- Imagine a hypothetical scenario in which the radioactive decay that powers Earth's internal heat engine abruptly ceases. What would be the most immediate and significant consequence for Earth's surface systems, on a timescale of years to decades?
Answer: A significant weakening and eventual collapse of the Earth's magnetic field, increasing the surface exposure to cosmic rays and solar wind.
When you encounter questions about Earth's internal processes, focus on understanding the interconnected systems and their different timescales. Earth's radioactive decay primarily drives two major systems: the geodynamo that creates our magnetic field and the convection that powers plate tectonics.
The correct answer is A because Earth's magnetic field depends on convection currents in the liquid outer core, which are driven by heat from radioactive decay. Without this heat source, the outer core would begin cooling within years to decades, weakening the convection currents that generate our magnetic field. This would dramatically increase our exposure to harmful solar wind and cosmic radiation.
Option B is incorrect because the mantle is enormous and would take millions of years to cool and solidify completely, not the years-to-decades timeframe specified. While tectonic activity would eventually cease, this wouldn't be the most immediate consequence.
Option C misunderstands Earth's energy budget. Weather patterns and ocean currents are primarily driven by solar energy, not internal radioactive heat. The sun provides vastly more energy to surface systems than Earth's internal heat, so these processes would continue largely unchanged.
Option D overestimates the contribution of internal heat to surface temperatures. Earth's internal heat contributes only about 0.03% of the energy reaching the surface compared to solar radiation. Surface temperatures would remain essentially unchanged on this timescale.
Remember: Earth system questions often test whether you understand the relative importance and timescales of different energy sources. Solar energy dominates surface processes, while internal heat powers deep Earth dynamics like the magnetic field.
- Earth's dynamic processes are driven by energy from two primary sources: solar radiation and internal heat. Which pair of phenomena is correctly matched with its dominant energy source?
Answer: Phenomenon 1: The water cycle (evaporation, precipitation). Source: Solar Energy.
Phenomenon 2: The rock cycle (plate tectonics, volcanism). Source: Internal Heat.
When you encounter questions about Earth's energy sources, think systematically about whether each process occurs at the surface (likely solar-driven) or within Earth's interior (likely driven by internal heat).
The water cycle perfectly illustrates solar energy at work. Solar radiation heats ocean water, causing evaporation that rises into the atmosphere, condenses into clouds, and falls as precipitation. Without the sun's energy, this cycle would cease. Meanwhile, the rock cycle's major processes—plate tectonics and volcanism—are powered by Earth's internal heat. The mantle's convection currents, driven by radioactive decay and residual heat from Earth's formation, move tectonic plates and drive volcanic activity.
Option B incorrectly attributes ocean tides to internal heat when they're actually caused by gravitational forces from the moon and sun. It also wrongly credits solar energy for Earth's magnetic field, which is generated by convection in the liquid iron outer core—a process driven by internal heat.
Option C reverses the energy sources. Metamorphic rocks form deep underground through heat and pressure from Earth's interior, not solar energy. Chemical weathering occurs at the surface where solar energy drives temperature changes and the water cycle that breaks down rocks.
Option D also gets it backward. Photosynthesis captures solar energy to convert carbon dioxide and water into glucose—it's fundamentally a solar-powered process. Mantle convection, as mentioned, is driven by internal heat, not solar energy.
Remember this pattern: surface processes (weather, erosion, photosynthesis) typically involve solar energy, while deep Earth processes (tectonics, volcanism, magnetic field generation) are powered by internal heat.
- Geothermal gradient refers to the rate of temperature increase with depth in the Earth's crust, while heat flow is the measure of heat escaping per unit area of the surface. In a stable continental region far from active plate boundaries, a geologist measures an anomalously high surface heat flow. Which is the most plausible explanation for this finding?
Answer: The underlying crustal rocks have an unusually high concentration of heat-producing radioactive isotopes like uranium and thorium.
When you encounter questions about geothermal phenomena, focus on understanding the sources of Earth's internal heat and how they vary geographically. Heat flow measures how much thermal energy escapes through the surface, while geothermal gradient describes how temperature changes with depth.
In stable continental regions, the primary source of heat is radioactive decay within crustal rocks. Uranium, thorium, and potassium isotopes undergo continuous radioactive decay, releasing energy as heat. When crustal rocks contain unusually high concentrations of these radioactive elements, they generate more heat locally, creating elevated surface heat flow even in tectonically quiet areas. This makes option D correct – radioactive isotopes provide a plausible internal heat source that would create the observed anomaly.
