Thermodynamics
Define thermodynamics
In geology, thermodynamics refers to the study of energy and heat transfer within the Earth’s interior, particularly how they influence the formation, transformation, and behavior of rocks and minerals. It is crucial for understanding processes such as magma formation, mineral crystallization, metamorphism, and plate tectonics.
Where does magma come from?
Magma originates from the Earth's mantle and crust due to the melting of rock under high temperature and pressure conditions.
How do you identify minerals in a hand sample?
Identifying minerals in a hand sample involves examining their physical properties using simple tests. Here are the key characteristics to observe:
1) Color
2) Streak
3) Luster
4) Cleavage or Fracture
5) Hardness
6) Crystal Form & Habit
7) Specific Gravity
What is the Gibbs Phase Rule
The Gibbs Phase Rule is a formula that helps determine the number of degrees of freedom (variables like temperature and pressure that can change without altering the system's state) in a system with different phases (solid, liquid, gas). It is expressed as:
F=C−P+2
What are the two ways that petrologists use thermodynamics?
1) Applying principals qualitatively → predict the effects of changing pressure (P), temp (T), or composition (X) will have on a stable melt/mineral/rock.
2)Quantitatively calculate whether a certain mineral (or assemblage) is stable at some particular P-T-X condition (based on experimental data)
Discuss the three different ways of creating melt on Earth
1) Decompression Melting – When mantle rock rises toward the surface (such as at mid-ocean ridges or hotspots), the pressure decreases, allowing the rock to melt.
2) Addition of Volatiles – Water and other volatile substances lower the melting point of rock, causing it to melt at subduction zones.
3) Heat Transfer Melting – Magma from deeper in the Earth can rise and transfer heat to surrounding rock, melting it in the process (often seen in continental volcanic regions).
How do you identify minerals in thin sections?
Identifying minerals in a thin section involves using a polarizing microscope to analyze their optical properties under plane-polarized light (PPL) and cross-polarized light (XPL). Here are the key characteristics to observe:
1) Color & Pleochroism (PPL)
2) Relief (PPL)
3) Cleavage & Fracture (PPL & XPL)
4) Interference Colors (XPL, Birefringence)
5) Twinning (XPL)
6) Extinction (XPL, Rotation Under Crossed Polars)
Describe what a phase diagram is
A phase diagram is a graphical representation that shows the stability of different phases (solid, liquid, gas) of a substance under varying conditions of temperature and pressure or composition. It helps predict how a material behaves when conditions change. By using a phase diagram, we can determine which phases are present at a given temperature and composition and predict how a material will transform as conditions change (such as cooling magma or crystallizing minerals).
What is Gibbs Free Energy?
Gibbs free energy (G) is a measure of the energy content of chemical systems. All chemical systems tend naturally toward states of minimum Gibbs free energy. Gibbs Free Energy is a thermodynamic property that determines whether a reaction, such as mineral crystallization or melting, will occur spontaneously.
G = H - TS
What is partial melting? Can you explain in detail how it works? What happens, for instance, when peridotite melts?
Partial melting is the process where only a portion of a rock melts while the rest remains solid. This occurs because different minerals within a rock have different melting points. When a rock undergoes heating, minerals with lower melting points will melt first, while those with higher melting points remain solid.
- Peridotite melting
Peridotite is a dense, ultramafic rock composed primarily of olivine and pyroxene, with minor amounts of plagioclase or spinel/garnet depending on depth. As the dominant rock in the Earth's upper mantle, it serves as the main source of basaltic magma. When peridotite undergoes partial melting, it typically occurs through decompression melting, a process where the rock ascends towards the surface, causing a drop in pressure that lowers its melting point. During this process, minerals with lower melting points, such as pyroxene and feldspar, melt first, while olivine remains solid for longer, though some portions may contribute to the melt. The resulting magma is enriched in silica compared to the original peridotite, leading to the formation of basaltic magma. This magma rises toward the surface and may erupt as basalt lava. The solid residue left behind becomes depleted in elements that entered the melt and is often referred to as harzburgite or dunite, depending on the remaining pyroxene content.
How do you determine the optical figure of a given mineral?
To determine the optical figure of a mineral (uniaxial/biaxial, positive/negative), follow these steps using a polarizing microscope:
1. Find a Suitable Grain
Choose a mineral grain in thin section that is well-centered, not too thick, and shows high interference colors under cross-polarized light (XPL).
For best results, find a grain cut perpendicular to its optic axis (showing concentric or oval interference patterns).
2. Use a Conoscopic Lens (Bertrand Lens)
Insert the Bertrand lens or use the focusing accessory to observe an interference pattern (optical figure).
This produces a conoscopic image known as an interference figure.
3. Identify the Optical Type
Uniaxial Minerals (e.g., quartz, calcite): Show a single isogyre (cross) that rotates symmetrically.
