{"id":3362,"date":"2019-06-19T21:29:46","date_gmt":"2019-06-19T21:29:46","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/?post_type=chapter&p=3362"},"modified":"2019-12-30T00:56:11","modified_gmt":"2019-12-30T00:56:11","slug":"relative-dating-of-earth-materials","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/chapter\/relative-dating-of-earth-materials\/","title":{"raw":"Relative Dating of Earth Materials","rendered":"Relative Dating of Earth Materials"},"content":{"raw":"

Relative Dating?<\/span>\r\nFirst and foremost-- <\/span>\r\nDon't date your relatives!\u00a0 In many places illegal, and generally just a bad idea!<\/span><\/h3>\r\nOK, seriously,\r\nGeologist refer to \"relative dating\" when we're deciding which rocks (or layers) are older, and which rocks (or layers) are younger.\r\nWe don't know how old or young, and we don't really know how much time has elapsed between rocks or layer deposition.\r\nAs we'll learn shortly, there are a number of basic rules for evaluating relative dating.\r\nBut before we get to these rules,\r\nit's worth defining \"absolute dating<\/strong>\" as the business of actually quantifying how old (or young) a rock actually is, in terms of years.\r\nFor example, when we say that a rock is 200 million years old, we've used an absolute date.\r\nFinally, absolute dating is NOT exact dating.\u00a0 Every absolute date (like every scientific measurement) has an associated error.\u00a0 It's not uncommon to say that a rock is 200my +\/- 5my, meaning that the rock is very likely somewhere between 195 and 205 my old (my= million years).(In most cases, but not all-- absolute dating relies on what we call isotopic or radiometric dating techniques.\u00a0 This is the use of radioactive decay as a clock.)\r\n

RELATIVE DATING RULES<\/h2>\r\nThese rules were derived long ago.\u00a0 Generally ascribed to Nicolaus Steno, who lived in the mid-1600's and was one of the first to recognize fossils as relics of past life (as opposed to something that had \"grown\" in the rock itself!).\r\n

\u00a0Steno wrote a short book (a prodromus) with a rather odd name-- \u00a0\"\"<\/a><\/span>\r\n\u00a0 \u00a0 \u00a0 \"De solido intra solidum naturaliter contento dissertationis prodromus\"\u00a0<\/span>\r\nEven if you translate the Latin, it's still a bit strange!\r\n<\/span>\"Dissertation Concerning a Solid Body Enclosed by Process of Nature Within a Solid<\/em>\"\r\n<\/span>What he wrote about though makes sense in the context of the mysteries of nature, in his day.\r\nThe book was about,<\/span>\r\nHOW DO YOU GET A PEBBLE INSIDE OF A ROCK<\/em> (part of what we now call a conglomerate)?<\/span>\r\nHOW DO YOU GET LAYERS BETWEEN LAYERS<\/em> (horizontality and superposition, see below)?<\/span>\r\nand, HOW DO YOU GET A SHELL OR BONE INSIDE OF A ROCK<\/em> (what we call a fossil!)?<\/span><\/p>\r\n\r\n

THE RULES<\/h1>\r\n

Original Horizontality<\/h3>\r\nThe idea here is pretty simple.\u00a0 As a consequence of gravity, most sediments are deposited in horizontal layers.\r\n\r\n\"\"<\/a>\r\n

Superposition<\/h3>\r\nAgain, not too complex!\u00a0 Those sediments that were deposited first are at the lower levels of a sequence, and hence the oldest.\u00a0 Those deposited last are at the upper levels and are the youngest.\u00a0 Note:\u00a0 These rules generally apply to sediments deposited out of water, but they could include air-deposition (volcanic ash fall) or lava flows.\r\n\r\n\"\"<\/a>\r\n\r\n
\r\n\r\nBut of course, once deposited and turned to rock (something we'll learn about later, with sedimentary rocks, and called \"lithification), THEN those layers may be tilted or otherwise deformed.\r\nCheck out the climbers in this picture--\r\n
\"TheThe Yellow Door 5.13a\/b Seal Rock Phil Gruber Lynn Hill Sport Climbing Rock Boulder Colorado Flatirons,, Climbing Magazine, Sept 2018<\/h5>\r\nHere, the layers have been tilted (uplifted, like shingles on a roof), but not overturned.\u00a0 Therefore, the rock layers to the right of the climbers are older than those to their left!\r\n

<\/h3>\r\n

<\/h3>\r\n

<\/h3>\r\n \r\n

<\/h3>\r\n

<\/h3>\r\n

Lateral Continuity<\/h3>\r\nLastly, regarding layering of sediments-- geologists often refer to an un-deformed sequence that has been merely eroded (not deformed or uplifted) as having lateral continuity.\u00a0 This just means that we can recognize the same layers across a region that has been eroded into valleys or hills.\r\n\r\n\"\"<\/a>\r\n\r\n \r\n

