Big Ideas in Volcanology: Volcanic Heat

Big Idea 1.
Volcanic heat comes from natural radioactivity...







The Big Ideas in Volcanology are several key concepts that a person studying volcanology should understand. The format of this project is based on the Earth Science Literacy Principles, a set of essential ideas that all Americans should know about geosciences. The depth to which one explores the Big Ideas in Volcanology depends on whether they study volcanism professionally, as a hobby, or somewhere in between. Here, we discuss the first Big Idea (Volcanic Heat) through seven basic sub-ideas supporting the fact that "volcanic heat comes from natural radioactivity", including some misconceptions, weaknesses, and implications of these concepts.

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1.1 Volcanism requires two prerequisites: material to melt, and a heat source.

Volcanism can be defined in many ways, but one definition is the surface manifestation of a planet’s internal heat processes. Coursing rivers of lava, destructive pyroclastic explosions, and quietly degassing fumaroles are all processes that are ultimately driven by heat. A common misconception about the origin of volcanism is that below our feet exists an enormous sea of molten rock (magma) that occasionally breaks through the surface, releasing heat and hot material. In fact, the only liquid layer of the Earth is the outer core and this is generally not where volcanic products originate (though read more about mantle plume sources and the D'' layer here). Source materials for volcanism can originate from crustal rocks, mantle material, or a combination of both. Most melts originate from solid mantle material that partially melts in response to a decrease in pressure. Between 20 and 300 km below the surface, temperature and pressure conditions allow a phase change from solid to liquid. After a melt is formed, it must rise through the crust to erupt at the surface in order to be produce volcanic activity. The pressure inside the Earth is due to the force of gravity, but where does the heat come from?


Next Big Idea: Heat Sources...


Internal structure of the earth.
Figure 1: The Earth's crust ranges in thickness from 0 - 70 km. Beneath the crust is a solid, but ductile layer called the mantle, which extends to a depth of 2891 km.  The outer liquid core surround the solid inner core, which begins at a depth of 5150 km. Click to enlarge.

Earth geotherm.
Figure 2: The dark line in this diagram illustrates how temperature increases with depth inside of the earth (this is called the geothermal gradient). The lighter line shows the position of the solidus (point at which liquid material solidifies). The shaded area represents solid regions. Note that temperatures in the outer core are higher than the solidus, so the outer core in liquid! Click image to visit external link/image source.


Residual heat sources.
Figure 3: The Earth still stores heat that was generated during initial formation. This heat comes from kinetic energy transferred by impacts and the subsequence gravity-driven accretion, friction caused by differentiation of the earth's structure (sinking of heavy elements like Fe, rising of light elements like Si), and the latent heat of crystallization released as the core solidified. Click to enlarge.

Radioactive decay.
Figure 4: Radiogenic heat is released as an isotope decays. There are several types of radioactive decay, but this example illustrates alpha decay, where an alpha particle (Helium nucleus, or two protons and two electron) is ejected from the parent isotope. Radiation (heat) is released simultaneously. Note that short-lived radioactive iostopes that existing during Earth's early history contributed a significant amount of heat that is considered "residual".



1.2 There are two main internal heat sources from the Earth: residual heat and radiogenic heat.

The heat sources that drive modern-day volcanism are 1) heat released as a byproduct of radioactive decay and 2) residual heat leftover from the formation of the Earth. The relative contributions of these two sources to the overall heat budget are still not yet completely understood, but this is an active area of research. Current estimates of the relative contribution of radiogenic heat are imprecise and are thought to be as high as 80%, but a recent article published in Nature argues that only about half of the Earth's internal heat comes from natural radioactivity. Is the other half be contributed completely by residual heat or are there another heat sources that we do not yet understand? As the Earth began to form about 4.6 billion years ago, heat accumulated from kinetic energy imparted by collisions, subsequent gravity-driven planetary accretion, differentiation (separation of the Earth into compositional layers), and from the latent heat of crystallization released as the core cooled. Some of this primordial heat is still stored in the Earth today and is released by volcanism.


Next Big Idea: Radioactive Isotopes...



1.3 The four isotopes that contribute the most radiogenic heat are 235U, 238U, 232Th, and 40K.

Heat is released when an atom of a radioactive element experiences decay. Decay is the ejection of a particle that occurs in order to bring the atom to a more stable configuration. Radioactivity is a natural, ongoing process and is not a human invention, although we have found numerous ways to exploit it. Regarding heat production inside the Earth, the most important radioactive isotopes are U-235, U-238, Th-232, and K-40. While U-235 produces the most heat per atom, K-40 is the most abundant radioactive isotope present in the Earth. The granite in your kitchen countertops is slightly (and safely!) radioactive, because granite contains a large amount of potassium. Early in Earth’s history, K-40 was much more abundant that it is today. This is due to its relatively short half-life – the amount of time it takes for the original number of parent isotopes to decrease by half. In contrast, the amount of Th-232 has decreased only slightly over the last 4.6 billion years, and as a result still continues to contribute a relative large amount of heat today. There are many other radioactive elements, and we may not currently have a good constraint of their relative past or present abundances inside the Earth and their relative heat contributions. How does the decrease in the abundance of radioactive isotopes affect the Earth’s heat budget?


Next Big Idea: Temperature through Time....







http://www.sciencedirect.com/science/article/pii/S0012821X08007711
Figure 5: This is Figure 8 from Arevalo et al. (2009). Relative radiogenic heat production for the four most important isotopes. K-40 produces the most heat, but this is because it is the most abundant radioactive isotope. In fact, U-235 produces the most heat per atom, so even though it exists in very small amounts, it still contributes significantly to the total radiogenic heat flux. Click image to visit external link.




