No abstract

We studied the tephra inventory of 14 deep sea drill sites of three Deep Sea Drilling Project and Ocean Drilling Program legs drilled offshore Guatemala and El Salvador (Legs 67, 84, and 138) and one leg offshore Mexico (Leg 66). Marine tephra layers reach back from the Miocene to the Holocene. We identified 223 primary ash beds and correlated these between the drill sites, with regions along the volcanic arcs, and to specific eruptions known from land. In total, 24 correlations were established between marine tephra layers and to well‐known Quaternary eruptions from El Salvador and Guatemala. Additional 25 tephra layers were correlated between marine sites. Another 108 single ash layers have been assigned to source areas on land resulting in a total of 157 single eruptive events. Tephra layer correlations to independently dated terrestrial deposits provide new time markers and help to improve or confirm age models of the respective drill sites. Applying the respective sedimentation rates derived from the age models, we calculated ages for all marine ash beds. Hence, we also obtained new age estimates for eight known but so far undated large terrestrial eruptions. Furthermore, this enables us to study the temporal evolution of explosive eruptions along the arc, and we discovered five pulses of increased activity: (1) a pulse during the Quaternary, (2) a Pliocene pulse between 6 and 3 Ma, (3) a Late Miocene pulse between 10 and 7 Ma, (4) a Middle Miocene pulse between 17 and 11 Ma, and (5) an Early Miocene pulse (ca. >21 Ma).

The Cão Grande Formation (CGF) on the western plateau of Santo Antão Island is part of the younger volcanic sequence that originated from both, basanitic and nephelinitic magmatic suites, respectively called COVA and COROA suites. Based on our detailed revised stratigraphy of the CGF, including two yet unknown tephra units, we can show that both suites produced multiple, highly differentiated eruptions over a contemporaneous period.Correlations of CGF tephras with marine ash layers provide distal dispersal data for Cão Grande I (CG I) and also identify two highly explosive, phonolitic eruptions that pre-date the A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT2 CGF tephra deposits known on land. Within the CGF, the lowermost, 220±7 ka old unit Canudo Tephra (CT; COVA suite) comprises phonolitic fall deposits and ignimbrites; it is partly eroded and overlain by debris flow deposits marking a hiatus in highly differentiated eruptions. The phonolitic CG I Tephra (COROA suite) consists of an initial major plinian fall deposit and associated ignimbrite and terminal surge deposits. This is immediately overlain by the phonolitic to phono-tephritic Cão Grande II (CG II; COVA suite), a complex succession of numerous fallout layers and density-current deposits. CG I and CG II have radiometric ages of 106±3 ka and 107±15 ka, respectively, that are identical within their error limits. The youngest CGF unit, the Furninha Tephra (FT; COROA suite), consists of three foidic-phonolitic fall deposits interbedded with proximal scoria deposits from a different vent.The phonolitic eruptions switched to and fro between both magmatic suites, in each case with a stronger first followed by a weaker second eruption. Each eruption evolved from stable to unstable eruption columns. During their terminal phases, both magma systems also leaked evolved dome-forming lavas next to the tephras. Distal ashes increase the CG I tephra volume to ~10 km 3 , about twice the previously published estimate. The tephra volume of CG II is ~3 km 3 ; CT and FT are too poorly exposed for volume estimation. The characteristics of the CGF tephra units outline hazard conditions that may be expected from future evolved explosive eruptions on the western plateau of Santo Antão.

No abstract

The rocks on the surface of the earth have different formation process, beginning with the lava of volcanic activity that flow on the surface of the earth and then cool down forming igneous rocks. Rocks derived by volcanic eruptions contain different types of minerals than those formed by sedimentary or metamorphic processes. The aim of study was to evaluate the relation of magnetic susceptibility with the elemental composition of rock and soil around the Danau Diatas. Magnetic susceptibility was measured using Bartington susceptibility meter MS2B and the element composition has been measured by X-Ray Fluorescence. The values of magnetic susceptibility of low frequency (χlf) obtained for rock are 2332×10−8 m3/Kg, 2161,7×10−8 m3/Kg, and 2791,6×10−8 m3/Kg, while the values of magnetic susceptibility of low frequency (χlf) obtained for soils are 802,4×10−8 m3/Kg, 779,8×10−8 m3/Kg, and 58,4×10−8 m3/Kg. Rock and soil samples contain several elements such as Fe, Mn and Ti, with Fe having the highest concentration and thus the highest potential influence on their magnetic susceptibility.