Geo Hour Exam 2 Flashcards
What is a sedimentary rock and why is it significant?
A rock formed from weathered fragments (sediment) that undergoes compaction, cementation, and recrystallization. It’s significant because it is the rock type most likely to contain fossils.
Define lithification.
Lithification is the process that transforms loose sediment into solid sedimentary rock through compaction, cementation, and recrystallization.
What are the main sedimentary structures, and what do they indicate?
Structures include bedding (horizontal layers), graded beds (coarse-to-fine transitions, often from turbidity currents), and cross bedding (inclined layers from dunes or delta deposits), each indicating specific depositional environments.
What does diagenesis entail in the context of sedimentary rocks?
Diagenesis refers to the chemical, physical, and biological changes that occur to sediments after deposition, within the upper few kilometers of the crust.
How do diagenesis and lithification differ?
Diagenesis includes all post-depositional changes (like recrystallization and chemical alterations), while lithification specifically is the conversion of sediment into rock via compaction, cementation, and recrystallization
How are chemical sedimentary rocks formed?
They form by precipitation of dissolved material—either by inorganic processes (like evaporation of seawater leading to evaporite minerals) or by organic processes (biochemical origins such as cemented shells).
What role does grain size, sorting, and texture play in identifying clastic sedimentary rocks?
They determine the rock’s texture; well-sorted sediments with distinct particle sizes can indicate specific depositional environments (e.g., terrestrial deposits like arkose).
What are the key steps in coal formation?
Coal forms through the accumulation of plant material under low heat conditions, which is later transformed (and sometimes further metamorphosed) into higher-grade coals.
What are the primary agents of metamorphism?
Pressure, temperature, fluids, and stress—all altering a rock’s mineralogy and texture while keeping it essentially solid.
How does metamorphic grade progress in rocks?
With increasing grade, rocks commonly progress from slate to phyllite to schist to gneiss, reflecting greater pressure and temperature conditions.
What is the difference between regional and contact metamorphism?
Regional metamorphism affects large areas under sustained pressure and heat (producing slate, schist, gneiss), while contact metamorphism is localized around igneous intrusions (often forming hornfels).
What are index minerals and why are they important?
Index minerals form under specific pressure and temperature conditions, serving as markers to determine a rock’s metamorphic grade and environment.
What principles underlie relative dating in geology?
The laws of relative time (such as superposition and cross-cutting relationships) and fossil correlation help establish the sequence of geologic events.
How is absolute dating performed?
Absolute dating uses radioactive decay (e.g., U‑238 with its known half-life) and accumulation clocks; although C‑14 dating is used, it’s more common in archaeology than in geology.
What do unconformities, like those in the Grand Canyon, represent?
Unconformities indicate time gaps in the geologic record—often where erosion has removed previously deposited layers, marking a break between older plutonic rocks and younger sediments.
What is a fault, and what are the different types?
A fault is a fracture in brittle rock along which displacement has occurred. Types include dip-slip (normal in extensional settings, reverse in compressive settings) and strike-slip (lateral displacement, such as the San Andreas Fault).
What distinguishes folds from faults in crustal deformation?
Folds are bends in layered rock (with features like limbs, axis, and axial plane) resulting from ductile deformation under compression, whereas faults are fractures with observable displacement.
What are the key parts of a fold?
A fold consists of limbs (the sides), an axis (the line of maximum curvature), and an axial plane that divides the fold symmetrically.
How are earthquake intensity and magnitude defined?
Magnitude measures the energy released (using scales like Richter), while intensity describes the observed damage and effects at specific locations.
Why do seismic waves produce shadow zones?
P-waves are refracted by changes in Earth’s density, and S-waves cannot travel through liquids, creating areas (shadow zones) where these waves are not detected.
How are ocean basins and continental margins characterized?
Ocean basins are mapped by echo sounding; passive margins (e.g., Atlantic) feature broad continental shelves with little tectonic activity, while active margins (e.g., Pacific NW) are near convergent boundaries with trenches and seismicity
What is continental rifting and why is it important?
Continental rifting involves the upwelling and tension that pulls crust apart, initiating the formation of a new ocean basin.
What are fault-block mountains and how do they form?
Fault-block mountains form when large blocks of crust are uplifted and tilted along faults, typically resulting in nearly parallel ranges averaging about 80 km in length.
What is the Wilson cycle?
The Wilson cycle describes the full cycle of continental breakup, ocean basin formation, and eventual collision that leads to mountain building.
How does the rate of oceanic spreading influence ridge elevation?
Faster spreading rates can affect the structure along the ridge axis (including the development of rift valleys), as seen at mid-Atlantic ridges spreading at about 1–5 cm/year.
How does the age of subducting lithosphere affect the location of a volcanic arc?
Older lithosphere has cooled more and leads to arcs located further from the ridge, while younger, warmer lithosphere results in arcs closer to the spreading center.
How is a tsunami generated at a convergent boundary?
At a convergent boundary, a megathrust earthquake can suddenly lift or drop the seafloor. This abrupt vertical movement displaces a huge volume of water, initiating tsunami waves.
What are the key features for identifying sedimentary rocks?
Look for features such as bedding (horizontal layers), graded and cross-bedding, texture (grain size and sorting), and specific markers like fissility and conglomerate fragments that help distinguish different rock types.
What are sedimentary facies and why are they important?
Sedimentary facies are bodies of sediment with distinct physical, chemical, and biological characteristics that indicate specific depositional environments. They help geologists reconstruct past environments and depositional processes.
What is metamorphism?
Metamorphism is the process of “changing form” in which a rock’s mineralogy, texture, and sometimes chemical composition are altered under elevated temperatures and pressures without significant melting.
Why is the parent rock important in metamorphism?
Every metamorphic rock originates from a parent rock—whether igneous, sedimentary, or even an earlier metamorphic rock—which undergoes physical and chemical changes during metamorphism.
What does metamorphic grade refer to?
Metamorphic grade indicates the degree of change a rock undergoes during metamorphism, progressing from low grade (low temperatures and pressures) to high grade (high temperatures and pressures).
How do tectonic settings influence metamorphism?
Tectonic settings create specific metamorphic conditions: contact (high T, low P), regional (high T and P), and hydrothermal (hot, ion-rich fluids).
What are the primary controls on metamorphism?
The primary controls include the composition of the parent rock along with four agents: Temperature, Pressure, Fluids, and Stress. Temperature and pressure are the main drivers of metamorphic reactions.
What is foliation and how does it develop?
Foliation is the planar alignment of platy or elongated minerals within a rock, created by directed stress during metamorphism. It leads to textures seen in rocks like slate, schist, and gneiss and is distinct from sedimentary bedding.
What role do index minerals play in metamorphic reactions?
They form at specific P and T conditions, marking metamorphic grade (e.g., kyanite, andalusite, sillimanite).
What are metamorphic facies?
Metamorphic facies are assemblages of minerals that indicate the range of pressure and temperature conditions during metamorphism, allowing geologists to interpret the metamorphic environment and grade.
How do fluids affect metamorphism?
Fluids (mainly water with volatiles like CO₂) enhance ion migration and recrystallization, accelerating metamorphic reactions. In some cases, they lead to metasomatism, where the bulk chemical composition of the rock is altered.
Why is retrograde metamorphism uncommon?
It’s rare because lost volatiles aren’t replenished.
