Dynamic Earth Class 2 12 January 2006 Any

Dynamic Earth Class 2 12 January 2006

Any Questions?

Uniformitarianism The present is the key to the past — James Hutton Geologic processes that we see in operation today have worked much the same way over geologic time — however, rates and intensity of processes may have changed.

Early estimates of Earth’s age n 1650: James Ussher -- Age: 6000 years (From reconstruction of the Bible) n By the mid-19 th Century, the age of the Earth was still only an educated guess, but the general feeling was that the Earth was at least 500 million years old, and probably much older.

Radiometric Dating n Use of radioactive decay to determine the age of a rock n First proposed ~1896 -1902

Isotopes Different forms of the same element containing the same number of protons, but varying numbers of neutrons i. e. : 235 U, 238 U 87 Sr, 86 Sr 14 C, 12 C

Naturally Occurring Isotopes of Carbon

Production and Decay of Radiocarbon

Beta Decay Electron Capture Alpha Decay

Radioactive Decay of Rubidium to Strontium

Radiometric Dating n Key principle: Half Life = time required for 1/2 of the nuclei in a sample to decay

Radioactivity and Absolute Time Decay of parent atoms Growth of daughter atoms

Radioactive Decay of Uranium 238 to Lead 206

Isotopic dating n Radioactive elements (parents) decay to nonradioactive (stable) elements (daughters) n The rate at which this decay occurs is constant and known n Therefore, if we know the rate of decay and the amount present of parent and daughter, we can calculate how long this reaction has been proceeding

Major Radioactive Elements Used in Isotopic Dating

Geologically useful decay schemes Parent 235 U Daughter 207 Pb Half-life (years) 4. 5 x 109 238 U 206 Pb 0. 71 x 109 40 K 40 Ar 1. 25 x 109 87 Rb 87 Sr 47 x 109 14 C 14 N 5730

Radiometric Dating n Shows that the earth is much older than people had previously suspected n Earth formed about 4. 6 billion years ago

Relative vs Absolute Age n Usually geologists first establish relative ages then try to get absolute age dates n Determining relative age relies on a number of geologic principles that were developed during the 17 th to early 19 th centuries

Principle of Superposition n In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom

Principle of Superposition Sedimentary rocks are deposited in a layer-cake fashion: Layer 4 Layer 3 Layer 2 Layer 1 Each layer is older than the one above and younger than the one below

Youngest rocks Oldest rocks Principle of Superposition

Principle of Superposition n Although this is really obvious, it was not stated until 1669 n This principle generally applies to volcanic rocks as well as sedimentary rocks

Principle of Original Horizontality n Layers of sediment are deposited in a horizontal or nearly horizontal position parallel to the Earth’s surface

Principles of original horizontality and superposition

Principle of Original Horizontality n Note that original horizontality is not strictly applicable to volcanic rocks because they are often deposited on slopes

Principle of Cross-cutting Relationships n Something (such as a dike or fault) that cuts across a layer must be younger than the layer

Layers of rock are said to be conformable when they are found to have been deposited essentially without interruption

Unconformity n Results from interruption of deposition n Represents a long period of time during which there either was no deposition, or earlier deposited material was eroded away

Sedimentation of Beds A-D Beneath the Sea

Uplift and Exposure of D to Erosion

Continued Erosion Removes D and Exposes C to Erosion

Subsidence and Sedimentation of E over C Unconformity: a buried surface of erosion

Formation of a Unconformity

Unconformity n Often not easy to recognize if the layers are all parallel n Much easier to recognize when there is a period of folding of the rocks before a period of erosion and renewed deposition

South rim of the Grand Canyon

South rim of the Grand Canyon 250 million years old Paleozoic Strata 550 million years old 1. 7 billion years old Precambrian

South rim of the Grand Canyon 250 million years old 550 million years old Unconformity 1. 7 billion years old

The Great Unconformity of the Grand Canyon

Siccar Point, Scotland: Hutton’s Classic Unconformity -- Old Red Sandstone (~345 my) overlies rocks that are ~425 million years old

Siccar Point, Scotland Buried and tilted erosional surface

Summary of Geologic Events in a small area

Relative Geologic Dating

These methods work well in small areas where we can see the relationships between rock layers. n What happens when we want to tell the relative ages of the strata on Oahu with respect to strata on Maui? n We have to figure out some way to correlate the layers of interest. n

Correlation n Process used to tie separated strata together n Based on matching physical features such as n Physical continuity - trace of rock unit n Similar rock types - marker beds, coal seams, rare minerals, odd color

Correlation n Within sedimentary layers there are often the remains of small animals (fossils) n Fossils are quite useful for correlating between two sections that are not laterally continuous

Principle of Lateral Continuity Layered rocks are deposited in continuous contact

Correlation n Fossils have evolved through time, so when we find a fossil of the same type in two different areas, we are pretty sure that the rocks are about the same age

Correlation n This technique is not very useful in Hawaii -- Why? n Fossils helpful in sedimentary rocks, but usually no fossils in volcanic rocks n Sometimes small amounts of sediment between layers of volcanic rock (such layers might have fossils), but most rocks in Hawaii do not have fossils

Radiometric dates provide absolute ages to the Geologic Column

Bracketing ages

Magnetostratigraphy n Technique that works best in volcanic rocks n Time scale based on polarity reversal of Earth's magnetic field n Major problem is that Earth's magnetic field has been constant for the past 700, 000 yrs (no reversals), so this doesn't work for very young rocks

Earth’s Magnetic Field

Magnetization of Magnetite

Lavas record magnetic reversals

Magnetic reversals over the past 20 million years

Magnetic time scale n 0 -700, 000 -- Normal n 700, 000 - 2. 5 my -- Reversed n > 2. 5 my -- Normal n Ko`olau lavas mostly reversed in polarity, so they must be older than 700, 000 yrs, but younger than 2. 5 my n Lavas on Kaua`i and in Wai`anae Range show normal polarity, so they must be older than 2. 5 my

The Geologic time scale n Divisions in the worldwide stratigraphic column based on variations in preserved fossils n Built using a combination of stratigraphic relationships, crosscutting relationships, and absolute (isotopic) ages

The Geologic Column and Time Scale

Tuesday n Geologic Time Scale (continued) n Homework #1 is due next Thursday, January 19