Option A misunderstands the relationship between gradient and heat flow. A low geothermal gradient means temperatures increase slowly with depth, which wouldn't concentrate more heat at the surface – it would suggest less heat overall.
Option B incorrectly assumes thicker crust automatically means higher heat flow. While thicker crust contains more total heat-producing material, it also acts as better insulation, often resulting in lower surface heat flow, not higher.
Option C confuses surface solar heating with geothermal processes. Solar energy only penetrates a few meters into the ground and operates on daily/seasonal cycles, completely separate from the deep crustal heat measured in geothermal studies.
Remember: when evaluating geothermal anomalies, always consider radioactive decay as a major heat source, especially in continental crust where granite and other felsic rocks concentrate uranium and thorium.
- The conversion of a large forested area into an urban center with extensive pavement and buildings can significantly alter local energy flows. What is the most likely consequence of this land-use change for the local energy budget on a sunny day?
Answer: A decrease in latent heat flux due to reduced evapotranspiration and an increase in sensible heat flux, resulting in higher surface and air temperatures.
When analyzing urban heat effects, focus on how different surfaces handle incoming solar energy through three key pathways: reflection (albedo), sensible heat (direct warming), and latent heat (cooling through evaporation).
Forests naturally cool their environment through evapotranspiration - trees and plants release water vapor that absorbs energy during the phase change from liquid to gas. This latent heat flux removes significant energy from the local system. When you replace vegetation with concrete and asphalt, you eliminate most evapotranspiration, drastically reducing latent heat flux. The solar energy that would have driven evaporation now goes into directly heating surfaces and the air above them (sensible heat flux), creating the urban heat island effect.
Option A incorrectly suggests concrete and asphalt have higher specific heat capacity than natural surfaces. In reality, these materials typically have lower heat capacity and heat up more quickly than soil and vegetation. Option B wrongly claims urban surfaces increase albedo - most pavement and roofing materials are actually darker than vegetation, absorbing more solar radiation rather than reflecting it. Option C mistakenly focuses on geothermal heat flow, but the dominant energy source affecting local temperature is solar radiation, not heat from Earth's interior.
The correct answer is D because urbanization fundamentally shifts the local energy budget from latent heat cooling to sensible heat warming, explaining why cities consistently run hotter than surrounding natural areas.
Remember: Urban heat questions often test whether you understand that vegetation cools through evapotranspiration, while impervious surfaces convert that same energy directly into heat.
- The theory of plate tectonics is fundamentally linked to the flow of energy within the Earth. Which of the following correctly identifies the primary energy source and the resulting heat transfer mechanism that drives the movement of lithospheric plates?
Answer: Energy Source: Radiogenic heat from the mantle and core; Mechanism: Convection of the plastic asthenosphere.
When you encounter questions about plate tectonics, focus on the Earth's internal energy budget and how heat moves through different layers with varying physical properties.
The correct answer is D because plate tectonics requires a massive, sustained energy source and an efficient heat transfer mechanism. Radiogenic heat comes from the decay of radioactive isotopes like uranium, thorium, and potassium throughout Earth's interior, providing continuous energy over billions of years. This heat drives convection in the asthenosphere—the partially molten, plastic layer beneath the rigid lithosphere. As hot material rises and cool material sinks in convective cells, it creates the forces that move lithospheric plates horizontally.
Option A is incorrect because solar energy only affects Earth's surface and atmosphere, lacking the intensity to drive deep crustal processes. Conduction through solid rock is also too slow to transfer sufficient heat over large distances.
Option B fails because tidal friction generates minimal heat compared to internal sources, and localized magma plumes, while real, cannot account for global plate movement patterns.
Option C mentions residual heat from Earth's formation, which does contribute some energy, but radiation cannot effectively transfer heat through the solid mantle. Radiation requires electromagnetic waves traveling through space or transparent media—not dense rock.
Remember this key principle: plate tectonics operates on a scale requiring both enormous energy input and efficient heat transfer. Only the combination of continuous radiogenic heating and asthenospheric convection meets both requirements. Look for answers that match the physical properties of Earth's layers with appropriate heat transfer mechanisms.
