Geology CH11

Steps to fossilization:

1. An organism dies 2. The remains get buried quickly by sediment. The soft parts decay. 3. With time, the hard parts get replaced with minerals. The sediment lay gets buried deeper and is compacted and cemented to form Sedimentary Rock. 4. The fossil is later exposed through erosion.

What might have caused the mass extinction at the end of the Paleozoic?

ANS: At this time, the Siberian traps formed. These flood basalts represent a massive volcanic eruption that could have triggered drastic and sudden environmental changes.

What are the major types of organisms that appeared during the Paleozoic?

ANS: Conodonts, trilobites, mollusks, brachiopods, and echinoderms.

What life forms appeared during the Mesozoic?

ANS: Dinosaurs, mammals, birds, large swimming reptiles, and pterosaurs are a few examples.

What major climatic and biologic events happened during the Pleistocene?

ANS: Much of the northern hemi sphere was in the grip of a glaciation (ice age), and there were numerous extinctions of large animals toward the end of the glaciation.

Describe the plate- tectonic conditions that led to the formation of the Sierran arc and the Sevier thrust belt. What happened during the Laramide orogeny?

ANS: North America had a convergent- plate boundary to the west that induced collisions with microcontinents and caused subduction of oceanic crust, which partially melted to form the Sierran volcanic arc. Compression induced the folds and thrust faults just interior to the arc itself. Faulting associated with the Laramide orogeny uplifted deep basement rock to produce the Rocky Mountains, whereas earlier orogenies produced only the more superficial faults of the fold- thrust belts behind the arc. Changes from the Sevier- style orogeny may have been a result of a shallowing dip of the subducting slab.

What continents formed as a result of the breakup of Pangaea?

ANS: North America, South America, Africa, Eurasia, Australia, Antarctica, and the microcontinent Madagascar are examples (answers may vary).

What supercontinent formed at the end of the Paleozoic, and what ocean formed when it broke apart?

ANS: Pangaea formed at the end of the Paleozoic. When it broke up, the Atlantic Ocean basin formed.

What may have caused the flooding of the continents during the Cretaceous Period?

ANS: Rapid Cretaceous rates of submarine volcanism thickened the mid- ocean ridges and produced numerous hot spots, which displaced water onto the continents. Additionally, glacial ice could not accumulate anywhere on Earth in the associated greenhouse warmth.

How did the Alleghenian and Ancestral Rocky orogenies affect North America?

ANS: The Alleghenian orogeny provided the final uplift to produce the Appalachian Mountains. This powerful collision between Laurentia and Gondwana influenced fault movement in what is now western North America. Uplifted western blocks (the Ancestral Rockies) are the first tectonic events in the region of the modern Rocky Mountains.

What caused the Himalayas and the Alps to form?

ANS: The Alps arose from Europe's collision with Africa; the Himalayas arose from India's collision with Asia.

What major tectonic provinces formed in the western United States during the Cenozoic?

ANS: The Basin and Range Province and the Snake River Plain Province are two important examples.

Describe the condition of the crust, atmosphere, and oceans during the Hadean Eon.

ANS: The crust would have been largely ultramafic magma and rock (which may have been subject to remelting). The atmosphere lacked free oxygen and would have been dominated by volcanically emitted gases (water vapor, nitrogen, and carbon dioxide). For at least part of the Hadean, temperatures were likely too hot to sustain a liquid ocean.

What could have caused the K-T mass extinctions?

ANS: The extraterrestrial impact that produced the Chicxulub crater was likely a factor. (The Deccan Traps eruption is another, unmentioned in the text - but we discussed the increase in volcanism during lecture.)

How did the Cambrian explosion of life change the nature of the living world?

ANS: These first abundant animals complicated the world food web through the introduction of more abundant filter and bottom (deposit) feeders, as well as large, mobile predators and the first biogenic reefs. Carnivores likely induced selection pressure favoring numerous defensive structures (thick shells, spines) and behaviors (burrowing, active swimming).

