Unit 8: Geologic Processes Inside Earth

Essential Questions

• Where does the heat from the interior of Earth come from?
• What is the difference between magma and lava?
• How magma are being formed?
• Why do volcanic eruptions occur?
• Why do rocks formed within Earth different from the rocks formed on Earth’s surface?

Review

• The crust is the uppermost and thinnest layer of Earth. It is classified into continental and oceanic crust.
• The mantle lies below the crust. Its uppermost part, together with the crust, forms the lithosphere. The lower mantle is called the mesosphere.
• Mantle convection transfers heat and drives surface processes; it is a key mechanism for plate movements and hotspot activity.
• The innermost layer is the core, composed mainly of iron with nickel; it has a liquid outer core and a solid inner core. The core is the hottest layer with temperatures reaching up to
• The Earth’s rocks are classified into igneous, sedimentary, and metamorphic based on minerals, texture, and formation processes.

Lesson 8.1: Earth’s Internal Heat

Objectives

• Describe where Earth’s internal heat comes from.

Warm-Up

• Moving Molecules activity:
  • Materials: three beakers (hot water, room temp water, cold water), food coloring, timer, water.
  • Procedure: assign hot, room temp, cold water to beakers, add a drop of food coloring, start timer, observe color change without stirring.
• Guide Questions:
  1) What happened to the molecules in each beaker? (Food coloring acts as a molecular movement detector.)
  2) Which beaker changed color first? What does that imply about kinetic energy?
  3) Accretion of dusts in space creates high kinetic energy which yields high heat energy. How is this related to the beaker that changed color first?

Learn about It

• Earth’s internal heat powers dynamic processes: plate movements, earthquakes, volcanism.
• Internal heat sources:
  • Residual heat: from extraterrestrial impacts and gravitational contraction.
  • Radiogenic heat: produced by radioactive decay of unstable isotopes.
• Residual Heat
  • Nebular theory: Earth formed by accretion of particles from a rotating cloud; as Earth grew, high-velocity impacts produced large kinetic energy that converted to heat.
  • Fig. 1: Accretion of fragments during planetary formation.
• Gravitational Contraction
  • Analogy: collapsing dusty cloud behaves like spinning skater; as radius decreases, rotational velocity increases (conservation of angular momentum), converting gravitational energy to heat.
  • Fig. 2: Collapsed cloud due to gravity.
• Radiogenic Heat
  • Radioactive decay of unstable isotopes releases heat as a byproduct.
  • Early Earth had more unstable elements; radiogenic heating was higher then than today.
  • Today, mantle enriched in radioactive isotopes remains a major heat source.
• Heat transfer from the core to the surface is mainly by convection.
• Global heat flow: Earth releases about $46\,\mathrm{TW}$ (terawatts).
• The total Earth thermal budget is debated: primordial heat vs. radiogenic heat as primary contributors; some theories propose Earth is cooling over time.
• Solar energy budget (Earth–Sun):
  • Albedo is about $0.3$, so 30% of solar energy is reflected.
  • The remaining 70% is absorbed by atmosphere, land, and oceans.
  • Absorbed energy drives winds and ocean currents, redistributing heat.
  • All absorbed energy must eventually be radiated back to space; energy in ≈ energy out.
• Thermal budget figure references:
  • Fig. 3: Radiogenic heat is a by-product of radioactive decays.
  • Fig. 4: Albedo 0.3; solar energy partitioning.

Key Points

• The internal heat of Earth fuels plate tectonics, earthquakes, and volcanism.
• Internal heat sources:
  • Residual heat (extraterrestrial impacts + gravitational contraction).
  • Radiogenic heat (radioactive decay).
• Earth’s thermal budget measures heat released at the surface and produced in the interior; energy in must balance energy out.
• Current views suggest Earth may be cooling overall, though debate continues.
• Solar energy budget: albedo ≈ 0.3; 70% absorbed; absorbed energy drives atmospheric and oceanic circulation; energy balance is necessary for climate stability.

