Understanding Geologic Time

Geologic Time Introduction

• Earth is 4.566 billion years old, which is a crucial discovery for understanding our place in nature and history.
• Human civilization occupies a tiny fraction (~0.0003%) of Earth's history when viewed against this immense timescale.
• Deep time refers to the vastness of geological time, which can be difficult to comprehend due to its scale.

Abbreviating Geological Time

• "ka" represents one thousand years (kilo-annum).
• "Ma" represents one million years (mega annum).
• "Ga" represents one billion years (giga annum).
• "kya" and "mya" are used to describe events that occurred thousands or millions of years ago, respectively.
• Numerical (absolute) dating: Assigning actual ages (years before present) to events.

Geology as a Historical Science

• Geology studies Earth and its history, reconstructing past events using evidence like rock layers, minerals, and fossils.
• Considered a historical science, similar to evolutionary biology, climatology, archaeology, and astronomy.
• Astronomy: the light from distant galaxies takes billions of years to reach Earth, so we're seeing them as they were in the past.

Foundational Concepts of Historical Geology

• Uniformitarianism: physical and chemical laws of nature have been constant throughout Earth's history.
• Uniformitarianism allows us to use observations of the modern world to understand Earth’s history.
• Intensities and rates of some geological processes have changed and catastrophic events have occurred but the chemical and physical laws of nature have remained constant over time.
• "The present is the key to the past," meaning studying Earth processes today helps understand past and predict future.
• Uniformitarianism is a fundamental idea in geology.
• James Hutton (1726-1797) developed the concept of uniformitarianism.
• Charles Lyell (1797-1875) popularized the idea in his book "Principles of Geology".

Principles of Relative Dating

• Relative dating determines the sequence of past geological events without knowing actual ages.
• Largely applied to sedimentary rock layers, which preserve much about life.
• Igneous rock can be included in the layers when it forms horizontally, in layers parallel to the Earth’s surface, as in lava flows, pyroclastic flows and ash falls.
• Determines whether one geological or paleontological event happened before or after another.

Principle of Superposition

• Nicholas Steno (1638-1686) studied relationships between rock layers, sediment, minerals, and fossils.
• The principle of superposition: rocks positioned below other rocks are the older than rocks above.

Principle of Faunal Succession

• William Smith (1769 – 1839) noticed the textural similarities and differences between rocks in different locations, and more importantly, he showed that fossils could be used to correlate rocks of the same age.
• Fossil A is always found below Fossil B, which is always found below Fossil C, and so on.
• Documenting sequences of fossils, that helps correlating rock layers (strata) from place to place.
• Rock layers in two different places were the same age as they include the same distinctive types of fossils.

Principle of Original Horizontality

• Sediments are deposited by gravity in layers parallel to Earth’s surface which also applies to volcanic igneous rocks that have been formed by lava flows, pyroclastic flows and ashfall deposits.
• Layers along basin edges may slope slightly.
• This principle helps "unfold the folds" and reconstruct deformed layers to understand their geological history.

Principle of Lateral Continuity

• Layers of sediment initially extend outward in all directions and are laterally continuous.
• Rocks separated by a valley or erosional feature can be assumed to be continuous.
• Limits are controlled by sediment amount, type, and sedimentary basin size and shape.
• Thickest deposits are found closest to the source.

Principle of Cross-Cutting Relationships

• Any geological feature that cuts across or disrupts another feature must be younger than the feature that is disrupted.
• Faults disrupting sedimentary rock layers show that the fault is younger than the layers.
• Hot magma can force its way into a fracture, or weakness, in pre-existing rock of the crust, these two clues help understand that the dyke is younger than the rock it cross-cuts.
• A new pattern such as tectonic cleavage/folding overprints a pre-existing structure.

Principle of Inclusions

• Any rock fragments included in another body of rock must be older than the rock in which they are included.
• A xenolith of granite included in basalt indicates the basalt intruded into the granite.
• Contact metamorphism (baking of the granite) also suggests the granite was in place before the intrusion, baking the rock prior to the instusion of hot magma/lava.

The Grand Canyon Example

• The Grand Canyon shows superposition (oldest at bottom, youngest at top).
• The principle of lateral continuity is illustrated by the Kaibab Limestone
• The exposed cliffs reveal 1.7 billion years of geological history.
• The diagram of the rocks exposed on the walls of the Grand Canyon, illustrating the principle of cross-cutting relationships, superposition, and original horizontality
• Principle of cross-cutting relationships: the metamorphic schist is the oldest, granite intrusion is younger.
• The numbered layers on the Grand Canyon walls illustrates the principle of superposition.
• The Colorado River carves through the Colorado Plateau, exposing the horizontal strata, that follows the principle of original horizontality

Unconformities – Missing Time

• Unconformity: An interruption in sedimentary layer deposition.
• Recognizing unconformities is important for understanding time relationships.
• Angular unconformity: Tilted Proterozoic rocks eroded flat before Paleozoic rocks were deposited.
• Nonconformity: Erosional surface separates igneous/metamorphic rock from overlying sedimentary rock.
• Disconformity: Occurs between parallel sedimentary layers.

