Lecture 6 Flashcards
Why do volcanoes often form at convergent plate boundaries?
At subduction zones, the sinking oceanic plate releases volatiles (like H₂O) into the overlying mantle wedge, lowering the melting temperature. This produces magma that rises to form volcanoes on the overriding plate. These volcanoes are often large (stratovolcanoes) and potentially very explosive.
What are hot spot volcanoes, and how do they form away from plate boundaries?
Hot spots arise from mantle plumes—columns of hot, buoyant material rising from deep in the mantle. When these plumes reach the lithosphere, decompression melting occurs, generating magma. Because tectonic plates move over stationary plumes, a chain of volcanoes can develop, such as the Hawaiian Islands.
What are some major volcanic hazards associated with eruptive materials?
- Lava flows – Can burn or bury structures; mafic flows (low viscosity) can travel far.
- Pyroclastic flows – Hot gas, ash, and rock fragments racing downhill at high speed (100–300 km/h) and temperatures of 500–1000 °C.
- Falling ash – Can accumulate quickly, causing roof collapse, contaminating water, and harming lungs.
- High-altitude ash – Ash plumes can damage aircraft engines and surfaces.
Besides eruptive materials, what other hazards can an eruption produce?
- Blast: Explosive lateral or vertical blasts can occur.
- Landslides: The volcanic edifice can become unstable.
- Lahars: Volcanic ash and debris mixed with water form fast-moving “wet concrete” flows.
- Earthquakes: Moving magma fractures rocks.
- Tsunamis: If volcanoes collapse or explosively erupt under/near water, large waves can form.
- Volcanic gases: Dangerous fumes (e.g., CO₂, H₂S) can asphyxiate or create toxic smog.
How does weathering fit into the rock cycle, particularly after volcanic rocks form?
Once igneous rocks (volcanic or intrusive) are exposed at Earth’s surface, weathering breaks them down into sediments. These loose materials can later be transported, deposited, buried, and eventually lithified into sedimentary rock—completing part of the rock cycle.
What is the difference between physical weathering and chemical weathering?
- Physical weathering (mechanical) breaks intact rocks into smaller clasts by force (e.g., jointing, frost wedging, root wedging, thermal expansion).
- Chemical weathering involves chemical reactions (e.g., dissolution, hydrolysis, oxidation) that alter or dissolve minerals when exposed to water or air.
Can you give examples of physical weathering processes?
- Jointing: Formation of cracks (joints) due to cooling or pressure release.
- Frost wedging: Water in cracks freezes and expands, breaking rock apart.
- Root wedging: Plant roots force their way into cracks.
- Thermal expansion: Rapid temperature changes cause outer layers to expand/contract.
What are some key chemical weathering reactions?
- Dissolution: Water dissolves minerals, especially salts and carbonates.
- Hydrolysis: Water reacts with silicate minerals (e.g., feldspar) to form clay.
- Oxidation: Iron-bearing minerals “rust” when exposed to oxygen.
- Hydration: Minerals absorb water, expand, and weaken the rock.
Why do physical and chemical weathering often work together?
Physical weathering produces more surface area (smaller pieces), allowing water and air to access fresh mineral surfaces, accelerating chemical reactions. Conversely, chemical weathering weakens rocks, making them easier to break physically.
What is differential weathering, and why does it occur?
Differential weathering is when rocks or minerals in the same outcrop weather at different rates. Variations in composition, grain size, or fracture density cause some layers or minerals to wear away faster than others, creating distinctive landscapes or formations.
How do clastic sedimentary rocks form from weathered material?
- Weathering: Produces clasts.
- Erosion/Transport: Water, wind, or ice carry clasts away.
- Deposition: Clasts settle out of the transporting medium.
- Lithification: Compaction and cementation bind clasts into solid rock.
What properties do geologists use to classify clastic sedimentary rocks?
- Grain size (e.g., clay, silt, sand, gravel).
- Grain shape (angular vs. rounded).
- Sorting (well-sorted vs. poorly sorted).
- Composition (quartz, feldspar, lithic fragments, etc.).
- Cement type (calcite, quartz, hematite).
What are some common clastic sedimentary rocks and their defining features?
- Conglomerate: Rounded gravel clasts; coarse-grained.
- Breccia: Angular gravel clasts; coarse-grained.
- Sandstone: Predominantly sand-sized grains (can be quartz-rich or arkosic).
- Siltstone: Fine-grained, mostly silt-sized particles.
- Shale or Mudstone: Very fine-grained, clay-rich.
Aside from clastic rocks, what other types of sedimentary rocks exist?
- Biochemical: Formed from biological activity (e.g., limestone from shells, chert from silica-secreting organisms).
- Organic: Consisting mainly of carbon-rich remains of plants or organisms (e.g., coal).
- Chemical: Formed from precipitation of minerals from water (e.g., evaporites like halite or gypsum).
How do geologists interpret past environments using sedimentary rocks?
Sedimentary features (grain size, composition, sedimentary structures, fossils) reflect the depositional environment (e.g., river channels, deserts, deltas, shallow marine). By analyzing these clues, geologists reconstruct ancient landscapes, climates, and life.