Web Links

• BBC: Journey to the Centre of the Earth – interactive animation.
  http://www.bbc.com/future/bespoke/story/20150306-journey-to-the-centre-of-earth/
• YouTube: Crust, Mantle, Core Song.
  https://www.youtube.com/watch?v=plNigVkMyL8
• MinuteEarth: Why is it Hot Underground?
  https://www.youtube.com/watch?v=mOSpRzW2i_4

Check Your Understanding

A. True/False
1) The internal heat of Earth fuels the planet’s dynamic processes including plate movements, earthquakes, and volcanism. – True
2) A great amount of kinetic energy is produced by accreting objects which were then converted to heat energy. – True
3) Rotational velocity is directly proportional to radius. – False (it is inversely proportional, in the skater analogy)
4) Collapsed clouds occur because accretion of more materials led to gravity causing contraction of Earth. – True
5) The collapsed cloud resulted in the conversion of gravitational energy into heat energy. – True
6) Unstable elements undergo radioactive decay. – True
7) The process of radioactive decay produces heat as a byproduct. – True
8) Earth’s thermal budget is the measure of the amount of heat that is released at the surface and produced in the interior. – True
9) The 30% of the solar energy that reaches the surface of Earth is absorbed by the clouds, atmosphere and light-colored areas. – False (it is reflected; albedo 0.3 refers to reflection; absorbed portion is 0.7)
10) The reflected energy drives wind and ocean currents. These currents distribute the heat throughout the planet since more sunlight shines on equatorial regions than polar regions. – False (reflected energy is not what drives winds; absorbed energy redistributed by winds/ocean currents)

B. A guided set of questions on the illustration (not shown here).
1) What is the total percentage of solar energy reflected? 30% (albedo 0.3)
2) What type of surfaces reflect incoming solar energy? Clouds, atmosphere, light-colored surfaces (deserts, ice, snow)
3) What is the total percentage of solar energy absorbed? 70%
4) Are all the energy absorbed radiated back to space? Yes, energy in = energy out in a steady state
5) How much energy is reflected by the atmosphere? A portion of the 30% reflection; breakdown varies by clouds and atmospheric conditions

Lesson 8.2: Magmatism: How Magma is Formed

Objectives

• Describe how magma is formed (magmatism).

Warm-Up

• Rising Magma activity:
  • Materials: small jar, large jar, water, food coloring, cellophane, rubber band.
  • Procedure: color hot water in small jar, cover with cellophane, poke holes, place small jar in larger jar with cold water, observe colored water movement.

Learn about It

• Magma is molten rock produced by partial melting of mantle and crust; contains liquids, gases, crystals, and rock fragments; composition depends on temperature and pressure.
• Formation of magma occurs via three mechanisms:
  • Temperature increase with depth (geothermal gradient) leading to melting.
  • Decrease in pressure (adiabatic decompression) lowering melting temperatures (decompression melting). Occurs at mantle plumes, beneath rifts, and beneath mid-ocean ridges.
  • Addition of volatiles (e.g., water, CO₂) which lower melting temperature; more volatiles reduce melting temperature further.
• Primary mantle magmas are basaltic in composition; as magma ascends, composition may change due to fractional crystallization and assimilation of country rock.
• Magmatism occurs at plate boundaries (divergent, convergent) and within plates (hotspots).
• Plate boundary types and magmatism:
  • Divergent: decompression melting, ridges/fissures, volcanic activity.
  • Convergent: subduction zones produce island arcs; numerous volcanoes; melting linked to subduction.
  • Transform: earthquakes; less magmatism than other boundaries.
• Magma classification by alkalis and silica content:
  • Tholeiitic magmas (tholeiites): large degree of melting; common at mid-ocean ridges (MORB); also from mantle plumes; comprise ~70% of ocean crust.
  • Calc-alkaline basalts: form along convergent boundaries above subduction zones; silica-saturated; higher K than tholeiitic.
  • Alkaline basalts: smaller degree of partial melting; alkali-rich and silica-depleted; common in intraplate settings (e.g., Hawaii); hotspot-associated
  • Carbonatites: not produced from silicate rocks; enriched in carbonate minerals; origin still debated.