- The generation of Earth's magnetic field by the geodynamo is a direct consequence of the planet's internal energy flow. Which statement most accurately describes the energy pathway that powers the geodynamo?
Answer: Heat from radioactive decay and secular cooling of the core drives vigorous convection in the molten iron of the outer core, and this motion of a conductive fluid generates the magnetic field.
The geodynamo is powered by the convection of the liquid iron-nickel alloy in the outer core. This convection is driven by heat flowing out of the core (from both primordial cooling and radioactive decay) and by compositional buoyancy released as the core crystallizes. The motion of this electrically conductive fluid generates and sustains the Earth's magnetic field.
* A is incorrect because the energy source for the geodynamo is internal, not external from the solar wind.
* C is incorrect because the temperatures in the core are far above the Curie point of iron, meaning it cannot hold a permanent magnetic field. The field is actively generated by electric currents.
* D is incorrect because while differential rotation is a component of the geodynamo process (the Coriolis effect is also critical), friction is not the primary energy source driving the convection.
- Heat is transferred in the Earth system by conduction, convection, and radiation. Which of the following correctly pairs a major Earth process with the dominant heat transfer mechanism responsible for its energy flow?
Answer: Heat moving from the Earth's solid inner core to the liquid outer core — Conduction
Heat must be transferred from the solid inner core to the liquid outer core to help drive convection in the outer core. Because the inner core is solid, heat transfer across the inner-core boundary occurs via conduction. This heat, along with other sources, then drives convection in the fluid outer core.
* A is incorrect because energy travels from the Sun to Earth through the vacuum of space, which can only happen via radiation.
* C is incorrect because heat is distributed through the oceans primarily by the physical movement of water (currents), which is convection.
* D is incorrect because the lithosphere is the rigid outer layer of the Earth. While the asthenosphere below it convects, heat moves through the solid lithosphere itself primarily by slow conduction.
- The greenhouse effect is a natural process that warms the Earth's surface. Which statement provides the most accurate physical explanation for how certain atmospheric gases cause this warming?
Answer: They are largely transparent to incoming shortwave solar radiation but absorb and re-radiate outgoing longwave (infrared) radiation, some of which is directed back towards the surface.
This choice accurately describes the mechanism of the greenhouse effect. Solar energy arrives as shortwave radiation (visible light, UV), which passes through the atmosphere. The Earth's surface absorbs this energy and warms up, then radiates energy back as longwave (infrared/thermal) radiation. Greenhouse gases (like CO2, H2O, CH4) absorb this outgoing longwave radiation. They then re-radiate this energy in all directions, including back down to the surface, effectively warming the lower atmosphere and the surface.
* A is incorrect because greenhouse gases interact with outgoing longwave radiation, not by reflecting incoming shortwave radiation.
* B is incorrect because this describes the action of the ozone layer in the stratosphere, which absorbs UV light, not the greenhouse effect in the troposphere.
* D is incorrect because it describes a completely wrong physical process and incorrectly links the warming to internal heat, not solar radiation.
- The graph provided shows the average annual solar radiation (insolation) received at the top of the atmosphere as a function of latitude. Which factor is the primary cause of the trend shown in the graph?
Answer: The curvature of the Earth causing sunlight to strike higher latitudes at a lower angle, spreading the same amount of energy over a larger surface area.
The graph shows that insolation is highest at the equator and lowest at the poles. The primary reason for this is the geometry of the Earth-Sun system. Because the Earth is a sphere, sunlight strikes the equatorial regions at a nearly perpendicular (high) angle, concentrating the energy in a small area. At higher latitudes, the same beam of sunlight strikes the surface at a more oblique (low) angle, so the energy is spread over a much larger area, resulting in lower insolation per unit area.
* A is incorrect because Earth's elliptical orbit affects the total solar energy received by the planet as a whole at different times of the year, but it does not create the latitudinal gradient.
* B is incorrect because albedo determines how much energy is absorbed by the surface, not how much is received at the top of the atmosphere, which is what the graph shows.
* C is incorrect because the greenhouse effect deals with outgoing longwave radiation, not incoming solar radiation.
- The ice-albedo feedback is a critical component of Earth's climate system. Which of the following scenarios correctly describes the operation of this positive feedback loop as it relates to solar energy flow?
Answer: An initial warming melts ice and snow, reducing the surface albedo. This leads to increased absorption of solar radiation, which causes further warming and more melting.
