Geologic Time ScaleGeologic Time ScaleCVE 3205CVE 3205Engineering GeologyEngineering GeologyWong Jee KhaiWong Jee KhaiIntroduction Introduction The concept that most geologic processes happen very slowly was propose !y James "#tton $%&2'(%&)&*+Geologists sort Earth,s history into a se-#ence o. events+/0osition in that se-#ence ienti.ies relative age. / Numerical age can !e etermine thro#gh analysis o. the pro#cts o. raioactive ecayRecord of Layered RocksRecord of Layered Rocks1ayere seimentary or volcanic roc2s contain important cl#es a!o#t past environments at an near Earth,s s#r.ace+Their se-#ence an relative ages provie the !asis .or reconstr#cting m#ch o. Earth,s history+The st#y o. strata is calle stratigraphy+The Laws of Stratigraphy The Laws of Stratigraphy 3ost seiment is lai own in the sea4 generally in relatively shallow waters4 or !y streams on the lan+Each new layer is lai own hori5ontally over oler ones+The law of original horizontality states that water(lai seiments are eposite in strata that are hori5ontal or nearly hori5ontal+Stratigraphic SuperpositionStratigraphic SuperpositionThe principle o. stratigraphic superposition states that any se-#ence o. seimentary strata was eposite .rom !ottom to top+Charles 1yell an other geologists o. the nineteenth cent#ry spec#late that it might !e possi!le to etermine n#merical ages !y #sing stratigraphic recor+Stratigraphic SuperpositionStratigraphic SuperpositionTwo ass#mptions m#st !e correct .or the metho to wor26/7t m#st !e ass#me that the rate o. seimentation was constant thro#gho#t the time o. seiment acc#m#lation+/7t m#st !e ass#me that all strata e8hi!it con.ormity4 meaning they have !een eposite layer a.ter layer witho#t interr#ption+Stratigraphic SuperpositionStratigraphic SuperpositionThe .irst ass#mption is .alse !eca#se it can !e o!serve toay that seimentation rates vary wiely .rom place to place an time to time+The secon an even more important ass#mption is .alse !eca#se seimentation can !e isr#pte perioically !y ma9or environmental changes4 s#ch as sea level changes an tectonic activity that lea to intervals o. erosion or non eposition+Kinds of UnconformitiesKinds of Unconformities:n #ncon.ormity is a s#!stantial !rea2 or gap in a stratigraphic se-#ence+Three important 2ins o. #ncon.ormities are .o#n in seimentary roc2s6/ Angular unconformity+The oler strata were e.orme an then c#t o.. !y erosion !e.ore the yo#nger layers were eposite across them+Kinds of UnconformitiesKinds of Unconformities/;iscon.ormity+7t is an irreg#lar s#r.ace o. erosion !etween parallel strata+: iscon.ormity implies a cessation o. seimentation an erosion4 !#t not tilting+7t is o.ten har to recogni5e4 !eca#se the strata a!ove an !elow are parallel+/