Key Points

• Magma is molten rock produced by partial melting of mantle/crust.
• Temperature increases with depth (geothermal gradient).
• Decrease in pressure causes melting by lowering melting temperatures (decompression melting).
• Volatiles lower melting temperatures and enable magma formation.
• Magmatism occurs along plate boundaries and within plates; plate boundary types:
  • Divergent: ridges; decompression melting.
  • Convergent: subduction; arc volcanism.
  • Transform: earthquakes; limited magmatism.
• Magmas are classified by alkalis-silica content: Tholeiitic, Calc-alkaline, Alkaline, Carbonatites.
• MORB (mid-ocean ridge basalts) are typical tholeiitic magmas; continental flood basalts are also tholeiitic in some settings.

Web Links

• AMNH interactive: Interactive: Different Magmas, Different Volcanoes.
  https://www.amnh.org/explore/science-bulletins/earth/documentaries/yellowstone-monitoring-the-fire-below/interactive-different-magmas-different-volcanoes/
• YouTube: Magma Rising Up Song.
  https://www.youtube.com/watch?v=tGm2ymM0Gaw
• YouTube: Different Types of Magma.
  https://www.youtube.com/watch?v=L2fo-d25_Xw

Check Your Understanding

A. Boundary labeling: Convergent, Divergent, Transform (cross-check with figure in text).
B. True/False practice:
1) The increase in temperature with depth is known as geothermal gradient. – True
2) As pressure is decreased, melting temperatures of materials increase. – False (decrease)
3) Volatiles are substances that cannot evaporate easily and can exist in gaseous form in the surface of Earth. – False (they evaporate easily and can exist as gas)
4) When volatiles mix with hot mantle rock, magma forms. – True
5) Rock's melting temperature increases when volatiles are introduced. – False
6) Once magma extrudes onto the surface, it is known as lava. – True
7) Magmatism occurs along plate boundaries or margins and sometimes within the plate. – True
8) Alkaline basalts are the only magmas not produced by silicate rock. – True (carbonatites are an exception)
9) Tholeiitic magmas are produced by a large degree of melting. – True
10) Magmas are classified according to the ratio between the alkalis and silica content. – True

Challenge Yourself

1) What is the temperature trend as you go deeper inside Earth?
2) How does volatiles amount affect melting temperature?
3) How are plate boundaries related to magmatism?
4) Why is adiabatic compression important?
5) Compare the four magma classifications (Tholeiitic, Calc-alkaline, Alkaline, Carbonatite).

Lesson 8.3: Volcanoes and Volcanism

Objectives

• Describe what happens after magma is formed (volcanism).

Warm-Up

• Volcano in a Beaker:
  • Materials: candle, matches, beaker, stove, sand.
  • Procedure: melt wax by candle, pour wax into beaker, cover with sand, pour water to settle sand, heat contents to eruption of hot wax.

Guide Questions

1) What do the wax, sand, and water represent?
2) What happened when the wax erupted and reached the water surface?
3) What is the difference between the wax beneath the sand and the wax that reaches the water surface? What does it represent?

Learn about It

• Volcanism: the process where magma rises to the surface as lava.
• Volcanism viewed as destructive (infrastructural damage, injuries, climate effects) and constructive (formation of atmosphere, ocean crust at ridges, island creation).
• Formation of volcanoes:
  • Divergent boundaries: ridges or fissures with decompression melting; volcanic activity along mid-ocean ridges and rifts.
  • Convergent boundaries: volcanic arcs; subduction melts produce elongated chains of volcanoes following trenches (examples: Mount Pinatubo, Mount Mayon).
  • Intraplate volcanism: hotspots; Mauna Loa and Kilauea in Hawaii as classic examples; hot mantle plumes source magma.
• Lava flows:
  • Move slowly down slopes; surface cooling forms lava tubes.
  • Pyroclastic flows are common in eruptions (not detailed here but implied by rock types).
• Lava flow types:
  • Pahoehoe: smooth, ropy surface.
  • Aa: jagged, angular surface.
• Columnar joints form as lava cools and contracts.
• Volcanic rocks formed from cooled lava include basalt, andesite, rhyolite; rocks are generally finer-grained than plutonic equivalents.
• Volcanic glass rock obsidian, pumice, and scoria can be ejected.
• Volcanism is also relevant on other planets (hotspots on Mars/Venus).
• Hotspots tend to produce basaltic plains; mantle plumes influence surface volcanism.

