Two rock magnetic methods can be used to determine the emplacement temperature of pyroclastic deposits. The first is by looking at the unblocking temperature spectra of the thermoremanent magnetization (TRM) and the second is through the repeatability of thermomagnetic behavior. Chachimbiro volcanic complex is an andesitic-dacitic stratovolcano located at the northern zone of the Ecuadorian volcanic arc. The lateral blast eruption that occurred at 3640-3510 BC originated from a ∼650 m wide and ∼225 m high rhyodacite dome. This satellite lava dome, located ∼6 km to the east of the main vent, erupted, resulting in a large pyroclastic density current (PDC). PDCs are hot mixtures of lithic fragments, gas and pumice, varying in size from fine ash up to metric blocks that descend the flanks of a volcano at great speeds, being the primary cause of death during explosive eruptions. The resulting PDC from this violent laterally directed explosion covered an area of 62 km^2, with the thickest parts of the deposit displaying as much as 15 m. We collected ~63 oriented block samples from 6 locations; their distances varying between 1.8 km to 6.7 km away from the source. Here we present the emplacement temperatures of the Chachimbiro pyroclastic deposits and the potential factors controlling them. Our rock-magnetic results indicate low titanium Ti-magnetite as the main magnetization carrier; maghemite being present in trace amounts. We have recognized that, based on the unblocking analysis of the TRM, the overall temperatures vary from 250 °C to 450 °C depending on the clast size and type. In general, our results suggest a minimum temperature of ~250 °C, with a large portion of the juvenile clasts having temperatures up to about ~450 °C. Furthermore, the analysis and the comparison of the Curie temperature executed in ~30 samples, against the emplacement temperatures obtained through the typical paleomagnetic studies will be presented. This work highlights the usefulness of paleomagnetism and rock magnetism to evaluate the emplacement temperatures of PDCs, thereby allowing to better assess the associated risk.

Data of the Earth’s magnetic field strength is diversely applicable from dating archeological artefacts or lava flows to understanding early earth evolution and the mechanisms of the geodynamo. Lava flows are commonly used as a means to obtain records of this paleointensity. Understanding the underlying paleomagnetic and rock magnetic properties and how they vary across a flow is crucial to ensure collection of good quality samples for analysis. In fact, the success of paleointensity as well as paleomagnetic analysis is strongly dependent on the rock-magnetic properties of the samples, and large variations may exist between samples even of the same unit, related mainly to varying cooling rates. The active Tungurahua volcano is one of the most prominent features in the Ecuadorian Eastern Cordillera with numerous basaltic andesite and andesitic lava flows exposed along its flanks. To appraise the relation between volcanic emplacement processes and rock-magnetic properties, we sampled a vertical transect in a ~20 m thick lava flow at Tungurahua volcano. A total of 55 oriented in situ sample from six sites distributed across Ulba Cascada lava flow were collected for this purpose. We will present petrographical analysis of each sample as a function of the depth within the lava flow, with an emphasis on the textural and structural characteristics of magnetic minerals as observed with transmitted, reflected light and scanning electron microscopes. Moreover, a detailed analysis of rock-magnetic properties such as magnetic susceptibility, hysteresis (remanent magnetization, saturation remanent magnetization, coercivity and back-field coercivity), accompanied with detailed stepwise alternating field and thermal demagnetizations will allow us to determine the direction of the magnetic field and assess the variations of magnetic properties with respect to the position within a lava flow. We will also discuss the correlations that may exist between grain size, oxidation state of the magnetic minerals and the emplacement processes of a lava flow, as well as the implications of all these results to paleointensity determinations.

Geophysical surveys are efficient ways to obtain information on areas that are promising for geothermal energy. One of the geophysical techniques commonly used is the magnetic method, which is useful to detect shallow structures and changes in magnetization due to processes related to geothermal activity, such as faulting and hydrothermal alterations. Despite the richness of available geothermal resources in South America and Ecuador, the use of these resources for electricity production is very limited. Chachimbiro, in northern Ecuador, is one of the potential sites for developing a geothermal power plant. Our objective is to provide complementary magnetometry data to improve the existing model of the geothermal area. We performed high resolution ground magnetometry survey of ~30 m spacing around the prospective drilling area in order to better understand the shallow structures above the reservoir. We also performed two additional survey lines with ~5 m spacing across possible fault locations. After necessary data reductions the magnetic anomaly map was compared with a digital elevation model and a geological map of the area. This helped to understand the distribution of the anomalies and their relation with the presence of high magnetic susceptibility materials, hydrothermal alterations and topography. Major anomalies observed in the magnetic profiles were compared with forward fault models, allowing us to distinguish topographic from fault effects. We then compared our new magnetometry results with previous geophysical models of the Chachimbiro geothermal system. The large long-wavelength negative anomaly on the Northeast side of the survey area seems to coincide with the suggested location of the clay cap, and can therefore be used to improve the existing models. The new magnetic exploration of Chachimbiro therefore shows the usefulness of this method to locate magnetic anomalies related to faulting and hydrothermal alterations.

Geophysical methods are very useful in archeological prospection by providing an inexpensive, non-invasive view of the subsurface, and, helping the archeologists to better target their excavation efforts. Ecuador’s past is very rich, with many archeological sites still unexplored. Manteño culture prevailed in the province of Manabí in a series of large coastal towns and along the river valleys and ridges of the Chongón-Colonche coastal mountains of what is now Ecuador from around 500 CE to 1532 CE. They were one of the last prehistoric cultures and the Inca Empire never conquered them directly, which meant their culture developed independently. Thousands of carefully arranged stone foundation have been documented across the abrupt landscape that has been intentionally modified for large scale agriculture. In this work, we present the results of Electrical Resistivity Tomography (ERT) and magnetometry surveys at the Río Blanco archeological area close to the coastal city of Puerto Lopez. The area includes one of the largest unexcavated archeological remains known in Ecuador. It consists of alluvial terraces modified by the Manteño people scattered with numerous ruins. The archeological structures are often delimited by buried rock blocks that sometimes outcrop in the surface. We made 2 ½ D ERT with dipole-dipole array configuration and ground magnetometry surveys at three locations which were identified earlier by archeologists as buried buildings, with one of them being previously partially excavated. The measurement grid for each structure was designed according to their size. For magnetometry, a base station measurement was taken after finishing each survey line in order to be able to remove diurnal variations from the magnetometry readings. All tested structures showed internal variations within them related to differences in electrical resistivity and magnetic susceptibility. According to our preliminary interpretation, some of these anomalies are from the wall rocks and some suggest the presence of buried objects as well as potential locations of fireplaces. The locations of the buried objects are intended to be later verified by archeological excavation.