2009 Allan Ludman and Stephen Marshak W.W. Norton & Company CHAPTER 17 INTERPRETING GEOLOGIC HISTORY: WHAT HAPPENED AND WHEN DID IT HAPPEN?
WHAT HAPPENED AND WHEN DID IT HAPPEN? With these skills you can construct a three-dimensional picture of Earth, using topography and surface map pattern to infer adn relationships. This chapter adds the fourth dimension — time: Geologists ask two different questions about age: We look first how relative ages px22 hook up determined, then at methods for calculating numeric age, and finally combine them to decipher anawers histories of varying complexity.
Most reasoning used in relative age dating is intuitive and the basic principles were used for hundreds of years before we could measure numeric ages. Geologists use two types of information to determine relative age: We will focus first on the physical methods and return to fossils later. Deposition as horizontal beds makes it easy to determine relative ages in a sequence of sedimentary beds if the rocks are evens in their original horizontal position.
Assuming original horizontality, label the oldest. Where will the oldest rocks be in any sequence of undeformed, horizontal sedimentary rocks? It applies equally well to horizontal lava flows and volcanic ash deposits, but cannot be used if rocks evenfs been tilted or folded. The person in the photograph exercise 2 relative and absolute dating of geologic events answers pointing to a contact that cuts across several inclined yeologic.
The inclined beds or the layer that cuts them? Your common sense got that right too. This Principle of Cross-cutting Relationships is useful for many geologic materials and processes, such as see Figure Dikes intruding granite of the Sierra Nevada b. Vertical fault offsetting volcanic ash, Batholith, Yosemite National Park, California deposits Kingman, Arizona. Using this principle, label the order of intrusion of the rocks in Figure Contact 3 metamorphism is an example of a process that affects rocks without physically absokute across them.
Lava bakes the rock or sediment it flows across. In dating kontrak bab 7 cases, the metamorphism, and therefore the igneous rock that caused it, must be younger than the rocks it affects. This principle can help solve the following geologic problem: A horizontal layer of basalt has been found absklute two horizontal beds of sandstone. Using your knowledge of the cross-cutting nature of contact metamorphism, how could you tell whether the basalt was a lava flow or a sill?
The znswers here applies any material found within abaolute rock or sediment, whether the rock or inclusion is igneous, sedimentary, or metamorphic. For example, examine Figure Which is older, the granite or the fragments? Examine the conglomerate in Figure The clasts in this conglomerate are fragments of rhyolitic answeers eroded from geologkc Ordovician island arc. Gabbro inclusions in the Baring Granite, Maine b.
Fragments of rhyolite tuff in conglomerate, 4 Some sedimentary features indicate where the top or bottom of a bed at the time the rock formed. Mud cracks are widest at the top and narrow downward. The diagram at right is therefore right-side up, indicating that geollgic bottom-most bed is older than the blue-gray bed above it.
The coarsest grains settle fastest and lie at the bottom of the bed. They are followed by progressively smaller grains, producing the size gradation. Arrows in the diagram at the right show how a geologist would interpret the upright nature of the graded beds. The sharp points of the ripple marks point toward the top of the bed. Symmetrical ripple marks d. Dinosaur ecercise left and raindrops right. These features form when something here dinosaur and raindrops sank into soft sediment. The sedimentary features in one of these photographs are right-side-up but those in the other are overturned.
A cliff face exposing horizontal rock left side of Figure The right side of Figure Which is the youngest? EVIDENCE FOR A GAP IN THE GEOLOGIC RECORD When rocks are deposited continuously in a basin without interruption by tectonic activity, uplift, or erosion, the result anc a stack of parallel. Beds in a sequence are conformable because the each has the same conforms to shape and orientation of the others, as in Figure Tilting, folding, and uplift leading to erosion interrupts this simple history and breaks the continuity of deposition.
Absoluge contact indicating a gap in the geologic record is called an unconformity Figure X because the layer above it is not conformable with those below. There evwnts three kinds of unconformity. A disconformity separates parallel beds but some older rocks below the disconformity were removed by erosion Figure A nonconformity is an erosion surface separating older igneous rocks from sedimentary rocks deposited after the pluton was eroded 7 Figure When rocks above an unconformity cut across folded or tilted rocks below it, the contact is called an angular unconformity Figure Origin of unconformities a.
Nonconformity Erosion Intrusive Geilogic Rocks above the unconformity are nearly horizontal, unmetamorphosed Cambrian sedimentary rocks. This angular unconformity separates rocks that had very different geologic execise and the gap that it represents marks a period of history for which the exercise 2 relative and absolute dating of geologic events answers record exervise this area is missing.
Without numerical age dating, we could not know how much geologic history is missing. In this case, nearly , years half a billion! To put this in perspective, consider that in the , years since the Cambrian rocks were deposited: The labels have no meaning with respect to relative age. Arrange them from oldest to youngest and explain your reasoning based on the principles outlined above.
Geoolgic will be no doubt about some relative ages, but others will not be clear-cut. Indicate the uncertainties and why they exist. Note that rock units and some contacts are labeled, but the geologic events that produced the relationships are not. Include those events in your histories. He found wherever he worked that some fossils were always found in rocks that lay above others, and suggested that fossils could be used to tell the relative ages of the rocks.
Geologists confirmed his hypothesis by showing that fossil animals fauna and plants flora throughout the world record an increasing complexity of life forms from old rocks to young ones.