Principle of Inclusion<\/h3>\r\nAnother fundamental relative dating tool-- included fragments, within an existing rock, are older than that rock.\r\nThis is similar to the idea that fossils (included fragments within a rock) were in existence before the formation of the rock.\r\nAlternatively, a conglomerate contains pebbles that had to have come from somewhere (hence older pieces of rock).\r\n\r\n\r\n

from John Merck,\r\nhttps:\/\/www.geol.umd.edu\/~jmerck\/geol100\/images.jpg<\/h5>\r\n

Cross Cutting<\/h3>\r\nImagine sticking a big fat spoon or fork or knife INTO a cake.\r\nIn order for that entity to be in your cake, it had to come after the cake was made.\r\nThings that CROSS-CUT existing layers are considered to have come after the layers were made.\r\nClassic cross-cuts would include either a dike (igneous intrusion, or \"squirt\" of magma through rock layers) or a fault (a breaking surface within a group of rocks).\r\n\r\n\"\"<\/a>\r\n
from John Merck,\r\nhttps:\/\/www.geol.umd.edu\/~jmerck\/geol100\/images.jpg<\/h5>\r\n

Unconformities<\/h1>\r\nFinally, we come to the subject of MISSING TIME.\u00a0 Layers or sequences of rocks are rarely a record of all that has past!\r\nUnconformities refer to missing time.\r\nMissing time occurs due to erosion (or associated \"non-deposition,\" i.e. no sediments being lain down).\r\nIn Figure \"a\" below, an igneous or metamorphic rock, formed, was uplifted and eroded, and then had sedimentary layers deposited atop.\r\nThis is a NONCONFORMITY<\/strong>.\r\n\r\nIn figure \"b\" below, a series of sedimentary rock has been tilted and then eroded (the dark line is the erosion surface) and then had sedimentary layers deposited atop.\r\nThis is an ANGULAR UNCONFORMITY<\/strong>.\r\n\r\nIn figure \"c\" below, a series of sedimentary rock has two layers that have a lengthy separation in time, marked by the lower layer having undergone a lengthy period of erosion (indicated by the wavy dark line).\u00a0 Then sedimentary layers were deposited atop.\r\nThis is a DISCONFORMITY<\/strong>.\r\n\r\nIn figure \"d\" below, we see a disconformity that is not well represented by an wavy erosion surface.\r\nThis is a PARACONFORMITY<\/strong>.\r\n\r\nNOTE-- both disconformities and paraconformities are quite difficult to discern in the field, where techniques\/tools to derive ages of rocks above and below are unavailable.<\/span>\r\n\r\n\"Figure<\/a>\r\n
Diagrams from Steven Earle, Physical Geology<\/h5>\r\n
IMPORTANT IDEA--\u00a0 Although we consider big chunks of missing time as unconformities, it's worth recognizing that even in an ordinary sequence of sedimentary layers, each discernible layer is separated from layers above and below by some sort<\/em> of time gap.\u00a0 In other words, every layer is bounded by what might be a paraconformity.\r\nWe reserve the various unconformity terms for \"sizable\" time gaps.<\/div>\r\n
<\/h5>\r\n

INDEX FOSSILS<\/h3>\r\n

Index fossils are those that were relatively wide-ranging in a geographic sense, relatively easy to identify, and represent an \"existence period\"\u00a0that is relatively short-lived.<\/p>\r\nIn the previous section (titled--\u00a0 Age of the Earth, Early Efforts) we learned that the early geologist Will Smith used the concept of Faunal Succession to recognize rock layers that were the same age, or of different ages.\u00a0 This is really all about index fossils, in that these fossils seem to be relegated to a particular period of time past!\r\n\r\nNot all fossils are index fossils.\r\nIn the diagram below, we see animals and plants that had enormous time spans-- these \"groups\" would make terrible index fossils.\r\nBut within each group (fish or mammals or ferns or grasses) there will be SPECIES that represent shorter periods of existence and would therefore make good index fossils.\r\n\r\nOnce \"dated\" using isotopic techniques, index fossils can be a tip-off as to the age of sedimentary rocks.\r\n\r\n\"\"<\/a>\r\n\r\n\"\"<\/a>\r\n\r\nExamples of index fossils from the USGS website,, see link,\r\n\r\nhttps:\/\/pubs.usgs.gov\/gip\/geotime\/fossils.html","rendered":"

