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INTRO GEOL: CHPT. 1
Geology is the study of the earth. There are many specialties within the discipline of geology.
The subsystems of earth include the atmosphere, biosphere, hydrosphere, lithosphere, mantle, and core. ALL of theses systems interact to produce a dynamic earth, which has evolved through an immense period of time - 4.6 billion years! Life began about 3.6 billion yrs. ago!
Geology has played an important role in history: Earth processes and sedimentary environments can concentrate deposits of gold, uranium, gems, coal, and oil in certain areas. Wars have been fought over control of natural resources such as oil, gas, gold, silver, and diamonds. Empires have risen or fallen on the distribution and exploitation of earth resources. The topography (shape of the earth's surface) has played a role in military tactics. Mountain ranges, rivers, and seas have served as political boundaries. Demise of civilizations has occurred through the misuse of local resources, volcanism, or through the abrupt change in climates.
EARTH ORIGINS: The solar system was formed under a collapsing, rotating cloud of dust and
gas, a result of gravity. The heated earth underwent differentiation, which resulted as the Earth
cooled, allowing the heavier materials to sink to the center and lighter materials to 'float' to the top.
THE EARTH: Because of this differentiation, the earth is composed of3 concentric layers:
A. Core - is metallic; mostly iron, some nickel
B. Mantle - dense rock called peridotite [containig iron and magnesium]
C. Crust - lighter rock such as granite & basalt; 6 to 40 miles in thickness.
The outer earth is also divided in another way:
Lithosphere - Includes the solid crust and thin layer of upper mantle (60 miles thick).
Asthenosphere - behaves plastically and flows slowly (is 60 to 150 miles thick).
PLATE TECTONICS
The earth is like a cracked eggshell. The lithosphere is broken into plates that ride atop the plastic asthenosphere. These plates are thought to move as a result of underlying convection currents that circulate in the mantle (sort oflike a solid chunk of flat rock riding atop flowing lava).
WHERE PLATES MEET:
At plate junctures occur three types of boundaries:
l.divergent - where plates move apart, such as undersea spreading ridges and rifting
continental valleys.
2.convergent - where plates collide - forming mountains, volcanic mountains, and
oceanic island arc systems
3.transform - where plates slide by each other (San Andreas fault).
ROCKS: THE ROCK CYCLE
The three types of rocks: igneous - created from cooling lava solidifying into rock (e. g., granite, basalt, obsidian) sedimentary - from erosion of other rock or chernicalprecip. (shale, sandstone, limestone) metamorphic - from intense heat and pressure (slate, marble, anthracite coal)
GEOLOGIC TIME:
A grasp of the immensity of geologic time is necessary to understand the long evolution of earth.
CHPT.2: PLATE TECTONICS
Plate Tectonic theory says that the Earth's surface is composed of PLATES. These rigid plates (the lithosphere) slide over the plastic asthenosphere. Intense geologic activity occurs at the plate BOUNDARIES - which explains the presence of mountain chains. earth~uakes, !!!!!. volcanoes. The theory explains geologic structures such as folds & faults. oncept born by combining continental drift and sea-floor spreading.
CONTINENTAL DRIFf: Proposed by Alfred Wegener. Hints came from similarity of ancient rock types, climate, and fossils from southern continents of 250-300 million years ago.
THE EVIDENCE FOR PLATE TECTONICS
- CONTINENTAL FITS: especially between South America and Africa. The match-ups between their mountains or coastlines (which includes the off-shore continental shelves) is near-perfect
- THE MATCH-UP OF THE SEQUENCE OF ROCK LAYERS between South America, Africa, Australia, Antarctica, and India. All show, in sequence, Paleozoic glacial till, shale and sandstone sedimentary rock, coal-containing rock, & Mesozoic basalt.
- SIMILARITY OF FOSSILS found in South America, Mrica, India, Australia, and Antarctica; such as Late Paleozoic Glossopteris plants, the Mammal-Like Reptiles Lystrosaurus and Cygnognathus, and the marine reptile Mesosaurus.
- SIMILARITY OF ORE VEIN DEPOSITS (for example, !!g) within South America, Mrica, North America, and Western Europe.
- GLACIAL STRIATIONS found on South America, Mrica, India, and Australia.