A positive feedback loop is one where an initial change is amplified by the system's response. The ice-albedo feedback works as follows: an initial warming causes highly reflective ice and snow to melt, exposing darker, less reflective land or ocean. This lower albedo surface absorbs more solar radiation, which leads to more warming, which in turn leads to more melting. This correctly describes an amplifying (positive) feedback.
* A is incorrect because it describes a potential negative feedback involving clouds (more warming -> more clouds -> more reflection -> cooling).
* B is incorrect because it misstates the effect of ice on albedo; expanding ice sheets would increase, not decrease, the planetary albedo, leading to further cooling (another positive feedback, but for cooling).
* D is incorrect because it adds an extra, separate feedback loop (carbon cycle feedback). The core ice-albedo feedback is a direct physical link between temperature, ice cover, albedo, and solar energy absorption.
- When considering the total energy budget at the Earth's surface, both solar radiation and geothermal heat flow contribute. Which statement best compares the magnitude and primary effect of these two energy fluxes?
Answer: The solar energy flux is several orders of magnitude greater than the geothermal heat flow, making it the dominant driver of climate, while geothermal heat is crucial for mantle dynamics.
The average solar energy absorbed by the Earth's surface is about 168 Watts per square meter (W/m²). The average geothermal heat flow is about 0.087 W/m². This is a difference of about 2000 times, or over three orders of magnitude. Therefore, solar energy completely dominates the surface energy budget and drives climate and weather, while the much smaller internal heat flow is the critical driver for processes within the Earth, like mantle convection and plate tectonics.
* A is incorrect because the fluxes are not remotely equal. Solar is vastly larger everywhere on the surface.
* C is incorrect because the solar flux is much greater, not the other way around.
* D is incorrect because the roles are reversed; solar energy drives weather, and internal heat drives plate tectonics.
- Earth experiences a net radiation surplus in the tropics and a net radiation deficit at the poles due to differences in incoming solar radiation. Which of the following describes the most significant mechanism by which the Earth system redistributes this energy?
Answer: The poleward transport of sensible and latent heat by large-scale atmospheric and oceanic circulation systems moves thermal energy from low to high latitudes.
The latitudinal energy imbalance is primarily balanced by the movement of fluids in the atmosphere and oceans. Warm air and water move poleward, carrying thermal energy in the form of sensible heat (temperature) and latent heat (water vapor). This is the fundamental driver of global climate patterns.
* A is incorrect because conduction through rock is an extremely slow and inefficient process for transferring heat over such large distances.
* C is incorrect because although seasons cause shifts in heating, there is still an annual average surplus in the tropics and deficit at the poles that requires transport to balance.
* D is incorrect because it describes an impossible physical process. Reflection from the poles sends energy back to space, not sideways to the tropics.
- A fundamental concept in Earth system science is the distinction between exogenic processes driven by external energy and endogenic processes driven by internal energy. Which statement most accurately contrasts the primary roles of solar radiation and Earth's internal heat in driving these processes?
Answer: Solar energy is the primary driver for atmospheric and oceanic circulation, powering the climate system, while internal heat is the primary driver for mantle convection and plate tectonics.
The correct answer accurately separates the roles of the two main energy sources. Solar radiation (an external source) provides the energy for the climate system, including weather, ocean currents, and the water cycle (exogenic processes). Earth's internal heat (an internal source) drives mantle convection, which in turn moves tectonic plates, causes earthquakes, and fuels volcanism (endogenic processes).
* A is incorrect because the roles are reversed. Internal heat drives mantle convection, and solar energy drives atmospheric circulation.
* B is incorrect because the energy contributions are vastly different; solar energy's contribution to surface processes is many orders of magnitude greater than that of internal heat. Internal heat does not directly drive erosional transport.
* D is incorrect because while chemosynthesis is an important process, the vast majority of Earth's biosphere is powered by photosynthesis, which uses solar energy.
- The diagram provided illustrates a simplified model of Earth's energy budget, with energy fluxes shown in relative units. According to this model, which process represents the largest transfer of heat away from the Earth's surface?
Answer: Emission of longwave (thermal) radiation from the surface that is subsequently absorbed by the atmosphere (part of the 86 units total surface radiation).