Relative Dating?<\/span>
\nFirst and foremost– <\/span>
\nDon’t date your relatives!\u00a0 In many places illegal, and generally just a bad idea!<\/span><\/h3>\n

OK, seriously,
\nGeologist refer to “relative dating” when we’re deciding which rocks (or layers) are older, and which rocks (or layers) are younger.
\nWe don’t know how old or young, and we don’t really know how much time has elapsed between rocks or layer deposition.
\nAs we’ll learn shortly, there are a number of basic rules for evaluating relative dating.
\nBut before we get to these rules,
\nit’s worth defining “absolute dating<\/strong>” as the business of actually quantifying how old (or young) a rock actually is, in terms of years.
\nFor example, when we say that a rock is 200 million years old, we’ve used an absolute date.
\nFinally, absolute dating is NOT exact dating.\u00a0 Every absolute date (like every scientific measurement) has an associated error.\u00a0 It’s not uncommon to say that a rock is 200my +\/- 5my, meaning that the rock is very likely somewhere between 195 and 205 my old (my= million years).(In most cases, but not all– absolute dating relies on what we call isotopic or radiometric dating techniques.\u00a0 This is the use of radioactive decay as a clock.)<\/p>\n

RELATIVE DATING RULES<\/h2>\n

These rules were derived long ago.\u00a0 Generally ascribed to Nicolaus Steno, who lived in the mid-1600’s and was one of the first to recognize fossils as relics of past life (as opposed to something that had “grown” in the rock itself!).<\/p>\n

\u00a0Steno wrote a short book (a prodromus) with a rather odd name– \u00a0\"\"<\/a><\/span>
\n\u00a0 \u00a0 \u00a0 “De solido intra solidum naturaliter contento dissertationis prodromus”\u00a0<\/span>
\nEven if you translate the Latin, it’s still a bit strange!
\n<\/span>“Dissertation Concerning a Solid Body Enclosed by Process of Nature Within a Solid<\/em>”
\n<\/span>What he wrote about though makes sense in the context of the mysteries of nature, in his day.
\nThe book was about,<\/span>
\nHOW DO YOU GET A PEBBLE INSIDE OF A ROCK<\/em> (part of what we now call a conglomerate)?<\/span>
\nHOW DO YOU GET LAYERS BETWEEN LAYERS<\/em> (horizontality and superposition, see below)?<\/span>
\nand, HOW DO YOU GET A SHELL OR BONE INSIDE OF A ROCK<\/em> (what we call a fossil!)?<\/span><\/p>\n

THE RULES<\/h1>\n

Original Horizontality<\/h3>\n

The idea here is pretty simple.\u00a0 As a consequence of gravity, most sediments are deposited in horizontal layers.<\/p>\n

\"\"<\/a><\/p>\n

Superposition<\/h3>\n

Again, not too complex!\u00a0 Those sediments that were deposited first are at the lower levels of a sequence, and hence the oldest.\u00a0 Those deposited last are at the upper levels and are the youngest.\u00a0 Note:\u00a0 These rules generally apply to sediments deposited out of water, but they could include air-deposition (volcanic ash fall) or lava flows.<\/p>\n

\"\"<\/a><\/p>\n

\n

But of course, once deposited and turned to rock (something we’ll learn about later, with sedimentary rocks, and called “lithification), THEN those layers may be tilted or otherwise deformed.
\nCheck out the climbers in this picture–<\/p>\n

\"TheThe Yellow Door 5.13a\/b Seal Rock Phil Gruber Lynn Hill Sport Climbing Rock Boulder Colorado Flatirons,, Climbing Magazine, Sept 2018<\/h5>\n

Here, the layers have been tilted (uplifted, like shingles on a roof), but not overturned.\u00a0 Therefore, the rock layers to the right of the climbers are older than those to their left!<\/p>\n

<\/h3>\n

<\/h3>\n

<\/h3>\n

 <\/p>\n

<\/h3>\n

<\/h3>\n

Lateral Continuity<\/h3>\n

Lastly, regarding layering of sediments– geologists often refer to an un-deformed sequence that has been merely eroded (not deformed or uplifted) as having lateral continuity.\u00a0 This just means that we can recognize the same layers across a region that has been eroded into valleys or hills.<\/p>\n

\"\"<\/a><\/p>\n

 <\/p>\n

Principle of Inclusion<\/h3>\n

Another fundamental relative dating tool– included fragments, within an existing rock, are older than that rock.
\nThis is similar to the idea that fossils (included fragments within a rock) were in existence before the formation of the rock.
\nAlternatively, a conglomerate contains pebbles that had to have come from somewhere (hence older pieces of rock).<\/p>\n

