The Oldest Rocks on Earth and Calibrating the Geologic Time Scale “What seest thou else in the dark backward and abysm of time?” Shakespeare

Mile Markers in Deep Time The fossil-based framework for Earth history is known as the GEOLOGIC TIME SCALE Units: Eras, Periods, Epochs, Ages

What do we do with rocks that don’t contain fossils? How do we determine the age of rocks that are “too old” to have abundant fossils? How do we determine the age of igneous and metamorphic rocks that do not contain fossils? How do we put numbers on age? What is the oldest rock on Earth and how do we know its age?

Oldest Rock on Earth Acasta Gneiss – NWT, Canada 3.96 Ga +/- 3 Ma How was this determined? “Revolutions still more remote appeared in the distance of this extraordinary perspective.” - Playfair

Oldest Minerals on Earth Acasta Gneiss –3.96 Ga +/- 3 Ma Oldest Minerals Sand grains of zircon Dated at 4.4 Ga +/- 8 Ma From sandstone in Australia

How do we know when these events happened? Fossils give us the relative age relationships (sequence of events) Only Radioactive Decay can be used to establish numerical age

How does it work? Some chemical elements are unstable They break down (radioactive decay) by undergoing changes in the nucleus –This is a Nuclear reaction (not chemical) Changes in nucleus produce –A new chemical element = daughter product –Energy in the form of radiation

How does it work? These nuclear reactions occur at a constant rate – this is the half life Since rate is constant, we can know “how long the clock has been ticking.” Time is calculated by measuring the number of atoms of the daughter product and the number of atoms of the parent element.

Where do we find these parent and daughter elements? Mineral crystals that are “closed systems” Minerals like zircon are ideal –Contain small amounts of Uranium –Very tough and durable – a “sealed container” Age that is calculated is the age of crystallization

Can we measure the age of all rocks? Ages are typically measured for igneous rocks such as layers of volcanic ash, lava flows and cross-cutting intrusions. Ages may also be measured for some metamorphic rocks – age of metamorphism. Ages are tied to the Geologic Time Scale which is based on fossils.

The Silurian – Devonian Boundary Boundary is defined at the Global Stratotype at Klonk in the Czech Republic Index Fossil is the Graptolite Monograptus uniformis uniformis Auxiliary Index is the conodont Icriodus woschmidti woschmidti

SUNY Oneonta Students and Calibrating the Age of the Silurian – Devonian Boundary

Calibrating the Silurian – Devonian Boundary Helderberg Group – Silurian – Devonian limestones in New York Coeymans Formation and Kalkberg Formation yield the index conodont I. woschmidti K-bentonites (volcanic ashes) in New Scotland Formation Microphenocrysts of zircon extracted from K-bentonite Uranium – Lead system used to determine age of Helderberg ash Occurrence within zone of lowest Devonian index fossil means age of ash approximates age of boundary S-D Boundary placed at 418 Ma – /- 1.0 Ma (Tucker, et. al. 1998)