Earth Science: Volcanology, Geologic Time, Paleontology, and Precambrian History

Comprehensive review flashcards covering volcanology and volcanic hazards, relative and radiometric dating principles, the geologic time scale, fossil preservation and major invertebrate/protist groups, and Precambrian planetary formation and early life evolution.

How does viscosity differ between pahoehoe and a'a lava flows?

Viscosity measures how easily a fluid flows; pahoehoe forms from lower-viscosity, runnier lava that flows smoothly, whereas a'a forms from higher-viscosity, thicker lava that breaks into rough, jagged blocks.

How do temperature and silica content control the viscosity of magma and lava?

Higher temperatures result in lower viscosity (runnier magma), whereas higher silica content introduces more silicate tetrahedra (more felsic), resulting in higher viscosity (thicker magma).

What are the key structural and eruptive differences between shield volcanoes, cinder cones, and stratovolcanoes?

Shield volcanoes are broad, low-viscosity stacks of fluid basaltic flows that collapse when empty (e.g., Kilauea); cinder cones have steep sides of moderate viscosity composed mostly of pyroclastic rock fragments with a'a oozing from the base (e.g., SP Crater); and stratovolcanoes (composite volcanoes) are large, steep, energetic, and deadly volcanoes built from high-viscosity lavas and pyroclastic deposits (e.g., Mount Fuji, Mount St. Helens).

What microscopic volcanic material from Mount Redoubt (2009) is shown in this scanning electron micrograph?

Volcanic ash (sharp shards of dense, abrasive rock and volcanic glass that pose severe hazards to machinery, roofs, and respiratory health).

What is a pyroclastic flow, and what speeds can it achieve?

A superheated mix of gas, ash, and rock fragments that travels at an average speed of $100\,\text{km/h}$ and can reach speeds up to $700\,\text{km/h}$.

What geological event caused this massive coral block to be displaced $1.8\,\text{miles}$ inland at Berouw following the 1883 Krakatau eruption?

A volcanic tsunami triggered by pyroclastic flows entering the ocean and caldera collapse, generating waves up to $46\,\text{m}$ high traveling at $560\,\text{km/h}$.

What is a lahar, and what triggers it?

A volcanic mudflow formed when volcanic ash mixes with melted snow, ice, or heavy rainwater.

What were the primary geographic features and devastating consequences of the August 26–27, 1883 eruption of Krakatau?

Located in the Sunda Strait between Sumatra and Java, Krakatau had three peaks (Rakata, Danan, and Perboewatan); on August 27 at 10:02 AM, its third blast produced the loudest sound on record (heard $5000\,\text{km}$ away), destroyed Danan and Perboewatan, killed an official death toll of $36{,}417$ people, and wiped out 165 villages.

What is the fundamental difference between relative dating and absolute dating?

Relative dating determines whether a rock unit or geologic event is older or younger than another without using numerical ages, whereas absolute dating places a specific numerical age (e.g., $55\,\text{Ma}$) on rocks or events.

What do the stratigraphic principles of Superposition, Original Horizontality, and Lateral Continuity state?

Superposition states that in an undeformed sedimentary sequence, younger strata stack on top of older strata; Original Horizontality states that sediments are originally deposited in flat, horizontal layers; and Lateral Continuity states that rock layers extend outward in all directions continuously until they thin out or terminate against a basin margin.

What does the Principle of Cross-cutting Relationships establish?

Any geologic feature, rock unit, intrusion, or fault that cuts through another rock body or structure is younger than the rock it cuts.

What are the three main types of unconformities, and how are they distinguished?

A disconformity is an erosional surface between parallel sedimentary beds; an angular unconformity features tilted or folded sedimentary strata overlain by younger, horizontal beds; and a nonconformity is a contact where younger sedimentary strata lie directly on top of older igneous or metamorphic crystalline basement rock.

What is the Principle of Faunal Succession, and how is it used in biostratigraphy?

Fossil species succeed each other in a specific, reliable, and recognizable vertical order through geologic time; biostratigraphy uses overlapping temporal ranges of fossil species to determine and correlate the relative ages of strata worldwide.

What early estimates for the age of the Earth were proposed by Ussher, Kelvin, Joly, and Lyell?

Archbishop Ussher (1650) calculated ~$6000\,\text{years}$ from biblical genealogies; Lord Kelvin (1865–1897) calculated ~$20\text{--}40\,\text{million years}$ based on thermal cooling; John Joly (1899) estimated ~$90\,\text{million years}$ from salt accumulation in oceans; and Charles Lyell (1830) inferred several hundred million years using uniformitarianism and sedimentation rates.

What is a radioactive half-life?

The exact time required for half of the unstable radioactive parent isotopes in a sample to decay into stable daughter isotopes.

What are the parent isotopes, daughter isotopes, and half-lives of the five common radiometric systems?

Uranium-238 to Lead-206 ($4.5\,\text{Ga}$); Uranium-235 to Lead-207 ($704\,\text{Ma}$); Potassium-40 to Argon-40 ($1.25\,\text{Ga}$); Rubidium-87 to Strontium-87 ($48.8\,\text{Ga}$); and Carbon-14 to Nitrogen-14 ($5730\,\text{years}$).

Why is Carbon-14 ($^{14}\text{C}$) dating not used to date ancient geologic rocks?

Because of its relatively short half-life of $5730\,\text{years}$, Carbon-14 decays too rapidly and is only useful for organic material younger than $67{,}000\,\text{years}$.

What condition must be met for a mineral to provide an accurate radiometric date, and why is zircon ideal?

It must be a closed system where no parent or daughter atoms enter or escape once crystallized; zircon ($\text{ZrSiO}_4$) is ideal because it strongly incorporates uranium impurities into its crystal lattice upon crystallization but rejects lead.

