Lecture 4 Flashcards
How do igneous rocks form?
Igneous rocks form when magma cools and crystallizes.
What are the two main categories of igneous rocks?
- Extrusive (volcanic)
- Intrusive (plutonic)
What is decompression melting?
Decompression melting occurs when pressure on hot mantle rock is reduced, causing it to melt.
What processes cause solid mantle material to melt and produce magma?
- Decompression
- Addition of volatiles
- Heat transfer
Where does decompression melting occur?
Decompression melting occurs at mid-ocean ridges and rift zones.
What is the role of volatiles in magma formation?
Volatiles like H₂O or CO₂ lower the melting temperature of the mantle.
Where is the addition of volatiles common in magma formation?
This process is common in subduction zones.
Why might hot magma cause additional rock to melt?
Rising magma can transfer thermal energy to surrounding rock, raising its temperature above the melting point.
What factors cause variation in igneous melts?
- Source-rock composition
- Partial melting
- Assimilation
- Magma mixing
What is partial melting?
Partial melting occurs when only some minerals in the source rock melt.
Why does partial melting often produce silica-rich magmas?
Minerals with lower melting points, often higher in silica, melt first.
What factors influence cooling time in the intrusive realm?
- Depth of intrusion
- Shape/size of magma body
- Circulating groundwater
What is fractional crystallization?
Fractional crystallization is the process where early-formed minerals crystallize first and may settle out of the melt.
What does Bowen’s Reaction Series describe?
Bowen’s Reaction Series describes the order in which common silicate minerals crystallize from cooling magma.
How are igneous rocks classified by composition?
- Ultramafic
- Mafic
- Intermediate
- Felsic
What is a phaneritic texture?
A phaneritic texture results from slow cooling, allowing larger crystals to grow.
What is an aphanitic texture?
An aphanitic texture results from fast cooling, yielding small crystals.
What are dikes and sills in intrusive igneous settings?
- Dike: A tabular intrusion cutting across rock layers
- Sill: A tabular intrusion parallel to rock layers
What is a batholith?
A batholith is a massive, deep-seated intrusive body formed by the coalescence of multiple magma chambers.
Why are volcanic igneous settings often associated with plate boundaries?
Volcanic settings occur at divergent boundaries (decompression melting) and convergent boundaries (volatile-driven melting).
What are the main reasons for the variety of igneous rock types?
- Variation in source-rock composition
- Partial melting processes
- Assimilation
- Magma mixing
- Fractional crystallization
- Cooling rates affecting texture