Key Points

• Volcanism is magma rising to surface as lava; volcanoes form along plate boundaries or within plates.
• Divergent boundaries: volcanism through decompression melting (ridges, fissures).
• Convergent boundaries: island arcs along subduction zones; many volcanoes.
• Intraplate volcanism: hotspots like Hawaii; volcanic activity far from plate boundaries.
• Lava flows: pahoehoe (smooth) vs. aa (jagged).
• Common volcanic rocks: basalt, andesite, rhyolite; volcanic rocks tend to be fine-grained; glassy rocks include obsidian, pumice, scoria.

Web Links

• Interactive map of active volcanoes and earthquakes: Volcano Discovery.
  https://earthquakes.volcanodiscovery.com/
• YouTube: Volcanic Eruptions Song.
  https://www.youtube.com/watch?v=R9PgmUcaCDM
• Harvard: Volcano Island game.
  https://www.cfa.harvard.edu/earthscope/volcano_island/

Check Your Understanding

A. Crossword-style clues (sample):
Across: 6) A process where magma rises to the surface of Earth as lava.
Across: 8) A hill or mountain where lava, pyroclastic materials, and gases erupt.
Across: 9) They have distinctive pattern of columns bounded by fractures.
Across: 10) A type of rock that forms when lava solidifies.
Down: 1) A type of volcanism wherein volcanoes are situated within the plate far from boundaries.
2) Lava flows having a smooth and ropy surface.
3) Plate boundaries that host large numbers of volcanoes.
4) A type of plate boundaries where volcanism manifests as ridges/fissures.
5) A type of plate boundaries where earthquakes occur.
7) Lava flows having jagged and angular corners.

B. True/False practice (statements aligning with Lesson content):
1) Divergent boundaries produce ridges with decompression melting. True
2) Intraplate volcanism is not related to hotspots. False
3) Tholeiitic magmas are typically produced by large degrees of melting. True
4) Carbonatites are not produced by silicate source rocks. True
5) Volcanoes form only at plate boundaries. False (intraplate volcanoes exist)

Challenge Yourself

1) How do divergent and convergent boundaries differently influence magma formation and magma chemistry?
2) Compare pahoehoe vs. aa textures in terms of eruption style and cooling rate.
3) Why does basaltic magma commonly form at oceanic ridges while rhyolitic magma is common in continental arcs?
4) How would you explain volcanic activity on other planets given the plate tectonics differences?
5) How do volcanism and weathering interplay to shape planetary surfaces over time?

Lesson 8.4: Plutonism

Objectives

• Describe what happens after magma is formed (plutonism).

Warm-Up: Marshmallow Crystals

• Materials: marshmallows, toothpicks, 3 plates.
• Plate 1: random marshmallows and toothpicks represent glassy igneous rocks like pumice/obsidian (rapid cooling, little to no crystallization).
• Plate 2: triangles of marshmallows/toothpicks represent crystals forming in magma; some crystals indicate partially crystallized lava like basalt/andesite in lava flows.
• Plate 3: connect triangles to form a large crystal representing large crystals in plutons formed deep underground.
• Guiding questions:
  1) What is the difference between the crystals in the three plates?
  2) Why do plutons have larger crystals?
  3) Why do some rocks not form large crystals?

Learn about It

• Plutonism (James Hutton): rocks formed by heat-driven processes inside the Earth; processes are slow and constant.
• Hutton vs Neptunism:
  • Neptunism posits granites originate as oldest precipitates from a primordial sea.
  • Plutonism posits granites as intrusive igneous rocks; granites cut across sedimentary layers, implying they are younger than sediments and formed from intrusion.
• Common plutonic rocks: gabbro, diorite, granite, peridotite (coarse-grained vs. volcanic equivalents).
• Plutons form as magma rises and displaces country rock; uplift/erosion expose structures.
• Structures:
  • Discordant: cut across bedding (e.g., dike).
  • Concordant: injected parallel to bedding (e.g., sill).
• Batholiths: largest intrusive bodies, lengths up to hundreds of kilometers and widths up to ~100 km; surface exposure > 100 km².
• Stocks: smaller plutonic bodies.

Key Points

• Plutonism emphasizes intrusion and interior Earth processes; rocks formed by heat-driven deep-crust/mantle processes.
• Neptunism vs. Plutonism debate historically shaped geology; plutonism underpins the intrusive vs. extrusive rock distinction.
• Plutons vary in size and shape; discordant structures cut across rocks (dikes); concordant structures run parallel to country rock features (sills).
• Batholiths are the largest intrusive bodies; stocks are smaller plutons.

