Igneous Rocks and Intrusive Igneous Activity

Bowen’s Reaction Series: A crucial concept that shows how different types of magma (mafic, intermediate, and felsic) originate from an original mafic magma through a process of crystallization.
Bowen’s reaction series: demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points
Igneous Rocks: Classification and characteristics of various igneous rocks are derived from their magma origins.
Intrusive Igneous Bodies: Overview of body types such as plutons that result from magma behavior.

Definition of Magma: Molten rock material located beneath the Earth's surface, functioning as the parent material of igneous rock.
- Density Characteristics: Magma has a lower density than the solid rock from which it forms, allowing it to rise to the surface.
- Cooling Mechanism: Most magma cools and solidifies below the surface, forming structures known as plutons.
- Surface Magma: When magma erupts onto the surface, it is termed lava; this can lead to the formation of lava flows.
- Pyroclastic Materials: Ejected materials resulting from explosive eruptions of magma into the atmosphere.

Classification based on Location:
- Volcanic (Extrusive) Igneous Rocks: Form from magma extruded onto the Earth's surface.
- Plutonic (Intrusive) Igneous Rocks: Form when magma crystallizes within the Earth’s crust.

Silica Content as a Defining Factor:
- Felsic: > 65% silica; characterized by an abundance of sodium, potassium, and aluminum. (eruptions = most rare, most violent)
- Intermediate: 53-65% silica; transitional between felsic and mafic.
- Mafic: 45-52% silica; features high levels of calcium, iron, and magnesium.
- Ultramafic: < 45% silica.
Common Elements in Earth’s Crust: Oxygen (O) and Silicon (Si); these combine to form Silica (SiO2) and silicate minerals.

Direct Measurements: using optical pyrometers from low-risk volcanoes, particularly in Hawaii.
- Tools Used: Optical pyrometers.
- Temperature vs. composition: ultramafic/mafic = highest temperatures, felsic = lowest.

Definition of Viscosity: The resistance to flow exhibited by molten magma.
- Low Viscosity: Flows quickly (e.g., water). = low resistance
- High Viscosity: Flows slowly (e.g., porridge, cold motor oil). = high resistance
- Factors Influencing Viscosity:
- Temperature: Higher temperatures = lower viscosity
- Composition: Silica content significantly affects viscosity.
  - Silica-rich (Felsic) Magma: High viscosity - flows slowly over short distances
  - Silica-poor (Mafic) Magma: Low viscosity - more mobile and can travel longer distances.

Formation of Magma: Generally occurs in the upper mantle or lower crust, where it accumulates in magma chambers at depths ranging from a few kilometers to several tens of kilometers.
Bowen’s Reaction Series:
- Crystallization: Sequence in which minerals crystallize from a cooling magma, influenced by their melting points (high-temperature minerals crystallize first).
- Partial Melting: Involves incomplete melting of rock types with varying melting points where minerals that melt will relinquish their specific elements to the liquid magma while others will remain solid.

Temperature Effects: The geothermal gradient causes temperature to increase within the Earth's upper crust by about 20°C to 30°C per kilometer.
Pressure Influence:
- Increased Pressure: Elevates the melting temperature of rocks.
- Decompression Melting: Occurs when confining pressure decreases, enabling melting at lower temperatures.
Role of Volatiles: Primarily water which facilitates melting at lower temperatures, particularly at tectonic boundaries such as where oceanic lithosphere descends into the mantle.

Decompression Melting: Occurs as the pressure decreases on hot rocks when tectonic plates separate.
Impact of Water: Present water lowers melting temperatures, leading to the formation of mafic magmas (30 to 52% silicate).

Convergent Plate Boundaries: Typically produce intermediate and felsic magmas due to partial melting of mafic rocks from the oceanic crust.
Volcanic Activity: Associated with the leading edge of the overriding plates.

Crystal Settling: The process by which crystallized minerals settle within the magma based on gravity.
Assimilation: The interaction between magma and surrounding country rock, where fragments may melt or fragment, incorporating them into the magma body.
Magma Mixing: The occurrence of two distinct magma bodies infiltrating each other, leading to intermediate compositions.

Cooling Process: As magma cools, ions systematically arrange into orderly crystalline patterns.
Size and Arrangement: The crystal size and arrangement dictate the texture of igneous rocks.

Textural Classification: Based on the appearance of rocks determined by the size, shape, and arrangement of interlocking minerals.
Factors Influencing Crystal Size:
- Cooling Rate:
- Slow Cooling → larger crystals.
- Rapid Cooling → smaller crystals.
- Very Rapid Cooling → glassy texture (unordered).
Igneous Textures:
- Aphanitic: Fine-grained due to rapid cooling.
- Phaneritic: Coarse-grained, slow cooling allows for crystal identification without a microscope.
- Porphyritic: Contains phenocrysts embedded in a finer-grained matrix (groundmass) due to two cooling rates.
- Glassy: No crystalline structure (e.g., obsidian).
- Vesicular: Contains gas cavities.
- Pyroclastic: Composed of fragmentary material from explosive eruptions.

Types of Plutons:
- Dikes: Discordant intrusions that cut across layering in country rock.
- Sills: Concordant intrusions that are parallel to layering.
- Laccoliths: Dome-shaped intrusions pushing up the overlying rock layers.
- Volcanic Pipes/Necks: Cylinder conduits connecting the crater with magma chambers, often left as resistant remnants post-eruption.
- Batholiths: The largest type of plutons, with >100 square km surface area; formed via forceful intrusion of magma.
- Stocks: Similar to batholiths, but smaller.
Xenoliths: Fragments of country rock that become enclosed within a magma body, resulting from the dislodging of overlying rocks.