Dating Rocks Using Fossils

• Paleontology: systematic assignment of relative ages to organisms from the distant past.
• Oldest undisputed fossils are from rocks dated around 3.5 Ga.
• Fossils can't provide numerical ages but can give time limits when combined with isotopic dates.
• Apply knowledge to determine the relative ages of rocks.
• William Smith’s principle of faunal succession is based on sequence of evolution on Earth.

Geologic Range

• Fossil identification to species/genus level and known time period provide a time range for the rock.
• Narrow time ranges if multiple fossils with known ranges are present.
• Index fossils: organisms that lived for relatively short periods of time over a wide geographic area.

Biozones

• Biozone fossils: Stratigraphic interval defined by specific fossils.
• Ammonoids: Complex suture patterns indicate younger fossils
• Foraminifera: Small, carbonate-shelled marine organisms useful as biozone fossils.

Numerical (or Absolute) Dating

• Numerical dating assigns actual dates (years before present) to geological events.
• Relative dating is concerned with determining the order of events in Earth’s past.
• The science of absolute age dating is known as geochronology, while the method is radiometric dating.

Radiometric Dating

• Hypotheses of absolute ages are determined from radioactive decay rates of isotopes in rocks.
• Element: Defined by the number of protons (atomic number).
• Isotopes: Variations in the number of neutrons.
• Isotopes are defined by their mass number (protons + neutrons).

Radioactive Decay

• Unstable isotopes shed energy, changing their number of protons and neutrons through the process of radioactive decay.
• The radioactive element will decay to form a different, stable element.
• The atomic nucleus that undergoes radioactive decay is known at the parent and the resulting stable element, the daughter product.
• We are dating a mineral in the rock not the rock itself.
• Ideal mineral accepts radioactive parent isotopes and excludes daughter products and is picky accepting some atoms into its crystals, but at the same time, they actively exclude the stable daughter atoms that result when the radioactive parents break down.
• In some cases, the radioactive element in question is an isotope of an element essential to the mineral’s growth, radioactive decay of 40K → 40Ar, both elements remain trapped within the closed system of the formed mineral.
• An example of this occurs in the mineral zircon (ZrSiO4), radioactive 238U atoms can substitute for Zr, due to the atomic characteristics of Pb, there is no “happy place” for Pb in the crystal, so Pb atoms will never enter the zircon crystal lattice while it is forming, radioactive decay of 238U → 206Pb occurs over time, both elements remain trapped within the closed system of the formed mineral.

Half-life Decay Constant Chart

• Zircon (also Monazite) contains radioactive 238U and decays to 206Pb with a half-life of 4.5 billion years which makes it useful in dating igneous, sedimentary, and metamorphic rocks.
• Muscovite, biotite, hornblende, potassium feldspar contain 40K and decays to 40Ar with a half-life of 1.25 billion years which makes it useful in dating igneous, and metamorphic rocks.
• Muscovite, biotite, potassium feldspar contain 87Rb and decays to 87Sr with a half-life of 48.8 billion years which makes it useful in dating igneous, and metamorphic rocks.

Radioactive Decay Curve

• At the start time (zero half-lives passed), the sample consists of 100% parent atoms (blue diamonds); there are no daughter products (red squares) because no time has passed.
• After the passage of one half-life, 50% of the parent atoms have become daughter products.
• After two half-lives, 75% of the original parent atoms have been transformed into daughter products (thus, only 25% of the original parent atoms remain).
• The ratio of parent atoms relative to daughter products in a sample will determine how many half-lives have passed since a mineral grain first formed.

Calculating Radiometric Dates

• Convert the number of half-lives that have passed into a numerical (i.e., absolute) age.
• Multiple the number of half-lives that have passed by the half-life decay constant of the parent isotope.

Carbon-14 (14C) Dating

• Virtually everyone has heard of Carbon-14 dating.
• However its main use is determining archeological dates of wood, charcoal, seeds, pollen, pottery, paper, natural fabric and more recent bone and shell material.

Mineral Ages vs. Rock Ages

• In igneous rocks all the minerals are the same age as the rock.
• Zircon crystal (igneous mineral) date from a granite (igneous rock) may be taken as representative of the age of the granite
• In a clastic sedimentary rock the minerals that make up a clastic sedimentary rock don’t form in place: they originated elsewhere and some time before.
• Detrital zircon have been very useful in tracking down the Earth’s oldest rocks.
• An original crystal can add new layers when it gets metamorphosed.
• The SHRIMP is a tool that allows the dating of different parts of a single zircon crystal.