Web Links

• Igneous Rocks Introduction video.
  https://www.youtube.com/watch?v=Uvft8XI4nao
• Once A Pluton (story).
  https://www.youtube.com/watch?v=W2xnZ-2HNW4

Check Your Understanding

A. Vocabulary-building activity: look up related terms and provide definitions. (Activity prompts)
B. Word puzzle: a cryptic-style fill-in activity (provided in the text).

Challenge Yourself

1) What is the difference between the plutonist and neptunist theories?
2) How would you compare plutonic and volcanic rocks?
3) Based on texture, what can you infer about plutonic rocks’ origin?
4) Why do plutonic rocks have large crystals?
5) What is the classification of plutonic rocks? Describe each.

Laboratory Activity

• Activity 8.1 Lava Viscosity:
  • Objective: demonstrate how intrusive processes form through a hands-on model.
  • Materials and Equipment: gelatin mold, chocolate sauce, syringe, aluminum pie plate, vertical supports.
  • Procedure:
    1) Prepare gelatin in a large bowl; poke holes in a perforated pie tin.
    2) Loosen gelatin edges in a hot water bath; place gelatin mold in tin.
    3) Set up vertical supports taller than syringe length.
    4) Use syringe to suck chocolate sauce and inject into holes; observe as sauce rises through gelatin model.
    5) Repeat until mold breaks apart.
  • Guide Questions:
  • How does the chocolate sauce movement resemble magma flow?
  • Why does flow direction vary with injection location?
  • How does this model compare to real magma movement in volcanoes?

Performance Task

Geologic Hazards

• Role: researcher/surveyor
• Audience: senior high school class
• Task: create a survey to inform before, during, and after eruption responses; gather community hazard data.
• Product: a survey instrument with clear questions and response options.
• Rubrics: evaluation criteria include purpose clarity, question clarity, implementation plan/timeline, pilot testing, sampling where applicable.

Standards and Criteria (Rubric Summary)

• Purpose: clarity of goal
• Questions and Response Options: clarity and lack of ambiguity
• Implementation Plan and Timeline: realistic schedule with pilot testing and sampling considerations
• Self Check: agreement with core statements (described in the unit)

Key Words (selected definitions)

• Aa: Lava flow with jagged, angular surface texture.
• Batholith: Large intrusive igneous body >100 km² surface exposure; hundreds of km in length, up to 100 km wide.
• Dike: Igneous body that cuts across bedding or other structures.
• Earth’s thermal budget: Heat released at the surface and produced in the interior.
• Geothermal gradient: Temperature increases with depth.
• Magma: Molten rock material produced by partial melting of mantle and crust; contains liquids, gases, crystals, and rock fragments.
• Magmatism: Magma formation and movement along boundaries or within plates.
• Pahoehoe: Lava flow with a smooth, ropy surface.
• Plutonism: Rocks formed by heat-driven processes inside the Earth.
• Sill: Nearly horizontal intrusive body formed when magma exploits weak spots between sedimentary beds.
• Volcanism: Process of magma rising to the surface as lava.
• Volcano: Hill or mountain where lava, pyroclastic materials, and gases erupt; can form along plate boundaries or within plates.

Wrap Up

• Geologic processes inside Earth involve a cycle of heat generation (residual + radiogenic), heat transfer (convection), magma formation (magmatism), and surface expression via volcanism and plutonism.
• Understanding these processes helps explain plate tectonics, volcanic activity, rock formation, and geologic hazards.

References (selected)

• Department of Geological Sciences, San Diego State University. How Volcanoes Work; The Earth’s Internal Heat Energy and Interior Structure.
• Hefferan, Kevin & O’Brien, John. Earth Materials.
• Korenaga, Jun. Earth’s Heat Budget.
• Tarbuck, J.T. & Lutgens, F.K. Earth Science (14th ed.).
• Taylor, A.H. The Foundation of Modern Geology; Plutonism.
• Volcanism on the Terrestrial Planets (Journal of Geological Education).
• Various web resources and museum/education sites cited in the unit.