How do we know that the Earth has a core?

A typical depiction of the
outer (red) and inner (yellow) core
of the Earth.
Most of us have wondered, at some point of our lives, why the cartoons depicting the Earth as a watermelon with a missing piece always show this ball in the center named the 'core'. How do we know that a distinct 'body' is actually down there, 2900 km below the surface?

Let's see: we know the total mass of the Earth through its gravitational interaction with the solar system. In 1797, Cavendish[1] measured the Gravitational constant G and the density of the Earth is ever since known to be about 5.51 times the density of water, nearly twice the average rock density in our planet's surface.

In 1898, Wiechert suggested[2] that this high Earth’s density could be explained by a core in the center made of nickel and iron (like many meteorites known at the time) surrounded by a shell or mantle of the lighter, silicon-dominated rocks that we see in the surface we inhabit.

Inge Lehmann was one of the key
discoverers of the inner core of the Earth
But only in 1906, Richard D. Oldham found that the increasing speed of seismic waves with depth within the Earth holds only down to 2890 km below the surface. Below that, mechanical waves propagate much slower, suggesting a different nature. Because this distinct material did not transmit shear seismic waves, it became clear that the core was liquid.

In 1936, Inge Lehmann found that the center of the core is indeed solid, or nearly-solid, since she detected weak shear waves travelling through it [4], using highly-sensitive seismometers recorded in New Zealand. This is known as the inner core.

Images of the tsunami following last week's earthquake in Chile.

Last week's earthquake in Chile provides a great opportunity for you to check if Oldham did everything right. You only need to get seismograms from seismic stations around the world (many of them have their data publicly available), and sort them according to the distance from the station to the EQ's epicenter, using the same time of reference, like in this image:
Left: Each horizontal line is a seismogram of the Chile earthquake recorded at different locations of the planet (check USGS: 2015-10-16; Mw=8.3). Each seismogram is plotted according to the distance of the measuring station to the earthquake (vertical axis). The red circle shows the signal gap due to the outer core.
Right: Same image, with the identification of the arrivals of the different seismic waves. 'P' waves are the compressional waves, they are first to arrive all around the planet's surface.
The horizontal axis shows elapsed time, measured since the EQ occurred.
The vertical axis shows the distance from the measuring station to the EQ.
The red circle shows the region (around 110 degrees from the source) where the first seismic waves are not recorded. 

Seismic shadow produced by an imaginary
earthquake occurring at the north pole. The
outer core, due to its slower seismic velocity,
refracts the mechanical waves of the earthquake,
shadowing a vast region of the planet, as seen
in the previous figure.
How the velocity of seismic waves changes with depth below the surface of the Earth.

In summary: the absence of wave reception in regions around 14,000 km (between 103 and 143 degrees) apart from the hypocenter demonstrates that there is a liquid core where seismic waves travel slow.
Isn't it amazing that nobody realized this before the 20th century?

Finally, remember that the outer core is where the magnetic field of the Earth is generated, by the thermal convection of conductive molten iron around a nearly-solid iron inner core. In fact the changes in the convection patterns in the outer core seem responsible for the rapid historical changes observed in the magnetic field. There is more about the magnetic field in this earlier post.

Convection in the iron-dominated outer core is
the most-accepted cause for the Earth's magnetic field.
Update: a new study finds that the core (and thus the magnetic field) was formed by the gradual cooling of the Earth only 1 to 1.5 billion years ago.

References (thank you nuclearplanet):
1. Cavendish, H., Experiments to determine the density of Earth. Philosophical Transactions of the Royal Society of London, 1798, 88, 469-479.
2. Wiechert, E., Über die Massenverteilung im Inneren der Erde. Nachr. K. Ges. Wiss. Goettingen, Math-Kl., 1897, 221-243.
3. Oldham, R. D., The constitution of the interior of the Earth as revealed by earthquakes. Q. T. Geol. Soc. Lond., 1906. 62, 459-486.
4. Lehmann, I., P'. Publ. Int. Geod. Geophys. Union, Assoc. Seismol., Ser. A, Trav. Sci., 1936, 14, 87-115.


Conferencias de divulgación geocientífica (50 aniversario del ICTJA)

Con motivo del 50 aniversario de nuestro instituto, cuatro investigadores del ICTJA participamos en el ciclo de conferencias divulgativas en Barcelona: "Las Ciencias de la Tierra en nuestra vida cotidiana", dentro del Cicle Dilluns de Ciència del CSIC-Catalunya.

