IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

0A2_Key-01

Houzan Hall

Date/Time: July 20 11:30-12:15

Andesites: their origin and the role in the Earth evolution
Yoshiyuki Tatsumi
Department of Earth and Planetary Sciences. Kobe University
andInstitute for Research on Earth Evolution, JAMSTEC
E-mail: tatsumi@diamond.kobe-u.ac.jp

One characteristic feature of the planet Earth is the bimodal height distribution at the surface (Fig. 1). Thisis caused by the difference both in density and thickness of the Earth’s crust. Two types of crusts, the oceanicand continental crusts, have been created at divergent and convergent plate boundaries, respectively, via.
plate tectonics. The bulk composition of continental crust is andesitic (60 wt.% SiO 2), in marked contrast withthe basaltic oceanic crust with ˜50 wt.% SiO2. This raises the question of how intermediate continental crust
forms from basaltic magmas produced in the mantle wedge of subduction zones.
The Izu-Bonin-Mariana (IBM) arc is a juvenile intra-oceanic arc and has a thick middle crust layer with Vpof ~6km/s, suggesting this arc to be an active site of both creation and growth of the continental crust.
Petrological modeling, including remelting of the initial basaltic arc crust and magma mixing between mantleand crust-derived melts, can successfully explain the layered crust-mantle seismic structure of the IBM. During
this process, the sub-arc Moho is chemical transparent and permeable to the refractory melting residue of arc
crust. This crust-mantle transformation or discharge of the ‘anti-continent’ could play the major role in thecreation of intermediate continental crust.
The continental crust in detail possesses compositions typical of calc-alkalic series that often coexists withtholeiitic series in a single volcano. Resolution of the genetic relationship between these two types of andesitic
magmas should, therefore, provide a better understanding of andesite genesis and arc crust evolution.
Reexamination of petrographical and geochemical characteristics of these two magma series in the NE Japanarc provides a new insight into andesite genesis: (1) tholeiitic magmas are produced via anatexis ofamphibolitic crust caused by underplating and/or intrusion of mantle-derived calc-alkalic basalt magmas into
the arc crust, and (2) The mantle-derived calc-alkalic basalt magma mixes with crust-derived tholeiitic melts toform calc-alkalic andesite magmas.
The essential cause of operation of plate tectonics that creates both andesitic continental crust andbasaltic oceanic crust and results in the bimodal height distribution is the temperature difference within themantle and mantle convection. The upper thermal boundary layer of this convection corresponds to thelithospheric plate, but behaves naturally as a stagnant-lid, i.e., the plate should not move. The presence ofliquid water at the surface, on the other hand, strongly reduces the yield strength and could cause the fracture
within the stagnant-lid, triggering the plate subsidence or subduction. Parameters that govern the presence
and absence of liquid water on the terrestrial planets are: the distance from the Sun and the mass of aplanet. A conclusion of this consideration would be that the Earth is a shore planet because of the presence ofthe ocean, which seems to be a conclusion of a Zen dialog.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

1

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

0A2_Key-01

Houzan Hall

Date/Time: July 20 11:30-12:15

Figure 1. Hypsometry of surface tomography of the terrestrialplanets.

©IAVCEI 2013 Scientific Assembly. All rights reserved.

2

IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

0A2_Key-02

Houzan Hall

Date/Time: July 20 12:15-13:00

Forecasting volcanic activity of Sakurajima
Masato Iguchi
Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University
E-mail: iguchi@svo.dpri.kyoto-u.ac.jp
thSakurajima is an andesitic volcano located on the southern rim of the Aira caldera. In the 20 century, threecharacteristic types of eruptions occurred: the 1914 gigantic flank eruption; the 1946 minor flank eruption; andfrequent vulcanian eruptions at the Minamidake summit crater, continuing from 1955 for more than 50 years.
The eruptive center has been shifted to the Showa crater, east of Minamidake since 2006 and the eruptive
activity is characterized minor but more frequent vulcanian eruptions. The magma plumbing system of
Sakurajima is approximated by a deep (10 km) magma reservoir beneath the Aira caldera, shallowreservoirs (3–6 km) beneath the central cones, and a conduit connecting one of the shallow reservoir to thesummit crater of Minamidake. From the inflation–deflation pattern and intensity of the vertical displacement, it73