- SEA-FLOOR SPREADING. MAGNEfIC REVERSALS have occurred in the earth's past. Paleomagneti~ evidence show mirror images of reversals across both sides of the Mid-Atlantic Ridge [J 1 J 1f 1- "1--]. [MAGNEfIC REVERSALS: Small magnetite crystals in a cooling lava act like tiny compass needles; they will align themselves to the existing magnetic field of the Earth, thus preserving a record of it's direction when the lava solidifies.]
The sea-floor age increases as you travel away from the Ridge on each side; also in mirror-image
fashion. [oldest I youngest !! youngest I oldest]
Why is there a difference between the ages of the sea floor and continents?? [Hint: subduction]
What is the DRIVING MECHANISM of Plate Tectonics? Convection currents in the mantle.
DIVERGING PLATE BOUNDARY EXAMPLES: A. At ocean-ocean boundaries - the Mid-Atlantic Ridge (MAR) including Iceland! B. Within continental interiors - The Great Rift Valley in Africa. The Red Sea.
CONVERGING PLATE BOUNDARY EXAMPLES : A. Ocean-Ocean (subduction). Results in island arcs systems - Japan, the Phillipines. B. Ocean-Continental (subduction) Results in volcanic mountain ranges - Andes Mts. C. Continent-Continent (no subduction). The Himilaya Mountains.
TRANSFORM BOUNDARIES EXAMPLES: A. San Andreas fault in California B. The transform faults along diverging ridges
IGNEOUS ROCKS I. INTRUSIVE
There are three kinds of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form from molten magma or lava. There are two groups of igneous rocks:
I. INTRUSIVE: These rocks form when MAGMA cools very slowly beneath the earth's
surface forming large emplacements called PLUTONS. Because they cool slowly over a million
years or so, the mineral grains are visible; and thus are classified as large-grained or coarse grained. These coarse-grained rocks include: Granite, Diorite, Gabbro.
II. EXTRUSIVE: Rocks formed when lAVA erupts on the surface (e.g., volcanoes, fissure
eruptions) and solidifies relatively quickly. [Extrusive rocks are covered in the next Chpt.]
- INTRUSIVE ROCK CATEGORIES-
Felsic Intermediate Mafic Ultra-Mafic
Silica (>65%) "li~ht, li~ht"
Silica (<.52%) "deep, dark, dense"
Silica «45%)
coarse-gramed PLUTONIC. GRANITE DIORITE GABBRO^ PERIDOTITE fme-grained VOLCANIC.... RHYOLITE (^) ANDESITE BASALT
FELSIC MAGMA -- The minerals are Si02-rich, with high content of SODIUM, POTASSIUM, and ALUMINUM [quartz, potassium and sodium feldspars, muscovite]. Light in color, weight. Solidifies lAST out of a melt.
MAFIC MAGMA - The minerals are Si02-poor, with high content of IRON, MAGNESIUM, and CALCIUM [olivine, pyroxene, hornblende, biotite, calcium plagioclase]. Dark color, dense, formed deeper down ("DDD"). Solidifies FIRST out of a melt. Minerals with high iron and magnesiuim content are called FERROMAGNESIAN minerals.
"Bowens Reaction Series" - As a magma body cools from approximately 1200 degrees down
to 800 degrees, there is a sequence of minerals that crystallize or solidifies out of the melt as the temperature cools. Olivine is the first to crysallize and quartz is the last [See chart!].
HOW DO MAGMAS OF DIFFERENT COMPOSITIONS EVOLVE??
- Crystal Settling - denser mafic minerals sink to the bottom; light, felsics float to top.
- Partial Melting- opposite of Bowen's Rx Series. Felsics melt first from a rock body.
- Assimilation - As a mafic magma moves into country rock, it melts and incorporates it.
- Mixing of magmas - e.g., a mafic and felsic magma combines to form an intermediate.
GRANITE makes up most CONTINENTAL crust. Why? Partial melting. Partial melting of a mafic magma will melt out only the felsic or intermediate minerals. BASALT makes up most OCEANIC crust. Why? Partial melting of the ultra-mafic upper mantle will melt out only the mafic lavas that emanate at sea-floor spreading. ANDESITE volcanoes are usually formed at subduction zones. Why? Melting and mixing of both mafic ocean crust with felsic continental crust forms an intermediate magma.
