Igneous Rocks
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.
What is metamorphism? What factors control it? Explain briefly.
Metamorphism is the process by which existing rocks undergo physical and chemical changes due to heat, pressure, and chemically active fluids, without melting. This transforms the mineral composition and texture of the rock while it remains solid.
The main factors that control metamorphism are:
Temperature – Higher temperatures can cause minerals to recrystallize into new forms.
Pressure – Increasing pressure, especially from tectonic forces, can realign minerals and form foliation.
Chemically active fluids – Hot, mineral-rich fluids can promote chemical reactions and mineral changes.
Time – Longer durations allow more complete and stable metamorphic transformations.
Each factor influences the type and degree of metamorphism experienced by a rock.
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
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 the texture and mineralogy of rocks change during metamorphism?
During metamorphism, the texture and mineralogy of rocks change due to heat, pressure, and fluid activity:
Texture changes include recrystallization, where minerals grow larger and interlock more tightly, and the development of foliation, where minerals align into layers or bands under directed pressure.
Mineralogy changes occur as original minerals become unstable under new conditions and transform into new metamorphic minerals (e.g., clay minerals becoming mica).
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)
Which of the following best describes a batholith?
A) A horizontal intrusion between layers of sedimentary rock
B) A massive discordant body of intrusive igneous rock larger than 100 km²
C) A surface depression formed by the collapse of a volcano
D) A small, dome-shaped intrusion that pushes overlying layers upward
Answer: B
What is an index mineral? How does it work to understand P-T conditions during metamorphism?
An index mineral is a mineral that forms only under specific pressure-temperature (P-T) conditions during metamorphism. Its presence in a metamorphic rock helps geologists estimate the grade (intensity) of metamorphism the rock has experienced.
Because each index mineral (e.g., garnet, kyanite, staurolite) forms within a known P-T range, identifying them in rocks allows geologists to reconstruct the metamorphic history and conditions under which the rock formed.
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.
Define and distinguish between a sill and a dike. Include their orientation relative to existing rock layers.
A sill is a tabular igneous intrusion that forms parallel to existing rock layers, meaning it intrudes horizontally between bedding planes. In contrast, a dike cuts across pre-existing rock layers, intruding vertically or at a steep angle. This difference in orientation is key to distinguishing the two in the field.
What is isograd?
An isograd is a geological line on a map that marks the first appearance of a specific index mineral in metamorphic rocks as metamorphic grade increases. It represents a boundary between different metamorphic zones and indicates where certain temperature and pressure conditions were first reached during metamorphism. Crossing an isograd means entering a new metamorphic grade with a distinct mineral assemblage.
Describe two key features under cross-polarized light that help you distinguish pyroxene from amphibole in thin section.
Pyroxene typically shows higher relief, blocky cleavage at near 90°, and lower birefringence, while amphibole has cleavage at ~60° and 120°, a more elongated habit, and higher birefringence with a distinct pleochroism in plane-polarized light.
Match the texture with its characteristic formation condition:
Texture Formation Condition
___ Glassy A) Slow cooling, visible crystals
___ Phaneritic B) Very rapid cooling, no crystals
___ Pegmatitic C) Extremely slow cooling, large crystals
___ Aphanitic D) Rapid cooling, microscopic crystals
Glassy – B
Phaneritic – A
Pegmatitic – C
Aphanitic – D
Explain the metamorphic facies concept. What happens to basalt as it goes through metamorphism in a subduction zone?
The metamorphic facies concept groups rocks based on the specific pressure-temperature (P-T) conditions under which they formed, each facies representing a set of stable minerals at those conditions. It helps geologists understand the metamorphic environment and history of a rock.
In a subduction zone, basalt is subjected to high pressure and relatively low temperature. As it undergoes metamorphism, it typically progresses through the blueschist facies (forming minerals like glaucophane) and, with greater depth, into the eclogite facies, where it transforms into eclogite, made of high-pressure minerals like garnet and omphacite.
Match the mineral to its diagnostic feature in thin section:
Mineral Diagnostic Feature
___ Garnet A) Isotropic or very low birefringence, no cleavage, high relief
___ Biotite B) One perfect cleavage, strong brown pleochroism
___ Plagioclase Feldspar C) Multiple twinning (polysynthetic), low relief
___ Quartz D) Undulatory extinction, no cleavage, low relief
Garnet – A
Biotite – B
Plagioclase Feldspar – C
Quartz – D