Presentation is loading. Please wait.

Presentation is loading. Please wait.

Ways to tell the age of a rock Relative Dating: Places events in geologic history in the proper order. The basis for the geologic time scale Mainly Sedimentary.

Similar presentations


Presentation on theme: "Ways to tell the age of a rock Relative Dating: Places events in geologic history in the proper order. The basis for the geologic time scale Mainly Sedimentary."— Presentation transcript:

1 Ways to tell the age of a rock Relative Dating: Places events in geologic history in the proper order. The basis for the geologic time scale Mainly Sedimentary Rocks Does not provide a true age Relative Dating: Places events in geologic history in the proper order. The basis for the geologic time scale Mainly Sedimentary Rocks Does not provide a true age

2 Ways to tell the age of a rock Absolute Dating: All you need is a tiny sample of material (mineral, bone) no larger than a grain of rice. Gives us the true age of a fossil or rock Mainly organic tissue or igneous crystals Measure the amount of unstable isotopes that have decayed to figure out age Absolute Dating: All you need is a tiny sample of material (mineral, bone) no larger than a grain of rice. Gives us the true age of a fossil or rock Mainly organic tissue or igneous crystals Measure the amount of unstable isotopes that have decayed to figure out age

3 Dating Rocks (determining their age, that is) Relative Dating –Superposition - The youngest rocks are on the top, oldest at the bottom.

4

5 Dating Rocks (determining their age, that is) Relative Dating –Superposition –Cross-cutting relationships - Geologic features that cut through and across rocks are younger than those rocks. Mostly Faults and Igneous intrusions

6

7 Dating Rocks (determining their age, that is) Relative Dating –Superposition –Cross-cutting relationships –Law of Inclusions - Rocks embedded in other rocks are older than those rocks they are embedded in.

8 http://www.earth.ox.ac.uk/~oesis/field/medium/xenolith-1365.jpg

9 http://bloginitiative.typepad.com/photos/uncategorized/crowd.jpg

10 Dating Rocks (determining their age, that is) Relative Dating –Superposition –Cross-cutting relationships –Law of Inclusions –Law of Original Horizontality (and Lateral continuity)

11 Hikingtripsreport.com

12 What are the relative age relationships shown here? How can you tell a sill from a lava flow? M&W4 Fig. 17.4; M&W5 Fig. 17.4

13 Dating Rocks (determining their age, that is) Relative Dating –Superposition –Cross-cutting relationships –Law of Inclusions –Law of Original Horizontality (and Lateral continuity) –Law of Unconformities

14 A DISCONFORMITY is a boundary between two layers of non-continuous ages. This boundary is usually marked by an erosional surface and is often irregular. M&W4 Fig. 17.8; M&W5 Fig. 17.8

15 An ANGULAR UNCONFORMITY is a disconformity between layers of different angles. The underlying layers are first tilted, then erosion scours away a new, horizontal surface. New, horizontal layers form on top

16 An NONCONFORMITY is a disconformity between different rock types, one of them sedimentary.

17 M&W4 Fig. 17.11; M&W5 Fig. 17.11

18 STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION Do three meters of strata at place A record the same amount of time as three meters at place B? Do three meters of strata at place A record the same amount of time as three meters at place B? How do we correlate events and the passage of time from one outcrop of rock to another and even around the world? How do we correlate events and the passage of time from one outcrop of rock to another and even around the world? Fossils! the main tool for correlating strata (and intervals of time represented by strata) from one rock outcrop to another outcrop.

19 STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION Different kinds of organisms have lived during different periods in Earth's history and then died off (or went extinct). This is called faunal succession. If a strata in different outcrops contain the same fossil assemblages, then the outcrops represent the same interval of time. These strata correlate. M&W4 Fig. 17.6; M&W5 Fig. 17.6

20 STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION Formations: The fundamental stratigraphic units that are used to correlate stratifed rocks are called formations. Formations have between one (and preferably all) of the following characteristics: a distinctive set of physical properties (sedimentary rock type, bedding, grain size) a distinctive fossil assemblage have a widespread (map scale) geographic distribution Geologists can thus construct a regional stratigraphy that represents much more geologic time than any single outcrop in any single location.

21 STRATIGRAPIC PRINCIPLES: FAUNAL SUCCESSION & CORRELATION From correlation of formations from different locations, the history of the entire region can be deciphered. Sequences of layers (from differente places) overlap, like when you create a panoramic photo from individual shots. M&W4 Fig. 17.14; M&W5 Fig. 17.14

22

23

24 GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION Marie Curie Ernest Rutherford Radioactivity was first discovered by Henri Becquerel in 1896 and Polish-French chemist Marie Curie discovered that radioactivity produced new elements (radioactive decay). Ernest Rutherford first formulated the law of radioactive decay and was the first person to determine the age of a rock using radioactive decay methods.

25 M&W4 Fig. 3.3; M&W5 Fig. 3.4 The number of protons (the atomic number) is fixed for any element and is unique for each element but the number of neutrons in atoms of different elements can vary. Atoms of an element having different numbers of neutrons are referred to as the isotopes (of that element). GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION

26 Radioactive decay occurs when an isotope of one element is transformed into a different element by changes in the nucleus. There are three different decay mechanisms: GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION M&W4 Fig. 17.18; M&W5 Fig. 17.18 ParentDaughter

27 How can we tell age based on the number of parent isotopes? Radioactive isotopes decay at a particular rate. We express this rate as the HALF-LIFE, which is the time it takes for HALF of the parent isotopes to decay.

28 GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION For radioactivity dating we use igneous rocks and minerals. The clock starts when radioactive atoms that are present in the magma get incorporated in the crystalline structure of certain minerals in the rocks. The crystals containing the parent atoms form and so we then have a container with parents that can begin decaying to form daughters. We can then use measure the parent-daughter ratio. This is our atomic clock that records the time since the rock crystallized. M&W4 Fig. 17.21; M&W5 Fig. 17.21

29 GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION To the oldest materials ever dated by the radioactive method are found in the Jack Hills of western Australia and are tiny zircon grains contained in sandstones and conglomerates. The zircons are 4.4 billion years old. Scanning electron microscope image of a Jack Hills zircon. Scale bar is 0.1 mm The very remote outback of western Australia--the Jack Hills

30 M&W4 Fig. 17.24; M&W5 Fig. 17.24 C 14 is an isotope of carbon that forms from Nitrogen in the atmosphere. Living things consume this radioactive carbon. Once dead, no new carbon is absorbed, and C 14 turns back into Nitrogen. The Half-Life of C 14 is 5,730 years. This method works best for fossils younger than 50,000 years. Why? (end)


Download ppt "Ways to tell the age of a rock Relative Dating: Places events in geologic history in the proper order. The basis for the geologic time scale Mainly Sedimentary."

Similar presentations


Ads by Google