Lugar: (mapa)
Sala d’Actes de la Residència d’Investigadors,

La conferencias de divulgación son los siguientes lunes:

2 Novembre, 18:30 h,
Charles Darwin, Lord Kelvin, els radioisòtops i el concepte de Temps
Dr. Santiago Giralt

9 Noviembre, 18:30 h
Tambora, 200 años de la erupción que cambió el Mundo
Dra. Adelina Geyer

16 Noviembre, 18:30 h
Megainundaciones, placas tectónicas y la formación del relieve terrestre
Dr. Daniel García-Castellanos

23 Novembre, 18:30 h
Interacció radiació-matèria per a estudiar-ho gairebé tot: nanomaterials, minerals exòtics, obres d’art, cadàvers,...
Dr. Jordi Ibáñez


Continental-scale evolution of topography and river networks. Tectonics and climate shaping Eurasia

[This post is about our recent publication on PLOS ONE]

How much does the erosion and sedimentation at the Earth’s surface influence on the patterns and distribution of tectonic deformation? This question has been mainly addressed from a computer modelling perspective, at scales ranging from local to orogenic. In the PLOS ONE paper published today, we present a model that aims at understanding this phenomenon at the continental scale, looking at the feedbacks between continental enlargement and climate aridification during the collision of continents.

60-million-year indentation of a continent from the south 
(at 50 mm/yr). 

Left: Topography and areas with precipitation
higher than 400 mm/yr (red shading). Note the 
orographic rainfall developing at the southern flank 
of the growing plateau. Wind blows from the southeast 
(towards the upper left corner).

Right: erosion and sedimentation rates, and
contours of crustal thickening rate due to tectonics. 

By Garcia-Castellanos & Jimenez-Munt, PLOS ONE, 2015. 
+ info here.

We couple a thin-sheet viscous model of continental deformation with a stream-power surface transport model. The model also incorporates flexural isostatic compensation that permits the formation of large sedimentary basins and a precipitation model that reproduces basic climatic effects such as continentality and orographic rainfall and rain shadow. We calculate the feedbacks between these four processes acting at different scales in a synthetic scenario inspired and scaled by the India-Asia collision. The model reproduces first-order characteristics of the growth of the Tibetan Plateau as a result of the Indian indentation.

Note that the southern continent (the indenter, India)
is chosen fixed to our reference frame, whereas
 the northern continent (Asia) is moving southwards). 

The initial topography is flat with small random noise
forming a network of lakes. 
The tectonic indenter in
the southern boundary represents India, while a rigid
block fixed around x=2500km represents the Tarim Basin. 

Wind blowing from SE at 7 m/s (relative humidity=1).
Red shading indicates orographic precipitation.

The continental deformation adopts a thin-sheet tectonic
+ info here.
What these simulations show is that, at large space and temporal scales, the climate dryness that develops in continental interiors triggers the trapping of sediment in closed basins within the continent, instead of exporting it to the continental margins. In the left panel you can see a large intramountain basin (comparable to the Tarim Basin) developing within Asia when a hard lithospheric region in predefined within the continent. The amount of sediment trapped in it is very sensitive to climatic parameters, particularly to evaporation, because it crucially determines its endorheic/exorheic drainage. We identify a feedback between erosion and crustal thickening leading locally to a <50 at="" climatically-enhanced="" concentrated.="" corners="" deformation="" flank="" growing="" in="" increase="" indenter="" is="" of="" orographic="" p="" place="" places="" plateau="" precipitation="" preferentially="" rates="" specially="" syntaxes="" takes="" the="" this="" upwind="" where="">
We hypothesize that this may provide clues for better understanding the mechanisms underlying the intriguing tectonic aneurysms documented in the Himalayas. At the continental scale, however, the overall distribution of topographic basins and ranges
seems insensitive to climatic factors, despite these do have important, sometimes counterintuitive effects on the amount of sediments trapped within the continent. The dry climatic conditions that naturally develop in the interior of the continent, for example, trigger large intra-continental sediment trapping at basins similar to the Tarim Basin because they determine its endorheic/exorheic drainage. These complex climatic-drainage-
tectonic interactions make the development of steady-state topography at the continental scale unlikely.


Erosion in northern Spain (Ebro Basin)

The previous post dealt with the erosion of the Ebro Basin after its colmatation with sediment, about 10 million years ago. The journal Geology has just chosen the following picture to illustrate the cover of their August volume:

Castildetierra is one of the many hills sculpted by erosion of the ancient
sediment infill of the Ebro Basin at Bardenas Reales (Navarra, Spain). 

