{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

DINOSAURS1copy - GEOLOGIC TIME AND GEOLOGIC PRINCIPLES...

Info icon This preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
GEOLOGIC TIME AND GEOLOGIC PRINCIPLES - INTRODUCTION The relative geologic time scale is a product of placing rocks in their proper sequence of formation to create a composite record of geologic events. History is a sequence of events placed in proper chronological order. Placing geologic events in their proper chronological order was first done by building composite sections in one area and correlating these sections to other areas where the record could be compared and improved upon. This process has built the geologic column as we know it today. In the later half of the twentieth century, radiometric age dating techniques allowed geologists to determine absolute ages for this time scale, creating an absolute geologic time scale. Selective determination of absolute radiometric ages of important sections has allowed geologists to place ages on the geologic column, producing the geologic time scale. We will examine how relative and absolute geologic ages are determined. Later we will see how these techniques are used to study the physical and biological history of the planet. - IMPORTANT PRINCIPLES The following are some of the important principles used to determine the relative ages of rock sequences. PRINCIPLE OF SUPERPOSITION: In any undisturbed succession of strata, oldest strata are at the bottom with successively younger ones above. A layer of rock is always older than the layer above it, unless they have been turned upside down. PRINCIPLE OF ORIGINAL HORIZONTALITY: Particles which settle under the influence of gravity settle to more nearly horizontal layers. Generally sediments deposited on land or in the sea form nearly horizontal layers. In some cases, they are inclined, as on the sloping face of a sand dune, but parallel to the surface of deposition. In most cases, if an area has strongly inclined or dipping sedimentary rocks, they were disturbed by forces from their original horizontal attitude. PRINCIPLE OF ORIGINAL LATERAL CONTINUITY: Strata extend in all directions until they thin against edges of basins they accumulated in. The original continuity of strata in a basin can be broken or faulted . For example, a river can carve a canyon into a basin filled with sediment leaving once continuous layers stranded on opposite sides of the canyon. According to the principle of lateral continuity, these opposite sides of the valley were once laterally continuous.
Image of page 1

Info icon This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
PRINCIPLE OF CROSS-CUTTING RELATIONSHIPS: Disturbed rocks or strata are older than those rocks which disturb them or cut through them. Liquid magma can be forced or intruded into preexisting rocks from their sources below. After these intrusions cool they form igneous rock that appears to cut across the other rock. Obviously the intrusive igneous rock is younger than the rock it cuts into. Faults also cut through rocks and those faulted rocks are older than the fault that cuts through them.The age of faults can often be determined by establishing the age of unfaulted strata overlying faulted strata because the unfaulted strata was deposited after the faulted rocks it buried.
Image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern