L1 - Petrology
Introduction to Petrology
Definition of Petrology
Petrology is the study of rocks. The term originates from the Greek words "petra," meaning rock, and "logos," meaning knowledge, reflecting its focus on understanding rocks.
The course aims to understand rock formation and history by observing mineralogy, chemistry, and by studying thin sections under a microscope. This involves detailed analysis of the minerals present, the chemical composition of the rock, and microstructural observations.
Petrography specifically refers to the description of rocks in thin section, which involves using polarized light microscopy to identify minerals and textures.
Course Schedule
The course involves two one-hour lectures and one two-hour practical session every week, providing both theoretical knowledge and hands-on experience.
Topics include melting processes, crystallization, magma interaction, and the use of phase diagrams to understand rock formation conditions. These topics are crucial for understanding the origins and evolution of different rock types.
Resources
Canvas provides lecture notes, practical handouts, and extra notes. There are also short videos available from the previous year, offering a variety of learning resources.
Textbooks for further reading:
Good resources to consolidate learning are available in college and department libraries, including ebooks. Key textbooks provide in-depth coverage of petrology principles and rock descriptions.
Melting Rocks
Mantle Geotherm
The mantle geotherm represents the temperature increase with depth in the Earth. It is a critical concept for understanding melting conditions.
It shows pressure (depth) on the y-axis and temperature on the x-axis, illustrating how these parameters change with increasing depth.
The temperature increases rapidly at shallow depths due to conductive heat loss in the lithosphere. This rapid increase is a result of the Earth's surface cooling and the slow transfer of heat through solid rock.
In the asthenosphere, convection mixes temperatures, leading to a gentler increase. Convection is more efficient at distributing heat, resulting in a less steep temperature gradient.
Lithosphere and Asthenosphere
The lithosphere is the rigid upper part of the mantle where heat loss occurs via conduction. It is composed of the crust and the uppermost part of the mantle.
The asthenosphere is where the mantle is still solid but convecting on geological timescales. This layer is more ductile and allows for the movement of tectonic plates.
Phases and Phase Diagrams
A phase is the mechanical state of a substance (solid, liquid, gas). In petrology, understanding these phases is crucial for interpreting rock formation.
A simple phase diagram indicates whether the mantle will be solid, liquid, or a mix at a given pressure and temperature. These diagrams are essential tools for predicting the state of mantle material under different conditions.
Solidus temperature: below which everything is solid. This is the temperature at which the first melt appears when heating a rock.
Liquidus temperature: above which everything is liquid. This is the temperature at which the last solid crystal disappears when heating a rock.
Mush region: transitional region between solidus and liquidus, where both solid crystals and liquid melt coexist.
A separate solidus and liquidus exist in the mantle as the mantle is composed of different minerals all of which with differing melting points
Under normal circumstances, the mantle is entirely solid because the geotherm lies within the solid region of the phase diagram. However, changes in pressure, temperature, or composition can cause melting.
Generating Melt
Melting can occur due to:
Decompression (Mid-Ocean Ridges): Reducing pressure allows the mantle to melt.
Heating (Mantle Hot Spots): Adding heat raises the temperature above the solidus.
Compositional Changes (Subduction Zones): Introducing volatiles lowers the solidus temperature.
Mid-Ocean Ridges
At mid-ocean ridges, plates pull apart, causing decompression melting. The mantle rises to a shallower depth, reducing pressure without adding heat, and intersecting with the mush zone, leading to melting. This process is responsible for the creation of oceanic crust.
Example: Iceland (Mid-Atlantic Ridge), a location where the ridge coincides with a mantle plume, enhancing melt production.
Mantle Hot Spots
Mantle hot spots involve hot upwelling convective cells from the base of the mantle, adding heat and shifting the geotherm to higher temperatures, causing melting. These are often associated with volcanic activity far from plate boundaries.
Example: Hawaii, where a stationary plume creates a chain of volcanic islands as the Pacific Plate moves over it.
Subduction Zones
In subduction zones, one plate subducts beneath another, releasing volatiles (e.g., water) into the overlying mantle. Water lowers the solidus temperature, causing melting without changing the thermal profile. This leads to the formation of volcanic arcs.
Example: The Andes, Japan, where the subduction of oceanic plates under continental or oceanic plates leads to significant volcanic activity.
Melt Composition and Implications
Typically, only partial melting occurs. The amount of melt controls its composition. The first melts are usually richer in incompatible elements. This is because the first melts are more silicic, which are more allowing of incompatible elements in their structures
Locations and Rock Types
Specific igneous rocks form at different locations due to varying melting processes. Different tectonic settings produce different magma compositions and, therefore, different rock types.
At spreading ridges, dikes, sills, and pillow basalts are common. Oceanic crust formation results from consistent melting of the asthenosphere.
Hot spots can produce batholiths and effusive volcanism (e.g., Hawaii), leading to large shield volcanoes and flood basalts.
Subduction zones lead to explosive volcanism due to water addition, which increases gas pressure in the magma.
Magma Ascent
Melts ascend to the surface due to lower density compared to the surrounding mantle. The density contrast drives magma upwards.
Basalt (more mafic) and granite represent two end-member compositions. Basalt is typical of oceanic crust, while granite is common in continental crust.
Mantle density ranges from to , significantly higher than melt densities, which typically range from to .
Density differences determine whether magma ponds in the crust or erupts at the surface. Magmas with lower density are more likely to reach the surface, while those with higher densities are more likely to pool at neutral buoyancy
Igneous Intrusions
Types of Intrusions
Country rock refers to anything that isn't the intrusion. It's the pre-existing rock that the magma intrudes into.
Intrusions can be:
Concordant: paying attention to existing geological structures and follow existing layers. These intrusions are parallel to the bedding or foliation of the country rock.