Photo: Larrión & Pimoulier.
Location: 42.2103 N, 1.5157 W
The soft alluvial clays that make most of this hill are interbedded with harder lacustrine limestones and fluvial sandstones. The same alternation prevails over much of the Ebro Basin (NE Spain). These strata record a 19-million-year-old lake and alluvial system in the centre of an endorheic Ebro basin (84,000 km2 in area). Subsequent basin capture and drainage integration towards the Mediterranean lead to erosional features like the one in the picture.

Despite the most recent sedimentary has been removed by erosion, our study could date this major drainage change at 12.0-7.5 million years ago, based on isostatic modeling constrained with paleomagnetic data. In these badlands at Bardenas Reales (Navarra), high erosion rates have been measured in the order of millimeters per year, but how these rates link to the long-term history of the region is unclear. Other places in the basin show Pleistocene erosion rates in the order of 0.1-0.4 mm/yr, whereas our results suggest an average erosion rate since the Miocene of 0.05-0.1 mm/yr.

Summary of the scientific article in Geology: Basins formed within mountainous regions often become perfect sedimentary traps that do not drain to the sea but to internal evaporitic lakes. When they do, their sediment layers ideally record the climatic, topographic, and tectonic history of the surroundings. And when these basins eventually overtop or overfill with sediment, they are rapidly excavated by the new outflowing fluvial network, exposing excellent stratigraphic outcrops. However, this erosion often removes the uppermost basin infill, and essential information about the late basin history is lost. We have estimated the timing and elevation of the maximum infill of the Ebro basin (NE Spain) by computing the rebound of the basin in response to erosion, adopting the common idea that the Earth's rigid outer shell (the lithosphere) rests on a fluid magmatic asthenosphere in an Archimedes-type equilibrium (isostasy). We combine these calculations with existing paleomagnetic ages of the sediment basin infill. The results show that the basin became overfilled between 12 and 7.5 million years ago, and that it reached a maximum elevation of up to 750 m above present sea level. The basin has been ever since incised at a rate close to 0.1 mm/yr and has been isostatically uplifted by up to 630 m at its center. This uplift may explain why the Ebro River, opposite to other large Mediterranean rivers, does not present a deep gorge excavated within its own basin during the desiccation of the Mediterranean (Messinian salinity crisis, 5.5 million years ago).

Evolución topográfica de la Cuenca del Ebro

[Este post hace divulgación de un trabajo que acabamos de publicar en Geology
[This is outreach material about our own research, now published in Geology

Viajando entre Navarra y Lleida habrás reparado seguramente en las capas casi horizontales de sedimento, omnipresentes en la Cuenca del Ebro. Se trata de sedimento depositado en el fondo de lagos y en ríos durante el Mioceno (hace entre 24 y 5 millones de años), proveniente de la erosión del Pirineo y, en menor medida, del Sistema Ibérico y la Cordillera Costero-Catalana.

1. Vista desde la cima de San Caprasio (Zaragoza), en el centro de la Cuenca del Ebro (cubierta por las nubes). A la derecha aparecen los sedimentos calcáreos más modernos preservados en la cuenca, datados en 13.6 millones de años. Foto: DGC.
2. Bardenas Reales (Navarra). Foto: PN Bardenas.
Soft alluvial clays interbedded with harder lacustrine limestones and fluvial sandstones predominate over much of the Ebro Basin in NE Spain. In these badlands at Bardenas Reales (Castildetierra, Navarra), high erosion rates have been measured in the order of millimeters per year, but how these rates link to the long-term history of the basin is unclear. These strata record a 15-million-years-old lake and alluvial system in the centre of an endorheic Ebro basin (84,000 km2 in area). Subsequent basin capture and drainage integration towards the Mediterranean lead to erosional features like the one in the picture. Our study dates this major drainage change at 12.0-7.5 million years ago. Location: 42.2103 N, 1.5157 W
3. La roca calcárea en lo alto del
Cabezo de Castildetierra (Bardenas 
Reales, Navarra) apenas protege a 
las margas de la erosión. Esas calizas 
se formaron en los lagos que ocupaban 
la Cuenca del Ebro hace entre 36 y 10 
millones de años. Foto: Carlos Sancho

Ese sedimento contiene importantes cantidades de yeso recristalizado (CaSO4·2H2O, Imagen 4), proveniente de la disolución de yesos más antiguos en el Pirineo. La acumulación de estas rocas evaporíticas indica que ese antiguo sistema de lagos del centro de la cuenca carecía de desaguadero, es decir, era un sistema endorreico en el que todo el agua recogida acababa siendo evaporada.