is estimated that magma has been supplied to the deep reservoir at a rate of 10 m /year. Since the93termination of the 1914 eruption, magma in the order of 10 m has been stored in the deep reservoir.
Considering eruptions in 20th century, we have 3 scenarios for future eruptive activity: 1) frequent occurrenceof vulcanian eruptions at the summit crater, 2) further more increase in eruptive activity at the Showa crater,
finally reaching effusion of lava, and 3) a large flank eruption at two sides of the volcano.
The shallow reservoir is inferred from vertical displacements, a tilt vector showing the crater side up before
the vulcanian eruptions, and an anomalously attenuated zone of seismic waves. The magma migrated fromthe deep reservoir to the shallow reservoir before the increase in vulcanian eruptivity at the Minamidake crater,
as demonstrated by the relocation of the upheaval center of the ground deformation. The increase inseismicity of A-type earthquakes and the migration of their hypocenters toward the shallow part also support
the migration of magma. The hypocenters of B-type earthquakes and explosion earthquakes with volumetricsources are concentrated beneath the crater. The hypocenters of A-type earthquakes generated by shear
fractures surround the origins of B-type earthquakes and explosion earthquakes. The separation of thehypocenters indicates the existence of a volcanic conduit connecting the shallow reservoir to the bottom of
the summit crater. The intrusion of magma into the conduit and smooth ascent up to the crater bottom induce
strombolian eruptions and swarms of B-type earthquakes. The magma at the uppermost part of the conduit
becomes a cap rock of the conduit against the following intrusive magma, which generates vulcanianeruptions. Upward tilt of the crater side and extensional strain are observed prior to both strombolian activities
and the vulcanian activities that follow and turn into downward and contraction strain, respectively, associatedwith the eruptions. The volatility of the magma has an important role in vulcanian eruptions, in the formation ofa gas pocket at the uppermost part of the conduit, and in sudden outgassing triggered by a pressure decreasein the conduit due to gas leakage.
Eruptive activity at the Showa crater has steadily increased since it resumed in June 2006, and 3660

©IAVCEI 2013 Scientific Assembly. All rights reserved.

3IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

0A2_Key-02

Houzan Hall

Date/Time: July 20 12:15-13:00

vulcanian eruptions occurred during the period from 2008 to 2012. An inflation event that started in October
2009 was caused by the major pressure source estimated to be located at a depth of 12 km beneath the Airacaldera and a minor source obtained a depth of 5 km at the northern flank of Kitadake in addition to a source
beneath Minamidake. Strain changes which indicate inflation were detected prior to explosions and theinflation strain lasted mostly 1 h. The strain changes were caused by a shallow pressure source less than 1.5km. The inflation occasionally continued for more than 7 h with an addition of inflation of a deep source (4 km),which corresponds to the magma reservoir beneath Minamidake. The conduit to the Showa crater may bebranched from the magma reservoir beneath Minamidake or from the major conduit connected to it. When
inflationary ground deformation progressed at a high rate, the eruptive activity reached a peak fromDecember 2009 to March 2010. This suggests that the accumulation of magma beneath the central cones of
the Sakurajima volcano progressed simultaneously to a discharge of magma. The simultaneous progress of
the accumulation and discharge of magma and the frequent occurrence of small vulcanian eruptions may berelated to the small open conduit.

Sakurajima volcano in magma intrusion stage
Increase in HCl/SO4 ratio in watersoluble content of volcanic ash
Increase in SO2 discharge rate
Minamidake

Increase in volcanic ash
Showa crater

Kitadake
Inflation of ground
Increase in underground CO2
Change of structure

Ejection of apart of magma

Magma reservoir beneath Minamidake

Magma reservoir beneath Kitadake

Accumulation of apart of magma
Magma reservoir
beneath the Airacaldera
Intrusion of basaltic magma©IAVCEI 2013 Scientific Assembly. All rights reserved.

4