Questions and Answers
What texture is characterized by the presence of large crystals called phenocrysts in a fine-grained matrix?
Which group of igneous rocks is characterized by having the highest silica content?
What type of volcanic rock is identified by its ability to float due to its low density and many vesicles?
Which texture indicates that lava cooled very quickly, not allowing for crystal formation?
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Which term refers to rock formed from volcanic ash and is usually characterized by a chaotic mix of crystals and fragments?
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What is the primary factor that distinguishes intrusive igneous rocks from extrusive igneous rocks?
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Which type of igneous rock is most likely to have a fine-grained texture?
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What characteristic is typical of extrusive igneous rocks compared to intrusive igneous rocks?
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What do the terms 'phaneritic' and 'aphanitic' refer to?
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Which process is primarily responsible for the formation of volcanic igneous rocks?
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What is a dike in the context of igneous rock formations?
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Which rock type corresponds to ultramafic compositions?
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How are sills different from dikes?
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Which process is associated with magma intruding and consuming surrounding rock?
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What does Bowen's Reaction Series primarily illustrate?
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Study Notes
Igneous Rock Formation
- Igneous rock forms when liquid rock freezes into a solid, with magma underground and lava at the surface.
- The Earth’s outer core is liquid; the mantle and crust are solid, but magma pockets exist near the surface.
- Magma is the source for volcanic activity and igneous rocks.
Types of Igneous Rocks
- Extrusive (Volcanic) Rocks: Formed from lava that cools quickly and has fine-grained (aphanitic) texture, often vesicular due to gas bubbles.
- Intrusive (Plutonic) Rocks: Formed from slowly cooled magma, characterized by coarse-grained (phaneritic) texture with visible crystals.
- Rocks can also be porphyritic, with both small and large crystals indicating multi-stage cooling.
Texture Classification
- Phaneritic Texture: Coarse-grained and visible crystals from slow cooling.
- Aphanitic Texture: Fine-grained and microscopic crystals from rapid cooling.
- Vesicular Texture: Contains gas bubbles (vesicles), found in scoria and pumice.
- Volcanic Glass: Results when lava cools too quickly to form crystals, e.g., obsidian.
- Pyroclastic Texture: Formed from explosive eruptions with chaotic mixtures of rock fragments; tuff is formed from tephra deposits.
Igneous Composition
- Classified into four main groups based on mineral and chemical composition:
- Felsic: Rich in silica (65-75%), with light-colored minerals like feldspar and quartz.
- Intermediate: Contains equal parts of light and dark minerals (55-60% silica).
- Mafic: Rich in iron and magnesium (45-50% silica), with dark minerals like pyroxene.
- Ultramafic: Very low in silica (40% or less), primarily olivine, often found in the mantle.
Igneous Rock Bodies
- Dikes: Vertical or angled intrusions that cut across existing rock layers.
- Sills: Horizontal intrusions that run parallel to sedimentary layers.
- Plutons: Large intrusion masses formed from cooled magma, can become batholiths if large.
- Laccoliths and Lopoliths: Concordant intrusions that bulge upwards or downwards, respectively.
Bowen’s Reaction Series
- Describes the temperature crystallization of minerals in igneous rocks from 700°C to 1,250°C.
- The series demonstrates the relationship between mineral crystallization, temperature, and composition.
- Mafic rocks crystallize at higher temperatures compared to felsic rocks.
Magma Components
- Composed of melt (liquefied minerals), solids (crystallized minerals), and volatiles (gases like water vapor, CO2).
- The behavior of magma is influenced by these components.
Geothermal Gradient
- The temperature increases with depth, averaging 25°C per kilometer in the upper crust due to residual heat and radioactive decay.
- Rock melting and magma generation can occur through decompression, flux melting, and heat-induced melting.
Volcanism
- Volcanoes form when lava solidifies at the surface and can originate from tectonic activity, forming at mid-ocean ridges and subduction zones.
- Classified as interplate (at plate boundaries) and intraplate (far from boundaries, often from hotspots).
Volcano Hazards and Monitoring
- The understanding of volcanoes has improved, aiding in monitoring and predicting eruptions, essential for public safety.### Interplate Volcanoes
- Generally less observed, located under 3,000-4,500 m of ocean, characterized by gentle eruptions.
- Iceland serves as an exception due to diverging oceanic plates, allowing hot mantle rock to rise and release pressure.
- Ultramafic mantle rocks (mainly peridotite) partially melt to produce basaltic magma, common in ocean floor volcanoes.
- Underwater eruptions create pillow basalts and support unique ecosystems around hydrothermal vents, utilizing chemosynthesis instead of photosynthesis.
Volcanoes at Subduction Zones
- Second most common volcanic location adjacent to subduction zones, a type of convergent boundary.
- Subduction emits water from hydrated minerals, causing flux melting in the mantle.