Categories of pluton emplacements: batholith, dike, sill, laccolith, stock, diapir.
VOLCANOES II. EXTRUSIVE IGNEOUS ROCK
Eruptions are the most impressive manifestations of Earth's dynamic processes. People in the Phillipines, Japan, Indonesia, Iceland, and Hawaii are fully aware of their effects.
Felsic Intermediate Mafic
VOLCANIC. ... RHYOLITE ANDESITE BASALT
fine-grained
Other volcanic rocks that are ejected out of a volcano include obsidian (glassy), pumice (frothy, vesicular glass), scoria (vesicular) and tuff (ash).
Know tenns: "aa" -large, blocky lava with a rough surface "pahoehoe" - smooth, ropy lava vesicular - cavities fonned from trapped gases in cooling lava columnar jointing - six-sided columns fonned as a result of shrinkage during lava cooling pillow basalts - bulbous masses of basalt fonned when lava is rapidly chilled underwater. crater - circular opening at the summit of a volcano, from which the gas and lava emanates. caldera - Fonned by volcano summit collapse when the underlying magma chamber is drained. pyroclastics - material ejected from a volcano (rock fragments, ash, bombs, etc.). pyroclastic flow (nuee ardente) - dense cloud of hot gas, ash, and rock fragments hurdling down the side of a volcano. pyroclastic sheet deposit - the result of huge pyroclastic flow deposits which may be a few meters to hundreds of meters thick. Also, from fissure eruptions. Ohter tenns plateau basalts/lava floods, flank eruptions, fissure eruptions.
VISCOSITY - is the resistance to flow of a lava ("gooiness" or "stickiness").
The two things that determine the degree of VIOLENCE of a volcanic eruption are:
1. viscosity - high viscosity (felsic, intennediate) lavas tend to cause violent eruptions
2. gas content (water vapor, primarily) - the more gas, the more explosive potential
When magma rises toward the surface, pressure is reduced, thus the contained gases begin to expand. In viscous magmas, expansion is inhibited and gas pressure builds up, setting the stage for cataclysmic eruptions.
- Shield - shield or dome-shaped; consist of basaltic flows; eruptions are relatively "qUiet." ~
- Cinder Cone - small cone shaped, explosive, emitting cinder fragments and gases. .cJ.
3. Composite/Stratovolcano - layered andesite, lahars (mud flows), pyroclasts/ash, and ~
perhaps lava domes. Your large, classic volcano... Mt. Pelee, Mt. Vesuvius, Krakatau, Mt. ~ Mazama (formed Crater Lake), Mt. Fujiyama.
Mafic lavas [basalt] are the least viscous ("runny") and are associated with quiet eruptions. Felsic lavas are very viscous (due to high silica content) and are associated with violent eruptions.
The two VOLCANIC BELTS: Circum-Pacific (60%) and Mediterranean (20%). About 20% of volcanoes occurs near mid-ocean ridges (Iceland) or ridge extensions (Mt. Kiliminjaro in Africa).
CHAPTER 7
SEDIMENTARY ROCKS
SEDIMENTARY ROCKS are rocks that have been eroded, chemically weathered, and transported as SEDIMENT....OR, they may have PRECIPITATED out of water (e.g., salts). Later, they are compacted and/or cemented as rock. Cements include CaC03 (limestone) SiO's ( silicas) and sometimes iron oxides & clays.
Chemical Weathering - the reaction of minerals with air and water. [e.g. Fe + 0 = FeO]
During transport, rock grains are ROUNDED, SORTED, and become SMALLER.
Sedimentary rocks are classified as:
(1) clastic - fragments cemented together [sandstone, mudstone] (2) chemical - precipitated out of water (carbonates and evaporites) (3) organic - accumulation of plant/animal material [coal]
What are some examples of each??
Grain sizes: gravel, sand, silt, clay. Is used to categorize clastic rocks.
