Lesson 3 of 6 · Magma, eruption & cryovolcanism
Volcanism: moving an interior to the surface
9 minute lesson · 3 checks along the way
Your goal for this lessonConnect eruption products with their source conditions while distinguishing an active eruption, an old volcanic landform, and a modeled heat source.
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Melting is local, and the material matters
is molten or partly molten rocky material below a surface; erupted molten rock is called . A planet does not need a completely liquid mantle to produce either. Rock can begin melting when temperature rises, pressure falls, or composition changes its melting behavior. The melt may separate from surrounding solid grains and collect in reservoirs before finding a route upward through fractures.
Eruption behavior depends on , dissolved gas, crystals, pressure, and the surrounding environment. Relatively mobile lava can spread in broad flows; more viscous material can build steep domes. Expanding gas can fragment material into an explosive eruption. These are interacting controls, not rules that every tall mountain must be volcanic or every smooth plain must have been flooded by the same kind of lava.
Check 1 of 3
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Compare heat sources without assuming one universal engine
Io provides a strong contrast to a quiet-looking asteroid. Its changing tidal distortion dissipates mechanical energy as heat, helping sustain exceptional volcanic activity. This is not heat arriving directly as sunlight, nor energy created from nothing. Orbital interactions help maintain the conditions that allow continued dissipation. The amount and location of heating depend on the moon’s orbit and material response.
A Juno analysis reported in 2024 found tidal deformation inconsistent with the proposed shallow global magma ocean used in that comparison; more localized magma storage remained plausible. Eruptions thus do not by themselves prove that all the subsurface is liquid. Different measurements test different parts of the story: thermal radiation constrains emitted heat, images reveal surface changes, and gravity and deformation constrain interior models.
Work through a decision
Estimate the size of a deposit without inventing a date
Consider an invented flow covering 12 square kilometers with an average thickness of 8 meters. Convert the thickness to 0.008 kilometers before multiplying: volume is approximately 12 × 0.008 = 0.096 cubic kilometers. This estimate assumes the mapped footprint and average thickness represent the deposit. Irregular buried topography or an uncertain average thickness can change the inferred volume substantially.
If that same deposit accumulated during 200 hours of actual emplacement, its average volumetric supply would be 0.096/200 = 0.00048 cubic kilometers per hour, or 480,000 cubic meters per hour. Neither a single footprint nor volume alone supplies the duration. A sequence of dated observations is needed to estimate an eruption rate, and a time average can conceal pauses or short bursts of much faster supply.
Check 2 of 3
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Recognize volcanism that does not involve red-hot rock
Dawn found Ceres’ isolated mountain Ahuna Mons, whose shape and other properties support formation by extrusion of , muddy material. This is : relatively cold volatile-rich material reaches the surface rather than familiar silicate lava. Salts alter the freezing behavior of water-rich mixtures. The interpretation combines landform shape, composition, and physical models; it does not follow from the word volcano in a picture label.
The archive collage places Ahuna Mons among craters and bright deposits elsewhere on Ceres. Separate panels have different scales and observing treatments, so a collage is not one continuous geological map. Its mountain records past emplacement; it is not a photograph of an eruption happening at that instant. Compare morphology with spectra, topography, and repeat observations before assigning a source material, a date, or present activity.
Dawn’s collage combines sites at different scales and includes false-color products. Ahuna Mons, at upper right, is a simulated perspective view made with enhanced-color images and twice the actual vertical relief. Its shape informs an emplacement model; it does not depict an eruption in progress.
Look closely
Find the isolated mountain at upper right. Can its apparent screen height be compared directly with the diameter of a crater in another panel?
See what to notice
No. The panels have different scales and treatments, and Ahuna Mons’ vertical relief is exaggerated. Physical dimensions require the source topography and scale; morphology, composition, and models jointly test its cryovolcanic interpretation.
Check 3 of 3
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Put it together
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