Geological events that occurred long ago can be arranged in the relative order in which they occurred. History of the Earth Older at the bottom. Why? In an undisturbed layer of rock, the oldest rocks are on the bottom and the rocks become progressively younger toward the top This happens because sediments are often deposited in horizontal beds, forming layers of sedimentary rock Relative Dating Relative dating is a strategy of estimating the Earths age by comparing rock layers and geologic features to each otherRelative dating is a strategy of estimating the Earths age by comparing rock layers and geologic features to each other Early geologists did not have high tech, like radiometric dating, which will be discussed laterEarly geologists did not have high tech, like radiometric dating, which will be discussed later Relative dating does not imply an exact amount of timeRelative dating does not imply an exact amount of time It points out that one event happened before another event It does not explain exact lengths of time between geologic events Relative Dating Principles There are six principles that guide relative dating Principle: a standard, belief, rule, or law; not principal, like Principal Galafaro 1.Principle of Superposition 2.Principle of Original Horizontality 3.Principle of Lateral Continuity 4.Principle of Faunal Succession 5.Principle of Crosscutting Relations 6.Principle of Inclusions Before We Begin: 3 Types of Rocks Can Create Rock Layers 1.Igneous: form from lava or magma 2.Metamorphic: form by intense heat and pressure 3.Sedimentary: form by compacting sediments together Most common rock layers Most likely to have fossils (To be discussed in more detail later in the school year) (1) Principle of Superposition In an undisturbed layer of rock, the oldest rocks are on the bottom and the rocks become progressively younger toward the top This happens because sediments are often deposited in horizontal beds, forming layers of sedimentary rock (1) Principle of Superposition Diagram Which layer is the oldest? THICKER gray layer at the bottom Which layer is the youngest? THINNER gray layer at the top Is that possible? Why or why not? Yes, the same gray type of material was deposited at different times. Older Younger As little sediments fall to the sea floor, they gather in flat layers. What would you name this principle? (2) Principle of Horizontality (2) Principle of Original Horizontality This principle states that layers of sediment are originally deposited horizontally under the action of gravity The Principle of Original Horizontality was proposed by the Danish geological pioneer Nicholas Steno (16381686) What principle might this describe? (3) Principle of Lateral Continuity The principle of lateral continuity states that layers of deposited sediments will first widen laterally in all directions In the example, the rock layers that are now separated by a valley or other erosional features, can be assumed to be originally unbroken. Even though the layers in the previous slide appear bent they continued sideways from and to each other. (4) Principle of Faunal Succession Area divided by several kilometers Although sedimentary rock material is different material marked by different colors, the period in Earths history can still be identified as the same by the specific fossils Usually index fossils What principal might this represent? (4) Principle of Faunal Succession States that sedimentary rock layers contain particular types of fossilized flora (plant-life) and fauna (animal-life) Fossils of the index nature may link time periods across broken and sometime different layers of rock Fossil forms were also thought to succeed each other in a specific and predictable order not so much anymore (4)Principle of Crosscutting Relations What principal might this represent? Charles Lyell in Principles of Geology (1830), the principle of cross-cutting relationships states that the geologic feature which cuts another is the younger of the two features. (5) Principle of Crosscutting Relations The Principle of Crosscutting Relations states that: Faults occur after the sedimentary beds have been deposited Igneous intrusions may occur after the sedimentary beds have been deposited What does that mean? Lava might pour through the cracks of the rocks (6) Principle of Inclusion States that fragments of rocks are included within other rock deposits: In sedimentary material, rock fragments are compacted and cemented together Called clasts In igneous material, the magma travels, picks up pieces of the rocks and the rocks are found inside the cooled magma Called xenoliths (6) Principle of Inclusion xenoliths clasts Principles In Action over Time: Uniformitarianism vs. Catastrophism Currently, the geologic processes that shape the planet are considered to be slow and gradual. Uniformitarianism has been accepted by scientists for the last 150 years. Uniformitarianism states that the slow and gradual speed of past geological processes is the same as the present slow and gradual speed i.e. Today, severe earthquakes and floods occur occasionally across the globe. This matches how geologic processes occurred in the past. Catastrophism explains that past geologic processes occurred fast and suddenly i.e. In the past, there were multiple global earthquakes and a single flood that shaped the earth.