Why can igneous rocks be directly dated radiometrically while clastic sedimentary rocks cannot?

Igneous minerals close their isotopic system when magma crystallizes, dating the rock formation itself; clastic sedimentary grains yield the age when their source igneous or metamorphic rocks crystallized, not the age when the sedimentary stratum was deposited.

In this stratigraphic section with datable volcanic ash layers (Ash A, B, and C) and fossil-bearing shale layers, how are the absolute ages of Shale 1 and Shale 2 determined?

By bracket dating: the age of Shale 1 is constrained between the radiometric dates of Ash A (below) and Ash B (above), while Shale 2 is constrained between Ash B and Ash C.

If a rock has undergone 3 half-lives, what percentage of the parent isotope remains and what percentage of daughter product is present?

$12.5\%$ parent isotope remains and $87.5\%$ daughter product is present.

What are the four eons of geologic time in order from oldest to youngest, and what are their starting ages?

Hadean (starts at $4.6\,\text{Ga}$), Archean (starts at $4.0\,\text{Ga}$), Proterozoic (starts at $2.5\,\text{Ga}$), and Phanerozoic (starts at $542\,\text{Ma}$).

What are the three eras of the Phanerozoic Eon, what do their names mean, and what dates mark their beginnings?

Paleozoic ('old life', began $542\,\text{Ma}$), Mesozoic ('middle life', began $251\,\text{Ma}$), and Cenozoic ('new life', began $65\,\text{Ma}$).

What are the 13 periods of the Phanerozoic Eon in chronological order?

Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene, and Quaternary.

What is the difference between a body fossil and a trace fossil?

A body fossil represents an actual anatomical part of the organism (e.g., bones, teeth, shells, leaves), whereas a trace fossil represents a physical record of biological activity (e.g., footprints, trackways, burrows, coprolites, feeding trails).

What key environmental circumstances favor the preservation of fossils?

Rapid burial in fine-grained sediment to prevent scavenging and decay, possession of mineralized hard parts, low-oxygen (anoxic) calm-water conditions (such as deep lakes), alkaline environments to avoid dissolution, and ash-fall smothering.

What is a Lagerstätte, and what are two classic examples?

A sedimentary deposit exhibiting extraordinary fossil richness and exquisite, soft-tissue preservation; examples include the Solnhofen Limestone in Germany and the Messel Shale.

Which famous transitional fossil discovered in the Solnhofen Limestone Lagerstätte of Germany is displayed in this image?

Archaeopteryx, displaying both avian feathers and non-avian dinosaurian skeletal traits.

What is a pseudofossil?

An inorganic mineral or sedimentary structure that visually mimics a fossil organism, such as mineral concretions, manganese dendrites, or septarian nodules.

What type of single-celled marine protist is shown in this scanning electron micrograph, characterized by spherical calcium carbonate armor plates?

Coccolithophores

How do foraminifera and radiolarians differ in shell composition and ecological deposition?

Foraminifera ('forams') produce multi-chambered shells made of calcite (calcium carbonate), whereas radiolarians build intricate shells of silica that accumulate on the deep ocean floor to form chert.

Which phylum of radially symmetrical invertebrates possesses the two basic body plans illustrated in this diagram: the polyp and the medusa?

Cnidaria (includes corals, sea anemones, and jellyfish).

What type of fossil cephalopod mollusc is shown here, recognized by its spiral shell and complex suture lines?

Ammonite

What extinct group of marine arthropods characterized by a three-lobed, segmented exoskeleton is shown in this fossil specimen?

Trilobite

Which class of echinoderm, commonly called a sea lily, is preserved in this fossil specimen?

Crinoid

What hydrothermal vent creature, discovered in 2005, is shown in this photograph cultivating chemosynthetic bacteria on its setae?

Yeti crab (Kiwa hirsuta)

How does the Nebular Hypothesis explain the formation of the solar system?

A large interstellar cloud (nebula) of gas and dust collapsed under gravity, flattened into a rotating protoplanetary disk, concentrated mass at the center to form the protosun, and allowed cooling dust particles of metal, rock, and ice to collide and accrete into planetesimals and planets.

What was the process of global planetary differentiation on early Earth?

Gravitational energy and collision heat completely melted the early Earth, allowing dense elements (mostly iron and nickel) to sink into the core while buoyant, lighter silicates rose toward the surface to form the mantle and rocky crust.

How did Earth's Moon form according to current scientific theory?

A Mars-sized impactor collided with the young proto-Earth, and the resulting ring of ejected debris (largely from the impactor's rocky mantle) accreted in orbit to form the Moon.

What were the primary sources of Earth's early atmosphere and oceans?

The early atmosphere was produced by volcanic outgassing (rich in volcanic gases, lacking free oxygen), while the early oceans formed largely from water delivered by comet bombardments along with condensed volcanic steam.

How did early continental crust grow through continental accretion?

Collisions of volcanic island arcs and microcontinents at convergent plate boundaries caused buoyant crustal fragments (terranes) to weld together against stable continental cratons.

What are stromatolites, and when does their earliest fossil evidence appear?

Layered biosedimentary structures created by mats of photosynthetic cyanobacteria that trap sediment, with earliest fossil examples dating back to $3.5\,\text{Ga}$ in the Archean Eon.

What was the Great Oxidation Event (GOE) at $2.4\,\text{Ga}$, and why was the accumulation of free oxygen in the atmosphere delayed?

The GOE was the accumulation of free oxygen gas in the atmosphere; it was delayed for over a billion years because oxygen produced by cyanobacteria was consumed by reacting with abundant dissolved iron in the oceans, precipitating Banded Iron Formations (BIFs).