SEDIMENTARY STRUCTURES: (How are they formed??)
bedding - general term for layering of sedimentary rock lamination - thin, horizontal layers of sediment formed in quiet (low-energy) waters cross-bedding - indicative of higher-energy water flow (e.g., rivers, beaches) graded bedding - coarse to fine-grain sequence; reflects flood, alluvial fan, turbidity current mudcracks - may indicate a change to a drier climate ripple marks - identifies a river or ocean shoreline deposition: higher-energy water flow imbrication - indicates flow direction; pebble slined up like "dominoes" worm burrows - may indicate shallow shoreline of lakes, rivers
ENVIRONMENTS of DEPOSITION: (What rock and sedimentary structures do we see?)
Alluvial Fans - high energy env.: breccia, conglomerate. Cross-beds, poor sorting. Glacial- Unsorted till.Till consists of angular rock fragments Braided Rivers - high energy: ripple marks, cross-bedding, breccia, conglom., sandstone Meandering Rivers - lower energy than braided rivers. sandstones, point bar deposits Aoodplains - medium to high energy; deposits can be sands, silts, clays (mud) Dunes - high-energy: wind-driven cross beds and sands (e.g., Zion Nat. Park) Lakes - Low energy env. Lamination. Silts, shales. Mudcracks along shoreline. Swamps - Low-energy water flow: lamination, shales, coal. Deltas - complex arrangement of mini-environs: sand, silt, mud; medium & low energy Beaches - High energy env.; rounded & well-sorted sands; cross-beds, ripple marks Lagoons - low energy env., laminar beds, bioturbation; fine-grained limestones, shales Tidal Flats -low energy; lamination showing alternating marine limestone & land shales Reef - high-energy; limestone and shell fragments Deep Marine - quiet waters; ooze consisting of fine-grained limestones, muds
SED J MENTA R Y
STRUCTURES
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METAMORPHIC ROCKS CHAPTER 8
Metamorphic rocks - formed from pre-existing rock under intense heat, pressure, & fluid
activity.
- Metamorphic rocks rovide clues for conditions at depth.
- They are a feature of the major mountain belts.
- They are important in providing evidence for what happens during subduction.
A large portion of the Earth's continental crust is composed of igneous and metamorphic rock. They a form a crystalline basement rock that underlie sedimentary rock. The exposed basement rock are called SHIELDS [Fig. 8.2] which are very stable and form the NUCLEUS of most continents.
What exactly happens during metamorphism?? When parent (original) rocks are metamorphised, small crystals can be re-crystallized or "squeezed" into larger one (e.g., calcite ---> marble); or more commonly, the parent rock is changed CHEMICAlLY and re-crystallizes into NEW minerals as seen below. Water is a catalyst for chemical reactions.
e.g. Clay shales ----> Mica. (^) Or,
Two minerals can be CaC03 + Si02 -----> CaSi03 + C "crunched together". (calcite) (quartz) (wollastonite)
TIME: How long does metamorphism take?? Metamorphic crystals like GARNET may grow
only 1.4 mID per million years!
TYPES OF METAMORPHISM:
1. Contact - from magma heat [fig. 8.5]
- Regional - from heat and pressure. Occurs over a large area.
CLASSIFICATION OF METAMORPIDC ROCKS: [Table. 8.2]
1. Foliated (planar texture) - slate, phyllite, schist, gneiss [fig. 8.9 - 8.11]
- Non-foliated - marble, quartzite, anthracite coal [fig. 8.14 - 8.15]
Progressive metamorphic grade: shale ---> slate --> phyllite ---> schist ---> gneiss
Many of our beautiful gemstones are formed under high pressure and temperature!
EARTHQUAKES
Know focus, epicenter, seismograph, seismogram.
Two types of seismic waves: body waves and surface waves.
- BODY WAVES [fig. 9.8]
A. Primary - "P" wave [rock vibrates parallel to the wave] These travel the fastest, 10,000 to 15,000 mph. B. Secondary - "S" wave [rock vibrates perpendicular It It] Cannot travel through liquid.
- SURFACE WAVE A. "L" wave. Does the most damage. Slowest waves.
RICHTER SCALE: A magnitude increase of one (e.g., from 6.0 to 7.0) equates to an increase in VIBRATION by a factor of 10 and an increase in ENERGY release by 30!
EFFECTS OF EARTHQUAKES:
l.GROUND MOTION
2. ARE
3. LANDSLIDES
- TSUNAMIS - creates wavelengths of 100 miles, traveling at 450 mph, and may result in a wave height of 100 feet! [Largest was 278 feet high!]