Discordant: not caring about existing geological structures. These intrusions cut across the existing structures. In other words, they crosscut the structure of the intruded rock
Concordant Intrusions
Sills are the most common type. Other types include lopoliths and laccoliths.
Lopoliths: flat base of bedding, curve downwards. These are large, basin-shaped intrusions.
Laccoliths: flat base of bedding, curve upwards. These intrusions cause the overlying rock to bulge upwards.
Discordant Intrusions
They are mostly vertical due to buoyancy and cut across existing layers. The density contrast between magma and country rock drives their ascent.
Plutons are moderate bodies of magma within the crust (smaller than batholiths). They represent individual magma bodies that cool and solidify.
Examples of Intrusions
Kilt Rock Sill on the Isle of Skye: follows bedding planes, demonstrating concordant intrusion. Cooling joints form hexagonal structures, similar to Giant's Causeway. These joints result from the contraction of the cooling magma.
Sills are laterally extensive and can intrude for 10s of km. They are typically basaltic because they require extensive melting and low viscosities.
Half Dome (Yosemite): a vast granite batholith, showcasing the scale of these intrusions.
Batholiths are >100km2 plutonic rocks, which are typically granitic in composition due to low density and high viscosity, allowing a buildup in the crust
Dykes: cut across sedimentary beds and can range from cms to 10s m in width and 100kms long, illustrating discordant intrusion.
Dykes and Chilled Margins
Intrusions cause baking of country rock and form chilled margins due to temperature differences. The heat from the intrusion alters the surrounding rock.
Chilled margins have no visible crystals and appear glassy in thin sections, while crystal sizes are coarser towards the center of the dyke due to slower cooling. This is a result of rapid cooling at the edges and slower cooling in the center.
Dyke Swarms
Multiple dykes can form sheeted complexes due to rifting. Example: coast in Arran, where numerous dikes run parallel to each other.
Oceanic Crust
Oceanic crust includes gabbro at the base, a sheeted dyke complex, and pillow basalts at the top. This structure is created at mid-ocean ridges.
Pillow Basalts
Pillow basalts erupt onto the seafloor in contact with cold water, forming a skin and inflating. The rapid cooling forms a glassy crust.
Extrusive Volcanism
Volcano shape depends on magma viscosity. High viscosity leads to steep-sided volcanoes, while low viscosity results in flatter structures.
Flood basalts: vast outpourings of lava (e.g., Siberian Traps, Deccan Traps). These eruptions cover huge areas with basaltic lava. Lava has low viscosity
Shield volcanoes: erupt material that was bent and not very viscous (e.g., Hawaii). These volcanoes have gentle slopes due to the low viscosity of the lava.
Composite volcanoes: erupt more viscous material. These volcanoes are steep-sided and prone to explosive eruptions.
Lava domes: lava cannot flow far, forming bulbous structures. These are often found within the craters of composite volcanoes.
Maars: explosive eruptions forming a hole in the ground. These are caused by the interaction of magma with groundwater. This eruption style is described as phreatomagmatic
Tuffs: volcanic material deposited like sedimentary rocks. These deposits can be formed from ash and other volcanic debris.
Recent Eruptions
Iceland (2021): Fisher eruption due to mid-ocean ridge and hot spot. This eruption demonstrated the complex geology of Iceland.
La Palma (Canary Islands, 2021): hot spot eruption destroying homes. This eruption highlighted the destructive potential of volcanic activity.
Mount Aso (Japan, October 2021): pyroclastic flows. Pyroclastic flows are fast-moving currents of hot gas and volcanic debris.
Viscosity
Viscosity varies significantly with composition. Higher silica content leads to higher viscosity.
Basalts/Komatiites: most runny, with low silica content.
Andesites: intermediate viscosity (subduction zones), with moderate silica content.
Dacites: even more viscous, with higher silica content.
Rhyolites: stickiest (subduction zones), with very high silica content.
Viscosity also depends on temperature and water content. Higher temperature decreases viscosity, while higher water content can either increase or decrease viscosity depending on the specific conditions.
Explosive Eruptions
Rhyolitic Magma
In rhyolitic magma, gas bubbles cause the melt to fracture due to its high viscosity, leading to ash formation. The high silica content hinders bubble escape.
Shattered magma forms shards of ash in Klinian eruptions. These eruptions are highly explosive due to the rapid expansion of gases.
Textures
Tuffs: ash deposits with angular fragments in thin sections. The angular fragments indicate explosive fragmentation.
Vesicles: gas bubbles trapped in sticky magma (e.g., pumice). These bubbles form as gas exsolves from the cooling magma.
Amygdales: vesicles filled with secondary phases. These fillings occur after the rock has cooled and been altered by fluids.
Low Viscosity Melts
Basaltic Lava
In basalts, eruption style depends on the relative speeds of melt and gas bubbles. The ease with which gas can escape influences the eruption style.
If they rise at the same speed, fire fountaining occurs. This is common in Hawaiian eruptions.
If gas escapes faster, a lava lake forms. The lava lake is a pool of molten rock.
Lava Flows
Two types of lava flow develop:
Pahoehoe: rope texture formed by a flow under the surface. This texture is smooth and undulating.
A a: a churned-up field with a sharp, crusty surface. This texture is rough and jagged.
Questions for Consideration
Why do we observe flood basalts but not flood rhyolites? This is due to the lower viscosity of basaltic magma, allowing it to flow over large distances.
Why are explosive eruptions associated with subduction zones? The addition of water lowers the melting point and increases gas content, leading to explosive eruptions.
Why is oceanic crust basaltic? The mantle source is depleted in silica, resulting in basaltic melts.