- Results in volcanic arcs with primarily silica-rich magma, leading to eruptions of intermediate-to-felsic rock like andesite and rhyolite.
- Example: The Ring of Fire surrounding the Pacific Ocean.
Volcanoes at Continental Rifts
- Form at continental rifts due to diverging lithospheric plates, as seen in the East African Rift Basin.
- Resulting eruptions are generally basaltic and can create flood basalts, cinder cones, and lava flows.
- Recent examples include cinder cone eruptions in the Black Rock Desert Volcanic Field, Utah, active 6 million years ago to 720 years ago.
Hotspots
- Source of intraplate volcanism where lithospheric plates move over a hot mantle plume.
- This plume creates basaltic volcanoes that can form islands over time, such as the Hawaiian Islands.
- Hotspots under continental plates can produce intermediate magma, leading to notable features like the Yellowstone caldera.
Volcanic Features and Types
- Stratovolcano: Steep, symmetrical, composed of alternating layers of pyroclastic material and lava; examples include Mount Rainier and Mount Fuji.
- Shield Volcano: Broad, low-angle, formed from low-viscosity basaltic lava; examples include Mauna Loa and Kilauea in Hawaii, as well as Olympus Mons on Mars.
- Lava Domes: Form from viscous felsic rock near volcanic vents; example includes Mount Saint Helens.
- Calderas: Large basin-shaped depressions from the collapse of a volcanic structure; examples include Crater Lake and Yellowstone calderas.
- Cinder Cones: Small, steep-sided volcanoes formed from ejected pyroclastic fragments; notable example is the Parícutin in Mexico.
- Flood Basalts: Vast, low-viscosity eruptions; examples include Columbia River Flood Basalts and Deccan Traps.
Volcanic Hazards
- Pyroclastic flows: Extremely dangerous, composed of ash, gas, and pyroclastic materials moving at high speeds; major cause of fatalities in eruptions like Mount Saint Helens and Mount Vesuvius.
- Landslides: Can be triggered by volcanic activity, resulting in fast-moving debris; associated with tsunamis when they enter oceans, e.g., Mount Krakatau.
- Tephra: Ejected materials from eruptions, including ash that can travel long distances, risking respiratory health and structural integrity of buildings.
- Heavy ashfall can disrupt transportation and impact air travel, as ash is harmful to aircraft engines.### Eyjafjallajökull Volcano Eruption
- In 2010, Eyjafjallajökull in Iceland erupted, producing a substantial ash cloud that disrupted air travel in northern Europe.
- This event marked the largest air travel disruption since World War II, leading to billions of dollars in losses for the global economy, though there were no injuries reported.
Volcanic Gases
- As magma ascends, decreased pressure enables dissolved gases to escape and pose hazards, including carbon dioxide (CO2), sulfur dioxide (SO2), and hydrogen sulfide (H2S).
- CO2 can accumulate in low-lying areas, making it dangerous; Mammoth Mountain in California is a high-risk area, exemplified by three ski patrol members dying from suffocation in 2006 due to CO2 from a fumarole.
- Limnic eruptions can occur suddenly; these happen when crater lake water, saturated with gases, is disturbed, leading to rapid gas release. A notable incident at Lake Nyos, Cameroon in 1986 resulted in nearly 2,000 fatalities from CO2 release.
Lahars
- A lahar is a volcanic mudflow formed from melting snow or glaciers, creating slurries of water, ash, and rock fragments.
- Lahars can flow at speeds of up to 80 km/h (50 mph) and present significant hazards, with cities like Tacoma located on prehistoric lahar routes from Mount Rainier.
- The 1985 lahar from Nevado del Ruiz in Colombia tragically buried Armero, killing approximately 23,000 people.
Monitoring Volcanic Activity
- Geologists employ various monitoring techniques to detect potential eruptions, focusing on earthquake activity, gas emissions, and changes in land surface elevation.
- Earthquake monitoring, particularly for harmonic tremors, can indicate magma movement leading to eruptions.
- Gas monitoring is critical; spikes in gas emissions may suggest that magma is rising and preparing to erupt.
- Tools like GPS units and tiltmeters are utilized to observe land surface changes and predict volcanic activity.
Igneous Rock and Magma Composition
- Igneous rocks are divided into intrusive (formed underground) and extrusive (formed from lava on the surface).
- Magma results from mantle material melting through decompression, flux, or heat-induced processes.
- The composition of magma is shaped by factors such as melting temperatures of minerals and processes like partial melting and magmatic differentiation.
- Volcanic hazards are significant, prompting extensive monitoring by geologists to mitigate risks to human populations.
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Description
Test your knowledge on igneous rocks with this quiz! Answer questions about crystal sizes, silica content, and volcanic rock types. Explore the fascinating processes that distinguish intrusive from extrusive igneous rocks.
