Historical Geology Introduction

What is Historical Geology? Earth’s History based on evidence preserved in rocks Immensity of time span difficult to appreciate –Changes occur over millions to billions of years –Human life span = years Reconstruct history through extrapolation

Uniformitarianism Very important to Historical Geology Defined as: Geologic processes operating today, operated in past Modern Ripples Fossilized Ripples Modern Mudcracks Fossilized Mudcracks Not inviolable –Geologic events last k’s to m.y. –Past conditions not similar to today

The Rock Record Read rock record to reconstruct past Grand Canyon

Law of Superposition Crucial to establishing older/younger rocks, therefore history of area Law defined as: In undisturbed section – youngest rock on top –Map formations, beds, etc.

Unconformities Rock record not continuous. Have sections missing Must identify the unconformities: Gaps in rock record –All geologic history not evident in rock –Have uplift and erosion, i.e. book with pages missing –Part of rock record always remains

Interpretation from Rocks Need to read sedimentary rocks –Non-marine, coal, marine siltstone, marine limestone Halite Coal

Fossils Remains of organisms—Very Important!! –Important for –time, –environments, –evolution of life –Record depth, temp, relative ages for correlation, and event timing

Early Concepts of Geologic Time  Greeks Thales (636 BC) knew of fossils, coastal erosion, sed. But couldn’t tell time Medieval times J. Lightfoot (1644) Relied on biblical accounts Earth formed (:OOAm, 17 Sept 3929 B.C Bishop Ussher ( ) Lightfoot wrong 23 Oct 4004 B.C deBuffon ( ) Cooling rates of spheres Earth 74,047 yrs Attempts to determine the Earth’s age Thales J. Lightfoot Bishop Ussher deBuffon

Early Concepts of Geologic Time Catastrophists –Believed in catastrophes –Changes in fossils + changes in creation –Fit biblical accounts of Noah’s flood –Therefore, Earth old, 5-6k years –Support from floods, earthquakes & volcanoes –common catastrophes Attempts to determine the Earth’s age

Early Concepts of Geologic Time Attempts to determine the Earth’s age Neptunists—Attempt to put rocks in sequence—Important! –Werner ( Controversial geologist (mineralogist/metallurgist) Rocks precipitated from H2O Divided Earth into formations –Oldest =primary = granite/gneiss –Transition+ greywacke, sedimentary rocks –Secondary rocks = flat=lying LS, SS, basalt –Tertiary rocks + alluvial sands & gravels

Early Concepts of Geologic Time Attempts to determine the Earth’s age Plutonists  Hutton ( ) –Theory of Uniformitarianism –Earth formed by processes acting today –Saw no act of Creation or Catastrophe –Wrote Theory of the Earth (1795) –Severely Criticized, Ignored biblical accounts  Hutton’s ideas popularized by Charles Lyell  Hutton not a good writer

Early Concepts of Geologic Time Attempts to determine the Earth’s age Plutonists –Lyell ( ) Supported Uniformitarianism Published Principles of Geology ( ) Local gaps in record –Erosion, not catastrophies Influenced Darwin

Age of Earth Uniformitarianism followers – very old Earth  Darwin –Observable geol. Record –Small part of Earth history  Jolly -Studied rate of ocean salt add’n - Earth is 100 m.y.  Thomson (Lord Kelvin) – Rate of Earth cooling – Earth is m.y.  Sody and Rutherford – Studied radioactivity  Boltword – Using radioactivity – Earth is 1640 m.y.

Actualism in Uniformitariansim Today follow “actualistic” model of Uniformitariansim Follow Uniformitarianism But, consider natural laws: –Erosion, climate, deposition  not same rates through time –Earth’s surface  different today –Conditions not the same  no epeiric seas today –Cataclastic events have happened  dinosaur extinction (meteor)