Early Earth Image
Figure 6: The early Earth was extremely hot and the surface may have been completely covered with a sea of lava! Click image to visit external link/image source.


1.4 Heat production is decreasing through time as the amount of radioactive isotopes decreases.

Currently, most heat production occurs in the crust. This is because incompatible elements (those whose size and/or charge make them difficult to include in minerals) migrate up to the crust where they decay. However, heat stored in the mantle lingers for a very long time because it is well insulted and because of the low thermal conductivity of the material that makes up the mantle (peridotite). Billions of years ago, the Earth was producing much more heat than it currently does today. This was partially due to the decay of radioactive isotopes with short half-lives, which no longer exist in any significant abundance. The temperature of the core is currently modeled to be around 5000 degrees C, but the early Earth would have been much hotter, resulting in increased volcanism and plate tectonics. At some point, the Earth was probably entirely molten. More heat is released from this incredibly hot Earth than is being supplied and so it gradually cools, and continues to cool today. The current rate of cooling is about 100 degree C per billion years. Since there are no significant external heat sources and radioactivity is decreasing, the Earth will continue to cool until it eventually reaches equilibrium with space. But don’t worry, it is much more likely that human life of Earth will be obliterated by an extraterrestrial impact, the loss of the magnetic field, or the inevitable expansion of our Sun!


Next Big Idea: Heat Transfer...





1.5 Heat is transferred by radiation, conduction, and convection.

Heat is transferred through three mechanisms: radiation, conduction, and convection. Radiation is the emission of thermal energy as electromagnetic waves. Conduction is the transfer of energy of atoms to neighboring atoms, and is the primary means of heat transport in solid materials. Convection is heat transport facilitated by the physical movement of fluids. Inside the Earth, heat is transported by convection in the mantle and outer core and to a lesser extent by conduction. Heat is radiated to space at the surface. Convection is driven by the geothermal gradient (increasing temperature with depth). In the mantle, thermal convection is an extremely important mechanism that drives plate tectonics. As previously mentioned, higher heat production in the Earth’s past may have resulted in increased mantle convection and therefore, increased plate tectonic and volcanic activity. Thermal convection also occurs in the outer core, where conducting liquid with the consistency of water convection and gives rise to the geomagnetic field. The geomagnetic field shields the Earth from solar winds, which would otherwise strip the Earth of its atmosphere (this is what happened to Mars, which now has a very weak magnetic field). Without heat-driven convection, and the resulting plate tectonics and geomagnetic fields, life may have never arisen on Earth. The total heat flow through the Earth from radiation, conduction, and convection is estimated to be at least 42x10^12 W (Stacey and Loper 1988). More recent estimates are around 4.4x10^13 W. (Lowrie 2001).


Next Big Idea: Detecting Heat...




Heat Transfer
Figure 7: Heat is conducted through three mechanisms, radiation, conduction, and convection. Radiation is the emission of thermal energy as electromagnetic waves. Conduction is the transfer of energy of atoms to neighboring atoms, and is the primiary means of heat transport in solid materials. Convection is heat transport facilitated by the movement of fluids. Click image to visit external link/image source.

Mantle convection.
Figure 8: Convection in the mantle drives plate tectonics. Earlier in Earth's history, when the planet was hotter and convection was more vigorous, it is possible that plate tectonics processes occured more quickly. Click image to visit external link/image source.




Thermal Image, Kilauea 1
Thermal Image Kilauea 2
Figure 9: Thermal infrared images of lava flows at Kilauea Volcano, Hawaii. Temperatures are represented by a false-color scale where white is the hottest and black/blue are coldest. Thermal monitoring volcanic eruptions is important both for understand the eruptive processes and for issueing hazard warnings. Images from Hawaiian Volcano Observatory Website (www.hvo.wr.usgs.org).



1.6 Surface manifestations of internal heat can be detected through thermal infrared imaging.

Lava, gasses, geothermal fields, and other heat sources can be detected using thermal infrared sensors. The ability to identify and quantify radiated heat is useful for both volcano monitoring and geothermal resource exploration. The hottest modern lavas are ~1300 degrees C, but some are much cooler (for example, carbonatites generally erupt at ~500 degrees C). The thermal infrared wavelength of the electromagnetic spectrum can be considered as 0.7 – 20 micrometers. Most sensors (satellites, ground-based cameras, aerial cameras, thermometers, etc) collect data in two regions, 3-5 um and 8-13 um (because of atmospheric windows at these wavelengths). Thermal imagery can be used to detect volcanic eruptions in remote areas and to monitor eruptions at volcanoes that pose an immediate risk.


Next Big Idea: Geothermal Resources...



1.7 Geothermal heat can be exploited as an energy source/natural resource.

Geothermal energy is a clean, safe, reliable, and sustainable source of electricity. Natural hot and warm springs can provide hot water for recreational or home usage. Geothermal power plants produce electricity by pumping water deep into the ground in areas where there is a high geothermal gradient (i.e. temperature increases rapidly with depth). The water is heated and turns into steam, which travels up a second borehole and spins a turbine to produce electricity. As of 2010, geothermal electricity account for only 0.3% of the total US energy production. However, in other countries such as Iceland and El Salvador, geothermal energy account for over 25% of total production. The average geothermal gradient is about 2.5 – 3.0 degrees C per 100 m, and geothermal wells can be realistically be drilled down to about 10,000 m. This gradient varies significantly and in geothermal areas can be greater than ten times the average gradient.



Heat flow in the US.

Figure 10: This heat flow map of the United States shows relative rates of temperature increase in the crust. An average gradient in above 25 degrees C increase per kilometer. In some areas, temperature increases drastically with depth, such as souther Oregon. Heat closer to the surface is more economical for energy production! Click image to visit external link.



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