The Age of the Earth • Prior to the 19th century, accepted age of Earth based on religious beliefs • ~6,000 years for Western culture (Biblical) • Old beyond comprehension (Chinese/Hindu) • James Hutton, the“father of geology”, realized geologic processes require vast amounts of time • Charles Lyell popularized Hutton’s concepts in book Principles of Geology • Uniformitarianism/actualism: same processes operating in past are operating at present - “The present is the key to the past”
History of Early Geology Catastrophism (James Ussher, mid 1600s) - He interpreted the Bible to determine that the Earth was created at 4004 B.C. This was generally accepted by both the scientific and religious communities. Subsequent workers then developed the notion of catastrophism, which held that the Earth’s landforms were formed over very short periods of time. Uniformitarianism (James Hutton, late 1700s) - He proposed that the same processes that are at work today were at work in the past. Summarized by “The present is the key to the past.” Hutton, not constrained by the notion of a very young planet, recognized that time is the critical element to the formation of common geologic structures. Uniformitarianism is a basic foundation of modern geology.
Geologic Time and Clocks Lifespan of a human ~ 100 years Human civilization ~10,000 years Modern humans ~100,000 years Stone tools ~1,000,000 years Age of oceanic crust ~100,000,000 years Precambrian Explosion ~540,000,000 years Oldest Rocks ~3.8 Billion years Age of the Earth ~4.3 Billion years Age of the Solar System ~4.6 Billion years Age of the Universe ~14 Billion years
Relative vs. Numerical Age • Relative age - the order of events or objects, from first (oldest) to last (youngest) • Determined by applying simple principles, including original horizontality, superposition, lateral continuity, cross-cutting relationships, inclusions, unconformities, and correlation of rock units and fossils • Numerical age- the age of events or objects, expressed as a number or numbers • Determined using radiometric dating (determining how much radioactive decay of a specific element has occurred since a rock formed or an event occurred)
Absolute Ages • Numerical dating- puts absolute values (e.g., millions of years) on the ages of rocks and geologic time periods • Uses radioactive decay of unstable isotopes • Only possible since radioactivity was discovered in 1896 • Radioactive isotopes decay in predictable manner, giving a characteristic half-life (time it takes for a given amount of radioactive isotope to be reduced by half)
Radioactive Decay N = N0 e-kt Where N is the amount of the radioactive element in the rock now; N0 is the amount originally in the rock, e ~ 2.718 (natural logarithm); k is the decay constant of the radioactive element, and t is time. Half-life when N/N0 = 0.5
Absolute Time: Radiometric Clocks Absolute (Radiometric) Dating: Using radioactive decay of elements to determine the absolute age of rocks. This is done using igneous and metamorphic rocks. Carbon-14 half-life ~ 5730 years Used in anthropology
Relative Age Determination • Contacts - surfaces separating successive rock layers (beds) • Formations - bodies of rock of considerable thickness with recognizable characteristics allowing them to be distinguished from adjacent rock layers • Original horizontality - beds of sediment deposited in water are initially formed as horizontal or nearly horizontal layers
Relative Age Determination • Superposition - within an undisturbed sequence of sedimentary or volcanic rocks, layers get younger from bottom to top • Lateral continuity - original horizontal layer extends laterally until it tapers or thins at its edges
Which are the youngest rock layers? Which are the youngest rock layers? What is the sequence of formation?
Relative Age Determination • Cross-cutting relationships - a disrupted pattern is older than the cause of the disruption • Intrusions and faults are younger than the rocks they cut through • Baked contacts - contacts between igneous intrusions and surrounding rocks, where surrounding rocks have experienced contact metamorphism • Inclusions - fragments embedded in host rock are older than the host rock
Which are the youngest rock layers? What is the sequence of formation?
Unconformities • Unconformity - a surface (or contact) that represents a gap in the geologic record • Disconformity - an unconformity in which the contact representing missing rock layers separates beds that are parallel to each other • Angular unconformity - an unconformity in which the contact separates overlying younger layers from eroded tilted or folder layers
Unconformities • Nonconformity - an unconformity in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock • Plutonic and metamorphic rocks exposed by large amounts of erosion • Typically represents a large gap in the geologic record
Which are the youngest rock layers? What is the sequence of formation? Where is the unconformity?