DISTRIBUTION OF EARTHQUAKES:
1. Circum-Pacific Belt. 80 % of earthquakes occur here.
Quakes, trenches, and andesitic volcanoes are closely associated.
2. Mediterranean-Himilayan Belt.
EARTHQUAKES AND PLATE BOUNDARIES:
Shallow quakes occur at diverging and transform boundaries.
Shallow and deep quakes occur at subduction zones (converging boundaries)
What is a P-wave shadow zone? What is an S-wave shadow zone? What causes the most damage in earthquakes? What is liquefaction ofthe ground? How much more wave amplitude and energy does an 8.6 quake have in comparison to a 6.6?
GEOLOGIC TIME SCALE (TIME ON EARTH)
ERA
65
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PERIOD
Quaternary
Tertiary
EVOLUTION OF LIFE
AS RECORDED BY FOSSILS
Origin and evolution of man
Evolution of mammals
" ... ;. Jurassic· (^55)
Triassic 35
PerlJ:lian 50
(Pennsylvanian)
Carboniferous 70
(Mississippian)
Devonian 55
Silurian 30
Ordovician 70
Cambrian 70
.. Precambrian" 4000
1
fwo
~O&'·
·m
215
400
500
550
··First birds.
,.DeVelopment of giant dinosaurs
First dinosaurs
First mammals
Wideextinctions
Dominance of mammal-like
reptile.s
First repti les
Dominance of amphibians
Firsfamphibians
Air-breathing fishes
Primitive land plants
First jawed fishes
First land-living animals
Jawless fishes
First vertebrates
Invertebrates widely established
Appearance of numerous fossils
Fossils rare
Algae
Origin of Earth
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PALEOMAPS OF THE PAST
How can we reconstrnct our planet over the last 4 billion years?? By combining evidence from (1) paleomagnetism (2) animal distribution (3) tectonic patterns (4) climatology (5) radiometric dating and (6) sedimentary data.
THE CLUES IN THE ROCKS
What is the evidence for:
C s~
- Glaciation? Tillites, glacial striations, dropstones. &;.'t ~
2. Ocean shorelines? A lateral sandstone-shale-carbonate {S - S· C} sequence. The ss
shale is terrestrial deposition and the carbonate is marine deposit. Ripple marks and well-sorted sandstone grains are found near ancient beach and surf zones. ~.Lr'I-1r1 C
- Sea transgressions? (higher sea levell The S-S-C migrating over itself; ..:::^ - __,-0-^ ~ ri. ' the s-s-c migrates landward. Fossils can give clues too.
4. Sea regressions: Reversal of the S-S-C migration; unconformities, evaporite deposits.
What causes these sea level changes? (1) Glaciation (2) Increase in Plate Tectonism; uplift/subsidence
- Conver,ina plates and thus mountain building: folds, reverse faults, metamorphic rock, granite lDtrusions; and andesite volcanoes in subduction zones. Clastic wedges. ~
6. Divergent plates or riftin..: nonnal faults. fault-block basins (horst & graben), tensional
structures such as joints, dikes, SIlls, & basalt lava flows. The presence of ocean crust - a
cross-section contains, from top to bottom. fine deep-sea sediment, pillow basalts, dikes, gabbro, and peridotite. An ocean crust (ophiolite) sequence is found in the Alps!
7. Climate clues
coal =swamps and wet climate tillites =cold clime red beds =at least a seasonally dry climate abundant plant fossils =wet, warm climate evaporites (salt, gypsum) =desertslhot shorelines fossils = may indicate a w~ or ~Iime
desert dunes (large cross-beds) =a hot, dry clime ~
- Sedimentary large crossbeds of sandstone/shale =a delta clues large-scale sandstone tross beds = a desert ~d mme
turbidi~ currents, graywacke + ooze = deep-sea fans
lime!lOne deposits = shallow marine waters Stromatolites = shallow marine waters
Fossil reefs = shorelines within 30 degrees north or south of the equator.
Evaporites may mark a cut-off lagoon.
- FossD distribution: may show the arrangements of continents (e.g., Gondwana). Also can identify a marine vs. a terrestrial environment.
