trans-
secting “graben” ?
Bouguer
Gravity map

8. N S
(Ármannsson et.al. 1987)
N-S lithological section

9. Resistivity 200 m a.sl. (1D inv. TEM)
Resistive
core
High T
(> 240°C)
alteration
Mostly conf.
within inner
caldera

10. Drilled wells in
Krafla
(Location of section
on next slide)

11. Resistivity, alteration and temperature
_
_
_
_
_
l
l l l l l
500
m a.s.l
0
0 1000 2000 3000 4000 5000m
- 500
- 1000
- 1500
KG-8
KG-10
KJ-15
KJ-19
KJ-20
KJ-16
KJ-18
Caldera
rim
Alteration
Resistivity
> 100 m
10 - 100 m
1 - 6 m low resistivity cap
High resistivity core
Ω
Ω
Ω
250
TEM sounding
Temperature °C
Unaltered rocks
Smectite - zeolite zone
Mixed layered clay zone
Chlorite zone
Chlorite-epidote zone
200
250
300
KraflaHveragil
Cooling

12. Different thermal character

13. The role of permeability

14. 3D inversion
of MT
S-wave shadows
(purple lines)
Sesmicity
(grey dots)

15. IDDP-01

16. 3D inversion of MT at Krafla

17. 3 km/s
P-wave velocity structure of Krafla
(Brandsdóttir et al., 1997)

18. Recent seismic tomography
(Schuler et al., J. Geophys. Res., in press)
Caldera
IDDP-1
S-wave
shadows
Just earthquakes
Earthquakes and
refraction

19. Heat sources
Gravity and extend of
high temperature alteration
at 200 m a.sl. (200 m depth)
Intrusions north of the
graben (WNW-ESE and
NNE-SSW dykes?)

20. ~E4°S
~E22°S
18°
d
d c
c = 0.31*d
Closer look at the
fissure swarm
Different spreading
directions N and S
of Krafla
Leads to NNE-SSW
spreading component
in Krafla

21. Conceptual model
• Repeated dike intrusions within the inner caldera and
to the north of the buried “graben”
• Not only NNE-SSW dikes. The extensional component
along the fissure swarm favours WNW-ESE dikes
• During the 5 ky (8 to 3 ky BP) a two phase geothermal
system developed in the northern part of the inner caldera
• When the spreading moved back to the eastern part,
permeability (along the fissure swarm) increased
dramatically at shallow depth west of Hveragil and
the upper isothermal system developed
• Repeated dike intrusions maintained suitable permeability
for the deeper two phase system
• Permeability did not increase much east of Hveragil,
hence two phase system there

22. Conceptual model (cont.)
• Intrusions (maybe at the end of glaciation10 ky BP), at
the SW rim of the inner caldera also produced a geothermal
system
• Repeated intrusions have not maintained the heat sources
at Hvíthólar. Just east of the active rift, the system is in
its final stage with two phase conditions at shallow depth
but temperature reversal below
• Further to the west, within the presently active rift,
extensive cooling has taken place
• 3D inversion of MT suggests partially molten rocks at the
edges of the S-wave shadows
• WNW-ESE dike injected in the southern part of the dike
complex in Mývatn Fires and the northern part in
Krafla fires ?

23. The role of re-melt of altered basaltic rocks
in heat transfer from basaltic intrusions
• Some evolved central volcanos, like Krafla and Askja, show
bimodal eruptive behaviour. For long periods they produce
basaltic magma, but occasionally they produce more silicic
magma or pure rhyolite.
• Isotopes show that the rhyolite has not differentiated form
primitive melt but is a re-melt of (epidote, amphibole)
altered basaltic rocks.
• Hot (~1100 °C) basaltic intrusion partially melts the altered
basalt producing hot silicic melt rich in volatiles.
• The melt has low viscosity and is buoyant and migrates
upwards

24. The role of re-melt (cont.)
• The rhyolite melt cools and degases as it migrates up
• Viscosity increases
• Can be trapped by minor structural obstacles at
relatively shallow depth (~2 km)
• Rhyolite melt has been encountered at least in two
wells in Krafla at 2-2.5 km depth (K-39 and IDDP-01)
and also in the Menengai caldera in Kenya.
• The shallow emplaced rhyolite produces a zone
of superheated steam
• Re-melted rhyolite transports heat
from deep basaltic intrusions to
shallow depths