To answer this question, one must identify all the energy fluxes moving away from the surface and compare their magnitudes. The diagram shows: reflected solar radiation (7 units), thermals (7 units), evapotranspiration (24 units), and total surface radiation (86 units). The largest of these is the 86 units of longwave radiation emitted by the surface. A small part of this (6 units) goes directly to space, but the vast majority is absorbed by the atmosphere.
* A is incorrect because 7 units is much smaller than the 86 units of emitted longwave radiation.
* B is incorrect because the 6 units radiated directly to space is a small fraction of the total heat leaving the surface.
* C is incorrect because the 24 units of latent heat transfer is significant but less than half of the total longwave radiation emitted by the surface.
- An observer studying a specific location on Earth's surface wants to find direct evidence of the planet's endogenic energy flow. Which of the following observations would provide the most direct evidence of a significant local flux of Earth's internal heat?
Answer: A mid-ocean ridge with active hydrothermal vents and fresh pillow basalts, indicating magma upwelling from the mantle below.
Endogenic energy is heat from within the Earth. Mid-ocean ridges are locations where tectonic plates are pulling apart and hot magma from the mantle rises to the surface. Hydrothermal vents and fresh volcanic rock (pillow basalts) are direct surface manifestations of this internal heat being transferred to the surface.
* A is incorrect because erosion by rivers is driven by the water cycle, which is powered by solar energy.
* B is incorrect because wind is a result of atmospheric circulation, which is also driven by differential heating from solar energy.
* D is incorrect because fossil fuels are derived from ancient organic matter, which stored energy from the sun through photosynthesis. This represents ancient solar energy, not internal heat.
- An exoplanet is discovered that is the same size as Earth and orbits a Sun-like star at the same distance. However, this planet has a much denser atmosphere with significantly higher concentrations of carbon dioxide and methane than Earth. How would the planet's surface energy budget most likely differ from Earth's?
Answer: The planet's surface would be significantly warmer than Earth's due to an extremely strong greenhouse effect caused by the high concentrations of absorbing gases.
When you encounter questions about planetary energy budgets, focus on how atmospheric composition affects the balance between incoming solar radiation and outgoing thermal radiation. The key is understanding how different gases interact with radiation at different wavelengths.
This exoplanet receives the same solar input as Earth since it's the same size and distance from a similar star. However, the significantly higher concentrations of CO₂ and methane create a much stronger greenhouse effect. These gases are highly effective at absorbing outgoing longwave (infrared) radiation that the planet's surface emits, trapping heat in the atmosphere. While some incoming solar radiation might be reflected by the denser atmosphere, the greenhouse effect from the absorbing gases would far outweigh any increased reflection, resulting in much warmer surface temperatures.
Option A correctly identifies this enhanced greenhouse warming. Option B incorrectly assumes perfect balance between greenhouse warming and atmospheric reflection - in reality, CO₂ and methane are much more effective at trapping outgoing heat than reflecting incoming sunlight. Option C overestimates the reflective properties of the dense atmosphere; while density might increase some reflection, the greenhouse gases would dominate the energy budget through heat trapping, not reflection. Option D misunderstands how dense atmospheres work - they actually help redistribute heat more efficiently, reducing temperature extremes between day and night.
Remember: when analyzing planetary atmospheres, greenhouse gases like CO₂ and methane are far more important for their heat-trapping ability than their reflective properties. Higher concentrations almost always mean warmer surface temperatures.
- A massive stratospheric volcanic eruption injects a large volume of sulfate aerosols into the upper atmosphere. What is the most likely primary effect of this aerosol layer on the Earth's energy budget?
Answer: It increases the planetary albedo by reflecting a greater proportion of incoming solar radiation back to space, leading to a net cooling effect at the surface.
The primary effect of sulfate aerosols in the stratosphere is to scatter and reflect incoming shortwave solar radiation. This increases the overall reflectivity (albedo) of the planet, reducing the amount of solar energy that reaches the surface and is absorbed by the Earth system. The result is a net cooling effect at the troposphere and surface.
* A is incorrect because while the stratosphere may warm slightly from absorption, the dominant effect at the surface is cooling due to the reflection of solar energy.
* B is incorrect because sulfate aerosols are not effective greenhouse gases; their primary interaction is with incoming shortwave radiation, not outgoing longwave radiation.
* D is incorrect because while aerosols do act as cloud condensation nuclei, sulfate aerosols tend to make clouds more reflective (brighter), not less, which would contribute to cooling, not warming.