4. Yeso cristalizado visible en los niveles intermedios de San
Caprasio. Unos 18 millones de años de edad. Foto: DGC 
Tras ese largo periodo endorreico, el sistema lacustre rebosó o resultó colmatado de sedimento, formándose el actual río Ebro, que ha erosionado y transportado al delta más de 30,000 km3 del antiguo relleno de la Cuenca del Ebro. Hoy, el sedimento preservado más elevado está precisamente en la zona central de la misma, en la Sierra de Alcubierre, 20 km al este de Zaragoza y a 840 m sobre el nivel del mar (Imagen 1).
¿Porqué están más altos esos sedimentos en el centro de la cuenca, si los lagos deberían ocupar la zona topográficamente más baja? ¿Y porqué no rebosaron antes los lagos hacia el Mediterráneo, si la actual divisoria de la cordillera Costero-Catalana tiene lugares de menos de 500 m de altitud?

En ausencia de deformación tectónica (las cadenas montañosas circundantes ya se habían acabado de formar), los movimientos verticales de la superficie de la Tierra están generalmente relacionados con la isostasia: La erosión de la cuenca del Ebro supuso una descarga y un levantamiento (un rebote isostático) de la litosfera terrestre, que descansa sobre el manto como si se tratara de un iceberg en el océano.
5. Hundimiento isostático que sufre la litosfera (en gris) sobre la
astenosfera fluida (blanco) cuando sobre ella descansa una carga (verde),
para 4 escenarios en los que la litosfera es progresivamente más delgada y débil.
En el escenario de una litosfera muy gruesa y rígida no se producen movimientos verticales
de reajuste. En el caso más débil, cada columna del sistema se reajusta localmente y
tiene el mismo peso si se mide hasta un nivel de compensación en la astenosfera. Autor: DGC
En nuestro artículo de esta semana, hemos calculado estos movimientos verticales de la litosfera terrestre para poder estimar el volumen de sedimento erosionado que falta en la cuenca del Ebro. Comparándolo con el volumen actualmente acumulado en el delta del Ebro, hemos podido establecer la edad en la que se produjo la colmatación, el relleno máximo de la cuenca, entre 7.5 y 12.0 millones de años, así como la altitud original que alcanzó la cuenca: 535–750 m sobre el nivel actual del mar.
6. Animation (reload page if necessary): Estimated topographic evolution of the Ebro Basin (NE, Spain) since 10 million years ago until present

7. Las cuencas endorreicas (zonas que no drenan sus aguas al mar) suelen presentar sistemas lacustres que son extremadamente sensibles a las variaciones climáticas, pues en ellos la superficie lacustre se debe adaptar para compensar la lluvia recogida con la evaporación en su superficie.


Microblogs will not become a source of scientific knowledge

This is the contribution I submitted to the newly created Journal of Brief Ideas, which defines itself as a research journal exclusively for articles of 200 words or less: 
Title: Microblogs will not become a source of scientific knowledge
The hypothesis that scientific knowledge can grow out of clearly-written ideas capsuled in 200 words with no antecedents, no references, no methods, and no results, is contradictory with the notion of science itself and therefore does not need to be refuted. However, the profusion of microblogging social networks that may be tempted to introduce DOI's as a way to make their tweets and posts citable, threatens to blur the boundaries with other sources of knowledge. Here, I expose the following brief ideas: 1) that any useful contribution to discern ideas that work from ideas that don't, will need, also in the future, enough words to describe how those ideas were tested; 2) that the routine activity of both scientists and non-scientists will tend to keep discerning the systematic, reproducible studies from bar conversations, online forums, and magically-revealed knowledge; and 3) that consequently the present brief, unreferenced publication will get no credit, even if it turns out to be the first to correctly predict the fail of short unreferenced notes as a source of scientific knowledge. And yet, my last 20 available words call for further transgressive ideas for this field, scientific publishing, that will be hardly recognisable in a decade.

Sadly enough, the editors have not accepted this 'article' for their 'beta journal', on the basis of a lack of a scientific advance in the field:
And I fully agree with their point. All they will attract is ideas, hypotheses, opinions; Scientific advance is much more than that and it will not be achieved in the format proposed. And to be consistent, the Journal of Brief Ideas should either ban most of the contributions they receive (like this) or stop calling itself a scientific journal. It is the reputation of the scientific method (slippery as this can be) that is at risk.