\"image\"<\/p>\n

from John Merck,
\nhttps:\/\/www.geol.umd.edu\/~jmerck\/geol100\/images.jpg<\/h5>\n

Cross Cutting<\/h3>\n

Imagine sticking a big fat spoon or fork or knife INTO a cake.
\nIn order for that entity to be in your cake, it had to come after the cake was made.
\nThings that CROSS-CUT existing layers are considered to have come after the layers were made.
\nClassic cross-cuts would include either a dike (igneous intrusion, or “squirt” of magma through rock layers) or a fault (a breaking surface within a group of rocks).<\/p>\n

\"\"<\/a><\/p>\n

from John Merck,
\nhttps:\/\/www.geol.umd.edu\/~jmerck\/geol100\/images.jpg<\/h5>\n

Unconformities<\/h1>\n

Finally, we come to the subject of MISSING TIME.\u00a0 Layers or sequences of rocks are rarely a record of all that has past!
\nUnconformities refer to missing time.
\nMissing time occurs due to erosion (or associated “non-deposition,” i.e. no sediments being lain down).
\nIn Figure “a” below, an igneous or metamorphic rock, formed, was uplifted and eroded, and then had sedimentary layers deposited atop.
\nThis is a NONCONFORMITY<\/strong>.<\/p>\n

In figure “b” below, a series of sedimentary rock has been tilted and then eroded (the dark line is the erosion surface) and then had sedimentary layers deposited atop.
\nThis is an ANGULAR UNCONFORMITY<\/strong>.<\/p>\n

In figure “c” below, a series of sedimentary rock has two layers that have a lengthy separation in time, marked by the lower layer having undergone a lengthy period of erosion (indicated by the wavy dark line).\u00a0 Then sedimentary layers were deposited atop.
\nThis is a DISCONFORMITY<\/strong>.<\/p>\n

In figure “d” below, we see a disconformity that is not well represented by an wavy erosion surface.
\nThis is a PARACONFORMITY<\/strong>.<\/p>\n

NOTE– both disconformities and paraconformities are quite difficult to discern in the field, where techniques\/tools to derive ages of rocks above and below are unavailable.<\/span><\/p>\n

\"Figure<\/a><\/p>\n

Diagrams from Steven Earle, Physical Geology<\/h5>\n
IMPORTANT IDEA–\u00a0 Although we consider big chunks of missing time as unconformities, it’s worth recognizing that even in an ordinary sequence of sedimentary layers, each discernible layer is separated from layers above and below by some sort<\/em> of time gap.\u00a0 In other words, every layer is bounded by what might be a paraconformity.
\nWe reserve the various unconformity terms for “sizable” time gaps.<\/div>\n
<\/h5>\n

INDEX FOSSILS<\/h3>\n

Index fossils are those that were relatively wide-ranging in a geographic sense, relatively easy to identify, and represent an “existence period”\u00a0that is relatively short-lived.<\/p>\n

In the previous section (titled–\u00a0 Age of the Earth, Early Efforts) we learned that the early geologist Will Smith used the concept of Faunal Succession to recognize rock layers that were the same age, or of different ages.\u00a0 This is really all about index fossils, in that these fossils seem to be relegated to a particular period of time past!<\/p>\n

Not all fossils are index fossils.
\nIn the diagram below, we see animals and plants that had enormous time spans– these “groups” would make terrible index fossils.
\nBut within each group (fish or mammals or ferns or grasses) there will be SPECIES that represent shorter periods of existence and would therefore make good index fossils.<\/p>\n

Once “dated” using isotopic techniques, index fossils can be a tip-off as to the age of sedimentary rocks.<\/p>\n

\"\"<\/a><\/p>\n

\"\"<\/a><\/p>\n

Examples of index fossils from the USGS website,, see link,<\/p>\n

https:\/\/pubs.usgs.gov\/gip\/geotime\/fossils.html<\/p>\n","protected":false},"author":58829,"menu_order":4,"template":"","meta":{"_candela_citation":"[]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-3362","chapter","type-chapter","status-publish","hentry"],"part":3294,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/pressbooks\/v2\/chapters\/3362","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/wp\/v2\/users\/58829"}],"version-history":[{"count":21,"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/pressbooks\/v2\/chapters\/3362\/revisions"}],"predecessor-version":[{"id":3620,"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/pressbooks\/v2\/chapters\/3362\/revisions\/3620"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/pressbooks\/v2\/parts\/3294"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/pressbooks\/v2\/chapters\/3362\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/wp\/v2\/media?parent=3362"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/pressbooks\/v2\/chapter-type?post=3362"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/wp\/v2\/contributor?post=3362"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/colorado-wmopen-geology\/wp-json\/wp\/v2\/license?post=3362"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}