28.1 Understanding Earth *Geologyis the study of rocks and materials that make up Earth and the processes that shape it. In1666, Nicholas Steno (1638-87) noticed that shark’s teeth resembled mysterious stones called “tonguestones” that were found in local rocks.
28.1 Understanding Earth Steno’sexplanation helped him develop ideas about how rocks and fossils form. These ideas are used in a technique called relative dating. *Relative dating is a way to put events in the order in What happened here and in what order? which they happened.
28.1 Understanding Earth *The approximate age of each layer of a rock formation can be determined by applying Steno’s idea called superposition. A stack of newspapers illustrates superposition. *Superposition means that the bottom layers of rock are older than the layers on the top.
28.1 Understanding Earth *Original horizontality states that sediment particles fall to the bottom of a basin, such as a riverbed, in response to gravity and result in horizontal layers.
28.1 Understanding Earth *Lateral continuity is the idea that layers of sediment extend in all directions when they form and before they become rock layers. The idea of lateral continuity states that layers of rock are continuous unless a geologic event like a river interrupts the layers or an earthquake them.
28.1 Understanding Earth Anotherimportant idea, developed by Scottish geologist James Hutton (1726-97), is that the "present explains the past." *The idea of cross- cutting relationships states that a vein of vein rock is younger than the rock that surrounds a vein.
28.1 Understanding Earth Sometimes rock pieces called inclusions are contained in another rock. *During the formation of a rock with inclusions, sediments or melted rock surrounded the inclusion and then solidified. *Therefore, the inclusions are older than the surrounding rock.
28.1 Understanding Earth *Over geologic history, many animals and plants have lived and become extinct. *Their remains have become fossils. *The idea of faunal succession states that fossils can be used to identify the relative age of layers of a rock formation.
28.1 Calculating Earths Age William Thompson Kelvin (1824-1907), known for proposing the absolute temperature scale that came to be named after him, meticulously calculated Earth’s age to be between 10 million and 100 million years. Lord Kelvin’s calculation was not accurate because he did not realize that Earth has internal heat from the core and radioactive decay.
28.1 Calculating Earths Age Earth’s age was estimated by measuring the radioactive decay of uranium to lead. With improved techniques and evidence from tree rings and glaciers: *The age of Earth is estimated to be about 4.6 billion years.
28.1 The Layers of the Earth Earth’s surface is covered with a thin crust. There are two kinds of crust: — continental — oceanic
28.1 Convection inside Earth The rocky material of the mantle moves in very slow convection currents. This movement is related to density and temperature differences in the mantle. Hot material is less dense and rises. Cold material is denser and sinks.