Correlation • Correlation - determining the time-equivalency of rock units • Within a region, a continent, between continents • Physical continuity • Physically tracing a continuous exposure of a rock unit • Easily done in Grand Canyon • Similarity of rock types • Assumes similar sequences of rocks formed at same time • Can be inaccurate if very common rock types are involved • Correlation by fossils • Fossil species succeed one another through the layers in a predictable order (faunal succession) • Similar fossil assemblages (groups of different fossil species) used
Stratigraphic Methods Lithostratigraphy (correlating rock layers by using rock types) Biostratigraphy (correlating rock layers by using fossils) Magnetostratigraphy (correlating rock layers by using magnetic reversals) Chemostratigraphy (correlating rock layers by using chemical or isotopic methods for correlating; e.g., oxygen isotopes or (iridium spike at end of Cretaceous) Chronostratigraphy (correlating rock layers by using absolute and/or relative time)
Geologic Time Scale • Standard geologic time scale • Worldwide relative time scale • Subdivides geologic time based on fossil assemblages • Divided into eons, eras, periods, and epochs • Precambrian - vast amount of time prior to the Paleozoic era; few fossils preserved • Paleozoic era - “old life” • appearance of complex life; many fossils
Geologic Time Scale • Mesozoic era - "middle life" • Dinosaurs abundant on land • Period ended by mass extinction • Cenozoic era - "new life" • Mammals and birds abundant • We are currently in the Recent (Holocene) Epoch of the Quaternary Period of the Cenozoic Era • Most recent ice ages occurred during the Pleistocene Epoch of the Quaternary Period
Combining Relative and Numerical Ages • Radiometric dating gives numerical time brackets for events with known relative ages • Individual layers may be dated directly • Radiometric dating of units above and below brackets age of units in between • Geologic Time Scale • Divided into four Eons • Hadean, Archean, Proterozoic, Phanerozoic • Precambrian (all time prior to Phanerozoic) represents 87% of geologic time)
Age of the Earth • Numerical dating gives absolute age for Earth of about 4.56 billion years • Oldest age obtained for meteorites, believed to have been unchanged since the formation of the solar system • Earth and rest of solar system very likely formed at this time • Geologic (deep) time isvast • A long human lifetime (100 years) represents only about 0.000002% of geologic time
Why Study of Historical Geology? • Survival of the human species may depend on understanding how Earth’s subsystems work and interact and exploring the past is our only ‘laboratory’ for testing hypotheses. • Knowing what occurred in the past can help us to understand our origins and place in both the Earth and the Universe.
Latest Precambrian / Early PaleozoicSupercontinent Rodinia, centered about the south pole, breaks apart. North America (Laurentia), Baltica, and Siberia moved North. Marine Invertebrates.North America: arc on the south. Baltica and Siberia moved in from the SE. Texas (505-570 Ma): Flat plain; remnants of eroded collisional belt (Llano). Shallow marine seas across much of Texas. Sandy sediment onshore, limestone offshore. Trilobites, brachiopods. http://vishnu.glg.nau.edu/rcb/globaltext.html
two kinds cyanobacteria from the Bitter Springs chert of central Australia, a site dating to the Late Proterozoic, about 850 million years old. On the left is a colonial chroococcalean form, and on the right is the filamentous Palaeolyngbya. Cyanobacteria – green algae
Dickinsoniais known from Vendian rocks of south Australia and north Russia. It is often considered to be an annelid worm Ediacaran Fauna
Kimberella, one of the most fascinating Vendian fossils, has received a great deal of attention lately. It was hypothesized to be a box jellyfish (cubozoan) this unusual disc-shaped form with three-part (triradial) symmetry. Named Tribrachidium heraldicum, its affinities are still mysterious, although distant relationships have been proposed with either the Cnidaria (corals and anemones) or Echinodermata (urchins and seastars). Ediacaran Fauna
Nemiana is one of the simplest of all Vendian fossils, and is difficult to interpret. It seems to be an impression of a saclike body Ediacaran Fauna
specimen is only a small part of Anomalocaris, which was a large (up to 60cm or more) arthropod-like predator. Marrella splendens is a small "arthropod" somewhat reminiscent of a trilobite Burgess Shale Tuzoia is a "bivalved" crustacean grossly similar to certain types of modern brine shrimp.
Vauxia gracilenta has a branching morphology: a sponge Burgess Shale
Trace Fossils Arthropod tracks Worm trails Trilobite tracks
Latest Precambrian / Early Paleozoic Supercontinent Rodinia continues to break apart. Pieces move north. -Fish. -Glaciation. North America: Numerous plates and continental blocks move in from the south and east. The Taconic arc collides, forming the Taconic orogeny. Texas 438-505 Ma:Shallow marine seas across much of inland Texas. Warm-water limestone. Corals, brachiopods. http://vishnu.glg.nau.edu/rcb/globaltext.html
Middle / Late Paleozoic Remains of Rodinia (Gondwana) move northward to collide with Laurasia -- creating the super continent Pangaea and the Tethys Ocean. First land-plants. Baltica collides with North America in the Caledonian-Acadian orogeny. Texas 403-438 Ma:Shallow marine seas across much of west Texas - limestone. Corals, brachiopods. http://vishnu.glg.nau.edu/rcb/globaltext.html