- The inclination of the paleomagnetism (arrows) in rocks can locate where the continent was in relation to the north pole.
RECORD IN THE ROCKS: SUMMARY OF EARTH HISTORY
PRECAMBRIAN [4.6 bya - 550 mya]
{88% of earth history}
ARCHEAN EON - Oldest known rocks are from 4 bya. The only landforms appear to be
volcanic island-arcs systems. Plate Tectonics occurred at a faster rate due to a much hotter interior. Most rocks were volcanic or metamorphic.
Life consisted of single-celled bacteria; later there is evidence of stromatolites.
PROTEROZOIC EON - Begins about 2.5 bya. The Archean volcanic island-arc systems
collided to form mini-continents and then larger continents. Sedimentary rocks appear in the rock record as deposition created shallow continental shelves offshore. Glaciation occurred near the beginning and near the end of the Proterozoic.
The dominant large life structures were stromatolites worldwide. Consequently the oxygen in the atmosphere increased to 10%, followed by the appearance of redbeds and multi-celled organisms.
Land masses were Laurentia, East and West Gondwana, Siberia, and Baltica (W. Russia).
PHANEROZOIC EON [550 mya to present]
{12% of history}
PALEOZOIC ERA [550-245mya] - Early Paleozoic was tectonically quiet in North America,
with extensive shallow carbonate platforms offshore. Then, 4 mountain building episodes occurred along the east/southeast coast, a result of Laurentia colliding with Baltica to form Laurasia and then Laurasia colliding with Gondwana to form Pangaea during the Late Paleozoic.
Glaciation occurred in the Late Ordovician and Late Paleozoic. Great' coal' forests developed in
the Miss-Penn. Periods. Sea levels fluctuate. Invertebrates, amphibians, & first reptiles fluorish.
MESOZOIC ERA [245 - 65 mya] - Dominated by the break-Up of Pangaea. Three mountain
building orogenies occurred along the west coast during the Mesozoic. Warm and dry climates continued from the previous Permian Period into the Triassic and Early Jurassic Periods. Sea levels were at their highest in the Cretaceous Period. Dinosaurs dominate the landscape.
CENOZOIC ERA [65 mya to present] - Continued break-up of Pangaea. The Atlantic
Ocean is formed; Greenland separates from North America and Australia separates from Antarctica. India collides with Asia, forming the Himilayas. The Rocky Mountains continue to rise from the Late Mesozoic Laramide orogeny. Ice ages begin 40 mya. Mammals dominate the land.
and unmetamorphosed. with widespread assemblages of SEDlMENfARY rock such as carbonate
shales - which results from shallow marine deposition.
PROTEROZOIC CONTINENTS:
Formed by the collision and accretion of Archean cratons. thereby forming much larger cratons.
An example is LAURENTIA which shows the accretion of five Archean cratons between 1.
1.0 bya. Laurentia consisted of North America, Greenland, Scotland and parts of Scandinavia.
Other Proterozoic continents existed as well:
East Gondwana - Australia. India. Antarctica Baltica (western Russia) West Gondwana - Mrica. South America Siberia
GLACIATION: in Early [2.4 bya] & Late Proterozoic [750 mya] - the most extensive in history.
ATMOSPHERE: The oxygen content rose from 1% to 10%. Stromatolites. which became common 2.3 bya provided the oxygen as a waste product.
LIFE: As in the Archean Eon. it consisted of single-eelled organisms and colonial (one-celled) stromatolites - blue-green algae/cyanobacteria. Multi-celled organisms evolved about 1.0 bya
III. PHANEROZOIC EON:
- PALEOZOIC ERA
The Paleozoic Era lasted 305 my [550-245 mya] representing 7% of geologic time. Events were:
(a) 4 mountain building episodes (orogenies) along the 'east' coast
(b) Plate collisions forming Laurasia [Laurentia + Baltica] and Pangaea [Laurasia
+Oondwana] along with the 4 orogenies
(c) much of the U.S. was submerged under shallow warm seas
There were six major continents at the beginning of the Paleozoic Era:
- Laurentia - composed primarily of North America and Greenland
- Baltica - Russia west of the Urals and northern Europe
- Gondwana - Mrica. South America. Australia. India. Antarctica
4. Siberia 5. China 6. Kazakhstania - central Asia
For North America. situated on the equator and submerged to a large extent.the Paleozoic Era also
reflects the following in general:
-four orogenies [mt. building episodes] -numerous rise and fall in sea levels
-a great coal forest in the Penn. Period -large evaporite deposits
-reef building in the shallow seas -a drier climate in the Permian Period
-large carbonate platforms in warm shallow seas
The Early Paleozoic Era was relatively quiet and warm in North America. with passive continental
margins producing warm. shallow Carbonate (limestone) platforms dominated by stromatolites.