Were there multicellular organisms before the Cambrian?

ANS: Yes, multicellular animals and other multicellular life have been found in the late Proterozoic Ediacaran faunas.

Cenozoic life evolution:

After the K-T boundary, plant life recovered. Forests of angiosperms and gymnosperms reappeared. The 1st grasses appeared in the middle Cenozoic. Dinosaur descendants (birds) diversified and spread. The Cenozoic is known as the age of mammals. Mammals rapidly diversified to fill vacated niches. By the mid-Cenozoic, huge mammals appeared (Woolly mammoths. Giant beavers, Ground sloths). Late Cenozoic, human ancestors 1st appeared.

Paleozoic summary

Began with appearance of diverse hard-shelled organisms, Hard-shells vastly increased fossil preservation, Made possible a more complete archive of life on Earth. Summary: As the Paleozoic Era began, rifting yielded several separate continents. Sea level rose and fell, depositing sequences of strata in continental interiors. Continents coalesced again, to form another supercontinent, Pangaea. Early Paleozoic evolution produced many invertebrates with shells, and jawless fish. Land plants and insects appeared in the middle Paleozoic, and by the end of the eon, there were land reptiles and gymnosperm trees.

Cretaceous paleogeography:

Climate continued to warm. Seas flooded the continents. An ocean connected the Gulf of Mexico to the Arctic.

Why are there no whole rocks on Earth that yield isotropic dates older than 4 billion years?

Earth is a geologically active planet. Subduction, erosion, metamorphism, and melting destroy rocks.

Carboniferous and Permian life evolution:

Life continued to evolve, Dense tropical wetlands hosted vegetation and giant insects, Amphibians diversified, Reptiles appeared for the 1st time - amniote egg permitted reproduction away from water. The Paleozoic ended with the Permian extinction, 90% of all marine species disappeared.

Triassic and Jurassic Life Evolution:

New species filled niches vacated by extinction, Corals became dominant reef builders, Gymnosperms (cycads, conifers) proliferated, Reptiles diversified. By end of the Triassic the first true at dinosaurs appeared, Dinosaurs differ from other reptiles in significant ways (Legs are positioned beneath their bodies, bear evidence of warm bloodedness). By the end Jurassic giant sauropods were abundant. The 1st feathered birds (archaeopteryx) and the 1st ancestors of mammals appeared during the Triassic.

The Archean Eon (Pre-Cambrian)

Oldest undisputed bacteria fossils ~ 3.2 Ga, Rocks after 3.2 Ga contain stromatolites. Photosynthesis changed Earth's atmosphere, converts CO2 and H2O to organic matter and free oxygen. The Archean Eon began about 3.85 Ga, when the fi rst continental crust that still remains formed. This crust assembled out of volcanic arcs and hot- spot volcanoes that were too buoyant to subduct. The atmosphere contained very little oxygen, and the first life forms— bacteria and archaea— appeared.

Hadean Eon (Pre-Cambrian)

Planet cooled, the first oceans formed as rain from the skies. The Earth formed about 4.57 billion years ago. For part of the first 600 million years, the Hadean Eon, the planet was so hot that its surface was a magma ocean.

Mesozoic summary

Summary: In the Mesozoic Era, Pangaea broke apart, and the Atlantic Ocean formed. Convergent boundary tectonics dominated along the western margin of North America. Dinosaurs became prominent land animals through the Mesozoic Era. During the Cretaceous Period, the continents flooded. Angiosperms appeared, along with modern fi sh. A huge mass extinction event wiped out the dinosaurs at the end of the Cretaceous Period, probably due to the impact of a large meteorite.

Cretaceous life evolution:

Swimming reptiles and gigantic turtles swam the seas. Angiosperms (flowering plants) appeared and spread. Dinosaurs reached their evolutionary peak. Inhabited almost all environments on earth. Social herds of grazing dinosaurs roamed the plains. Large carnivores fed upon the herbivores. Pterosaurs soared overhead, birds began to diversify. Mammals developed larger brains and specialized teeth.