Biaxial Minerals (e.g., feldspar, mica): Show a two-isogyre figure that shifts position when rotated.
4. Determine Optical Sign (Positive or Negative)
Insert a gypsum plate (λ plate) to observe color changes:
Uniaxial Positive: The slow ray (higher refractive index) is aligned with the c-axis. The center of the figure turns blue with the gypsum plate.
Uniaxial Negative: The center turns yellow.
Biaxial Positive/Negative: Check isogyre movement and color shifts using the plate.
What is Solvus?
This occurs when a solid solution is no longer the lowest energy configuration for a given composition.
Why do different minerals crystallize at different temperatures in a cooling magma body?
Minerals crystallize at different temperatures due to Bowen’s Reaction Series, which reflects the thermodynamic stability of minerals at various temperatures. In a cooling magma, high-temperature minerals like olivine and pyroxene crystallize first because they are stable at hotter conditions, while low-temperature minerals like quartz and feldspar crystallize last as the magma cools further. This occurs because each mineral has a different Gibbs Free Energy (GGG) at a given temperature, meaning that as the system loses heat, only minerals with lower free energy at that temperature will remain stable and crystallize. This process, known as fractional crystallization, causes magma to evolve in composition, leading to the formation of diverse igneous rocks such as basalt, andesite, and granite.
How does magma differentiate after it is formed?
After magma is formed, it undergoes magmatic differentiation, a process that changes its composition as it cools and crystallizes. This occurs through several mechanisms, including fractional crystallization, magma mixing, assimilation, and partial melting. These processes influence the final type of igneous rock that forms.
What's the difference between felsic, intermediate and mafic rocks?
The difference between felsic, intermediate, and mafic rocks lies in their mineral composition, color, silica content, and density.
In a plagioclase phase diagram, if a magma with 60% Anorthite (An60) is cooling and reaches a temperature where it intersects the liquidus, what phases are present, and how do you determine their composition and proportions as cooling continues?
At the liquidus, the magma is fully liquid with a composition of An60. As cooling continues and the temperature drops below the liquidus, solid plagioclase begins to crystallize. The composition of the first crystals follows the solidus line, which means the solid plagioclase will be more An-rich than the liquid. The remaining liquid shifts in composition toward a more albite-rich (Na-rich) composition.
Describe as many thermodynamic concepts as you can.
1) A system: Some portion of the universe that you wish to study
–Isolated (no energy or matter transferred) Geologically rare
–Open (matter and energy can flow across boundaries) Example: Magma erupted onto sea floor
–Closed (heat can move but matter cannot) Example: magma chamber
2) Surroundings: The adjacent stuff. Changes in a system are associated with the transfer of energy –potential, kinetic, chemical, thermal, mechanical
3) Energy states of natural systems
–Unstable: Falling, jumping, etc.
–Metastable: On a “perch” that can be disturbed with an energy input
–Stable: At rest in a low energy state
4) Phase: A mechanically separable portion of a system
5) Reaction: A change in the nature or types of phases in a system.
What happens to a basaltic magma body during fractional crystallization? Explain which solids (crystals) form from magma and how the remaining liquid changes its composition.
During fractional crystallization, a basaltic magma body cools, causing minerals to crystallize in a specific sequence according to Bowen’s Reaction Series. High-temperature minerals like olivine and pyroxene crystallize first, along with calcium-rich plagioclase. These dense crystals settle to the bottom of the magma chamber, removing iron, magnesium, and calcium from the remaining melt. As cooling continues, amphibole and sodium-rich plagioclase begin to form, further depleting the magma of these elements. The remaining liquid magma becomes increasingly enriched in silica, potassium, and aluminum, gradually evolving from mafic to intermediate or even felsic compositions. This process contributes to the formation of a variety of igneous rock types, from basalt to andesite and eventually rhyolite, depending on the extent of crystallization and separation of minerals.
What's the difference between volcanic and plutonic rocks?
The difference between volcanic and plutonic rocks lies in their formation, texture, and crystal size.
What are two-component systems and what do these diagrams show?
In geology, two-component systems in phase diagrams represent the crystallization and melting behavior of mixtures of two minerals or compounds, such as plagioclase feldspar (albite-anorthite) or olivine (forsterite-fayalite). These diagrams show temperature on the y-axis and composition on the x-axis, with liquidus and solidus lines marking the boundaries between liquid and solid phases. As magma cools, minerals crystallize at temperatures defined by the liquidus, and their composition evolves along the solidus. The lever rule helps determine the relative amounts of solid and liquid at any given temperature. Special points like the eutectic point indicate the lowest temperature at which a liquid can exist before crystallizing into multiple solid phases simultaneously. These systems help geologists understand fractional crystallization, partial melting, and magma evolution in Earth's interior.