- The world's oceans play a crucial role in regulating Earth's climate by redistributing the excess solar energy received in the tropics. Which of the following oceanic processes is the primary vehicle for this large-scale poleward heat transport?
Answer: The slow, deep-ocean thermohaline circulation and faster wind-driven surface currents, which transport vast volumes of warm water to higher latitudes.
When you encounter questions about global heat transport, focus on the scale and mechanisms involved. Earth receives uneven solar heating—excess energy at the tropics must be redistributed poleward to maintain climate balance. This requires massive, sustained transport systems.
Ocean currents provide the primary mechanism for this heat redistribution through two interconnected systems. Surface currents, driven by prevailing winds, carry warm tropical waters toward the poles at relatively fast speeds. Meanwhile, the deeper thermohaline circulation—driven by density differences from temperature and salinity variations—creates a slower but massive global conveyor belt that moves enormous volumes of water and heat energy across ocean basins. Together, these currents transport approximately 40% of the global poleward heat flux.
Option A incorrectly focuses on tidal mixing, which operates locally and doesn't create the sustained directional flow needed for large-scale heat transport. Option B misunderstands wave mechanics—while waves transfer energy, they don't actually transport water masses over long distances; water particles move in circular motions rather than traveling with the wave. Option C describes heat absorption and conduction, but this process moves heat vertically downward into the ocean depths rather than horizontally toward the poles.
Remember that climate regulation requires sustained, directional transport of massive water volumes across thousands of kilometers. Only ocean circulation systems—both surface and deep—have the scale and persistence to accomplish this global heat redistribution effectively.
- Earth's climate system maintains a long-term energy balance, where energy input roughly equals energy output. Considering the total amount of solar energy absorbed by the Earth's atmosphere and surface, what is the ultimate fate of the vast majority of this energy?
Answer: It is radiated back into space from the top of the atmosphere as longwave (infrared) radiation.
When you encounter questions about Earth's energy balance, think about the fundamental principle that energy cannot be created or destroyed—it can only change forms and move between locations. Earth receives solar energy and must ultimately return that same amount of energy to space to maintain equilibrium.
The correct answer is A because virtually all solar energy absorbed by Earth's atmosphere and surface is eventually radiated back to space as longwave infrared radiation. Here's how it works: Solar radiation (mostly shortwave) heats Earth's surface and atmosphere. This absorbed energy then gets re-emitted as longer-wavelength infrared radiation. While some of this radiation is temporarily trapped by greenhouse gases, it ultimately escapes to space from the top of the atmosphere, maintaining Earth's energy balance.
Option B is incorrect because while solar energy does drive wind and ocean currents, these represent only temporary energy transformations. The kinetic energy in these systems eventually converts to heat, which is then radiated away as infrared radiation—supporting answer A rather than representing a separate fate.
Option C misrepresents the scale involved. Although photosynthesis does convert solar energy to chemical energy, and some biomass becomes buried, this represents an extremely small fraction of total solar input. Most organic matter decomposes, releasing its energy back to the atmosphere.
Option D confuses solar energy with geothermal energy. Solar heating affects only the very shallow surface layers and doesn't contribute meaningfully to Earth's internal heat, which comes from radioactive decay and primordial heat.
Remember: Earth's energy budget is dominated by the radiation balance—energy in equals energy out through space.
- Earth's internal heat engine drives processes like plate tectonics and volcanism. While several sources contribute to this heat, which of the following is considered the dominant source of Earth's internal thermal energy at the present time?
Answer: Radiogenic heat produced by the spontaneous decay of long-lived radioactive isotopes, primarily uranium, thorium, and potassium, within the mantle and crust.
Current models of Earth's internal heat budget indicate that radioactive decay of isotopes like 238U, 232Th, and 40K is the single largest contributor to Earth's internal heat production today, accounting for more than half of the total heat flow from the interior.
* A is incorrect because while primordial heat from accretion is still a very significant component, it is no longer the dominant source; heat is continuously being generated by decay.
* B is incorrect because tidal and frictional heating are minor, secondary sources of heat compared to radiogenic and primordial heat.
* D is incorrect because heat from core crystallization is a contributor, but its magnitude is estimated to be considerably less than the total heat produced by radioactive decay throughout the mantle and crust.