Glaciation occurred twice - once during the Late Ordovician and secondly in the Late Paleozoic in
southern Gondwana. Ocean current circulation was open.
MOUNTAIN BUILDING [raconic. Acadian. Allegheny. Ouichita]
1. Cordilleran Mt. Belt - passive during most of the Paleozoic.
- Appalachian Mt. Belt - 3 orogenies; the 2nd due to the collision of Laurentia with Baltica. forming Laurasia. The 3rd created the Appalachians when Pangaea formed.
- OUichita Mt. Belt - south-central U.S.A. Uplifted as Gondwana collided with Laurasia to produce the supercontinent PANGAEA. 2
- MESOZOIC ERA
THE MAJOR EVENT IN THE MESOZOIC WAS THE BREAKUP OF PANGAEA. The Mesozoic lasted 180 my (4 % of history!). America "grows'; due to accreted microplates. The breakup of the supercontinent Pangaea is documented by:
- fault-block basins 4. tensional structures such as dikes. sills. lava flows
- eventual fonnation of ocean crust between rifting plates
- Increased diversification of animals (dinosaurs), particularly in the Cretaceous Period
This major rifting caused sedimentation up to 18,000 feet thick in the fault-block basins! Also present are lava flows and intrusion of dikes and sills [e.g.,The Palisades in New York].
CLIMATE: [Wann throughout the world]
During the first half of the Mesozoic, a huge region of the Pangaea interior was dry - a carry over from the preceding Penni an Period. The evidence: a. Evaporite deposits - Large deposits in the Gulf of Mexico. b. Pennianffriassic red beds - e.g., the American SW, including Utah c. Desert dunes (e.g. 75 feet high) located in the large scale cross-beds [Zion Nat. Pk]
Sea level was lower during much of the Triassic/Jurassic but was very high in the Cretaceous.
OROGENIES (This time. the Cordilleranbelt underwent mountain building)
- Nevadan Orogeny - Late Jurassic. Thrust faults. Large batholiths of granite
- Sevier Orogeny - Cretaceous. Eastward from the Nevadan. Thrust faults
- Laramide Orogeny - Cretaceous-Tertiary. Even further eastward [the Rocky Mtns].
- CENOZOIC ERA
The Cenozoic encompasses the last 65 million years - only 1.4 % of earth history. CENOZOIC PLATE TECTONICS:
- Continuation of the separation of Pangaea, with the growth of the Atlantic Ocean basin.
- Spreading ridges include the Mid-Atlantic Ridge and the East Pacific Rise
- India travels north and collides with Asia; Australia separates from Antarctica
- Rifting begins in Africa in the Late Tertiary. The Arabian Plate separates from Africa.
- Orogenies occurred mainly in the Alpine-Himalayan and Circum- Pacific belts
OROGENIC BELTS:
- The Alpine Orogeny was caused by the northward movement of the African and Arabian plates. Fonned were the Alps, Atlas. Apennines. Pyrenees, Caucausus Mts.
- Himalayas - Collision of India into Asia 40-50 my a during the Laramide Orogeny. About 1200 miles of India lies under Asia. J. Circum-Pacific belt: island-arcs systems - Aleutians, Phillipines, and Japan.
- Andes Mts: fonned during plate convergence, includes granitic plutons.
South America was an island continent until very recently.
Volcanism: In the Pacific Northwest enonnous amounts of the Columbia River basalts (200, cubic km) were exuded, flowing from fissures over a 3.5 million-year period. Cascade Mtn. Range volcanism continues as a result of the subduction of the Juan de Fuca plate.
ICE AGES: Started 40 mya. Within the last 2 my. 4 major glaciations have occurred in U.S.
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