Presentation on theme: "Discovering Earth’s History"— Presentation transcript:
1 Discovering Earth’s History Chapter 12, Section 1
2 Rocks Record Earth History Rocks record geological events and changing life forms of the pastErosion has removed much of Earth’s rock record, but enough remains to study and interpret the pastThe geologic time scale revolutionized the way people think about time and how they perceive our planetWe have learned that Earth is much older than anyone had previously imagined and that its surface and interior have been changed by the same geological processes that continue today
4 A Brief History of Geology The primary goal of geologists is to interpret Earth’s historyUniformitarianism – the concept that processes that shaped Earth in the past are essentially the same as those operating todayThe forces and processes that we observe today have been at work for a very long timeThe acceptance of uniformitarianism meant the acceptance of a very long history for EarthRemember, to us it may seem that a landscape is unchanging in our lifetimes, but they are changing on a scale of hundreds, thousands, or even millions of years
5 Concept CheckHow do the laws that govern geological processes change through time?The same laws that operated in the past still operate today.
6 Relative Dating—Key Principles To establish a relative time scale, a few basic rules or principles had to be appliedThese principles were major breakthroughs, and their discovery and acceptance was an important scientific achievementRelative Dating – process by which rocks are placed in their proper sequence or orderRelative dating tells us the sequence in which events occurred, not how long ago they occurredNicolaus Steno, a Danish anatomist, geologist, and priest ( ), is credited with describing a set of geologic observations that are the basis of relative dating
8 Law of SuperpositionLaw of Superposition – states that in any undeformed sequence of sedimentary rocks, each bed is older than the layers above and younger than the layers belowThis rule also applies for other surface-deposited materials, such as lava flows and beds of ash from volcanic eruptions
10 Principle of Original Horizontality Principle of Original Horizontality – layers of sediment are generally deposited in a horizontal or nearly horizontal positionIf you see rock layers that are flat, it means they haven’t been disturbed and are still in their original positionIf the rocks are tilted, they must have been moved sometime after deposition
12 Concept CheckTo what rock type can the law of superposition and the principle of original horizontality be best applied?Undisturbed sedimentary rocks
13 Principle of Cross-Cutting Relationships Principle of Cross-Cutting Relationships – a rock or fault is younger than any rock or fault through which it cutsFor an example, look at the following figure
15 Inclusions Sometimes inclusions can help the relative dating process Inclusions are pieces of one rock unit that are contained within anotherThe rock unit next to the one containing the inclusions must have been there first in order to provide the rock fragmentsTherefore, the rock containing inclusions is the younger of the two
17 UnconformitiesUnconformity – a surface that represents a break in the rock record, caused by erosion or lack of deposition (angular unconformity, disconformity, and nonconformity)An unconformity represents a long period during which deposition stopped, erosion removed previously formed rocks, and then deposition resumedAn angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurredTwo sedimentary layers that are separated by an erosional surface are called a disconformityNonconformities mean the erosional surface separates older metamorphic or intrusive igneous rocks from younger sedimentary rocks
20 Concept Check What are the three basic types of unconformities? Angular unconformity, disconformity, and nonconformity
21 Correlation of Rock Layers In order to develop a geologic time scale that can be applied to the entire Earth, rocks of similar age in different regions must be matched upCorrelation – establishing the sequence of rocks of similar age in different areasBy correlating the rocks from one place to another, it is possible to create a more complete view of the geologic history of a region
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