Cenozoic paleogeography:

The final stages of the breakup of Pangaea. Global climate has gradually cooled since the Cretaceous. The Antarctic ice cap reappeared in the early Oligocene. Continued cooling led to the formation of grasslands. The Isthmus of Panama emerged 2.5 Ma (Isolated circulation between the Atlantic and the Pacific, Permitted the Arctic Ocean to freeze).

Cenozoic (65 Ma to present)

The most recent portion of geologic time Summary: In the Cenozoic Era, the collision of Africa and India with Asia and Europe formed the Alpine- Himalayan orogen. Convergent tectonics persisted along the margin of South America, creating the Andes, but ceased in North America when the San Andreas Fault formed. Rifting in the western United States produced the Basin and Range Province. Various kinds of mammals filled niches left vacant by the dinosaurs, and the human genus, Homo, appeared and evolved through the Pleistocene Ice Age.

Cambrian and Ordovician paleogeography

The rifted continents developed passive margin, Rising seas flooded expanses of continental crust, Epicontinental seas were shallow, warm, and sunlit, Thriving marine life was preserved as fossil-rich sediments.

The K-T (Cretaceous-Tertiary) boundary event.

There is abundant evidence of catastrophic change. Instantaneous global change in fossil assemblages. Sudden mass extinction of most species on earth. The dinosaurs that had ruled the planet for 150 Ma vanished. Catastrophic impact by a 10 km comet or meteorite. The Chicxulub crater lies beneath the northern Yucatan. Radiometric dating indicates the crater formed at ~ 65 Ma. Iridium in the clay is rare on Earth; common in meteorites. Iridium-enriched clay found at the K-T boundary worldwide.

Why can we find mineral grains that are older than the oldest rocks?

These oldest minerals grains are zircons found in sedimentary rocks. Isotopic dates for the zircons give the age of the original igneous rocks in which they formed. These original igneous rocks have been destroyed by the weathering process. The sandstones in which the zircon grains are found are younger than these isotopic dates (principle of inclusions), and no rocks have been found that are as old as these zircon grains.

Fossil

any remains, impression, or trace (tracks, trails, burrows, etc.) of a living thing of a former geologic age, as a skeleton, footprint, etc.

Silurian and Devonian life evolution:

Vascular land plants evolved and spread across Earth, Internal water transport systems, Woody tissues, Seeds., Land plants changed Earth. Fish rapidly evolve and proliferate. The 1st land animals followed plants (Scorpions, Spiders, Insects, Crustaceans).

Describe how the first continental crust might have formed.

Volcanic island arcs and hot- spot volcanoes may have become too buoyant to subduct, particularly after magmatic differentiation processes_

What evidence do we have that Earth nearly froze over twice during the Proterozoic Eon?

_Glacial deposits have been found in sedimentary rocks representing sea- level environments along the paleoequator_

How did the atmosphere and tectonic conditions change during the Proterozoic Eon?

_The atmosphere became oxygenated (and depleted of some carbon dioxide) as a result of photosynthesis. Plate- tectonic conditions became more similar to what is observed today, with larger oceanic plates and the development of broad, stable cratons in continental interiors_

Did supercontinents form in the Proterozoic?

_Yes; Rodinia formed late in the Proterozoic.

Paleozoic

ancient life

Cetaceans (whales) evolve from land dwelling mammals

artiodactyls (antelopes) and enter the water/seas in the early Eocene Epoch. They re-occupied niches that existed after the extinction of the marine reptiles at the end of the Cretaceous.

The Proterozoic Eon

began at 2.5 Ga, Archean cratons sutured together to form large cratons. Photosynthesis added oxygen to the atmosphere. By the end of the Proterozoic, soft- bodied marine invertebrates populated the planet, and continental crust had accumulated to form a supercontinent.

mesozoic

middle life

cenozoic

recent life