Earth History 870:035 Course goal: To describe the history of Earth and its inhabitants Most of Earth s history predates humanity, so it has not been observed Therefore, we will emphasize how scientists interpret evidence for reconstructing Earth history Earth History, Ch. 1 1
Major Events in Earth s History Origin of the Earth Formation of continents Origin of life (simple cells) Origin of multicellular life Invasion of land by plants and animals Formation of major mountain chains Assembly and break-up of supercontinent Pangaea Five great mass extinctions Age of dinosaurs Origin of mankind Earth History, Ch. 1 2
Earth is a system of highly integrated, interdependant processes geosphere atmosphere biosphere Earth History, Ch. 1 3
Earth s physical history and biologic history must be studied in tandem because they are so intricately linked: e.g., The global flora influences atmospheric O 2 and CO 2 levels, and thus climate Plants and animals play a role in both mechanical and chemical weathering of rocks, and in the deposition of some sedimentary rocks Physical factors influence biogeography, evolution, and extinction Earth History, Ch. 1 4
How do we study Earth s history? Uniformitarianism: Conceived by James Hutton, popularized by Charles Lyell: The principle that laws of nature are constant Actualism using knowledge of modern processes to interpret ancient products by applying uniformitarianism the present is the key to the past Earth History, Ch. 1 5
Uniformitarianism/Actualism illustrated Modern process: current ripples Ancient product: inferred current ripples Earth History, Ch. 1 6
Uniformitarianism/Actualism (some limitations) Although physical laws may be constant through time, rates sometimes vary! e.g., Earth s rotation is slowing down Some geologic processes are not observable e.g., those that occur deep within the Earth Some conditions change through time e.g., increase in atmospheric O 2 Some geologic features are created by catastrophic events that seldom occur e.g., huge meteorite impacts separated by millions of years Earth History, Ch. 1 7
Rock Review The pages of Earth History Mineral = a naturally occurring inorganic solid with a particular chemical composition and characteristic internal structure Rock = an aggregate of one or more minerals Earth History, Ch. 1 8
Rock types Igneous those rocks that have crystallized from molten magma Sedimentary those rocks that consist of cemented or compacted particles Metamorphic those rocks that form as a result of the alteration of pre-existing rocks by heat and/or pressure Earth History, Ch. 1 9
Igneous Intrusive igneous rocks solidified at depth within the Earth and usually have coarse crystals (e.g., gabbro) Extrusive igneous rocks solidified at the Earth s surface and have microscopic crystals (e.g., basalt) Earth History, Ch. 1 10
Intrusive igneous rock (granite) Earth History, Ch. 1 11
Sedimentary Sandstone and Shale are formed by cementation and/or compaction of grains from pre-existing rocks Limestone is formed by cementation of biologically secreted shell material Evaporites and other chemical sedimentary rocks are formed by inorganic precipitation of minerals from water Sedimentary rocks are usually layered or bedded. An individual layer is called a stratum (plural = strata) Earth History, Ch. 1 12
Sedimentary rocks (strata) Earth History, Ch. 1 13
Metamorphic Pre-existing rocks can be metamorphosed when subjected to heat and/or pressure. This alteration takes place without the rock becoming molten. Any rock can be metamorphosed Igneous metamorphic Metamorphic metamorphic Sedimentary metamorphic Earth History, Ch. 1 14
Metamorphic rocks (foliated) Earth History, Ch. 1 15
Classification of rock bodies A formation is a distinctive, mappable body of rock A formation may be subdivided into members Two or more formations may be united into a group Earth History, Ch. 1 16
The rock cycle Any rock can be transformed into another rock of the same kind or either of the other two kinds Earth History, Ch. 1 17
Steno s laws for sedimentary rocks Principle of superposition = in an undisturbed succession of rock, the oldest strata are at the bottom and progressively higher strata are progressively younger Principle of original horizontality = strata are horizontal or nearly horizontal at the time of deposition Principle of original lateral continuity = strata are continuous or nearly continuous within the confines of the basin of deposition Earth History, Ch. 1 18
Steno s Laws Superposition Original horizontality Original lateral continuity Earth History, Ch. 1 19
Ordering of geologic events Principle of intrusion states that an intrusive igneous rock body is younger than the rock it has intruded Principle of components states that when fragments of one rock body are found within another, the second rock body is younger than the first Principle of cross-cutting relationships state that when one geologic feature cuts another, the feature being cut is older than the feature cutting across Earth History, Ch. 1 20
Unconformities An unconformity is a surface of erosion or nondeposition that separates rock bodies Angular: rocks beneath the unconformity surface are tilted relative to those above it Disconformity: rocks beneath and above the unconformity surface are parallel, but the surface itself is irregular Nonconformity: the unconformity surface separates crystalline rocks below from sedimentary rocks above Earth History, Ch. 1 21
Unconformities Earth History, Ch. 1 22
Angular unconformity Earth History, Ch. 1 23
Dating the geologic record Relative ages of rocks can be determined by fossil succession Each interval of geologic history was characterized by an assemblage of plants and animals that was distinct from the assemblages of older and younger intervals Absolute ages can be determined by the orderly decay of natural radioactive materials Earth History, Ch. 1 24
The geologic time scale Earth s 4.6 billion year history is divided into three great eons Phanerozoic Eon Proterozoic Eon 0.54 billion Abundant, multicellular organisms Mostly unicellular organisms Archean Eon 2.5 billion 4.6 billion Organization of the Earth s internal layering Earth History, Ch. 1 25
Structure of the Earth Layers are distinguished on the basis of seismic velocity contrasts, which reflect differences in density Seismic boundary between crust and mantle is known as the Moho Earth History, Ch. 1 26
Lithosphere and Asthenosphere Lithosphere = the outer rigid layer consisting of the crust and uppermost mantle Asthenosphere = the underlying plastic layer Consisting of partially molten mantle crust Moho Lithospheric plates move relative to one another, driven by thermal convection within the asthenosphere. The movement of plates is called plate tectonics. mantle Earth History, Ch. 1 27
Crust/mantle vs. lithosphere/asthenosphere Earth History, Ch. 1 28
Plate tectonics Study of the interaction among lithospheric plates (processes and products) Earth s surface is made up of about a dozen large plates in constant motion relative to one another Convection is the driving force behind plate tectonics Earth History, Ch. 1 29
Lithospheric plates Earth History, Ch. 1 30
Convection heat from interior Earth History, Ch. 1 31
Interactions between the water cycle and the rock cycle The water cycle and the rock cycle are intricately linked Water plays a key role in weathering and erosion of rocks Water plays a key role in deposition of sediments Water is subducted along with lithospheric slabs along subduction zones, then recycled into magma Earth History, Ch. 1 32
heat from sun Water cycle Earth History, Ch. 1 33
Directional changes in Earth s history Evolution the irreversible progression over time of life forms from simplest cell to complex organism Decrease through time in Earth s internal heat Increase through time of atmospheric O 2 Earth History, Ch. 1 34