• Plate tectonics

Earth's surface is comprised of 15 tectonic plates that move at the rate of 2-15 cm. a year.

The supercontinent Pangaea formed 250 million years ago; its breakup has shaped the earth's continents and oceans.

• Our planet consists of layers.

• Core: a solid iron in the center is surrounded by molten iron in the outer core.

• Mantle: a thick rocky layer that supports a molten Asthenosphere in the upper mantle.

• Crust: the thin layer of rock at the surface where most life is found.

The Lithosphere spans the uppermost mantle and the crust, forming mobile tectonic plates.

• During the Silurian-Devonian periods (440 - 355 million years ago), much of North America lay under a shallow tropical sea, just south of the equator.

Photosynthetic organisms provided the bottom of the food chain.

Marine life such as trilobites, nautiloids, crinoids, corals, and sponges were abundant.

Their calcium-rich shells and skeletons accumulated over time into thick deposits that form the limestone bedrock of the continent.

• Algae and cyanobacteria are capable of photosynthesis: capturing energy from sunlight and storing that energy in carbohydrates (sugars).

After they died, their energy-rich remains were buried under layers of sediment.

Some of these deposits were not completely decomposed, and under heat and pressure they eventually turned into fossil fuels such as crude oil and natural gas.

Thus the petroleum and oil shale being harvested today on the Great Plains are "fossil sunbeams" that may be 450 million years old.

• Limestone is sedimentary rock - formed by deposits of calcium-rich shells and skeletons of marine animals.

Some of these ancient animals left fossil imprints of their body in the stone.

Cephalopod

Crinoid

• As the North American landscape became drier (and warmer) during the Carboniferous period (355 - 300 Million years ago), land plants invaded the soggy soil.

Huge swamps were dominated by giant sphenopsids (horsetails or scouring rushes such as these calamites), tree ferns, and leafy conifers.

Remains of these plants were buried in sediment, eventually forming deposits of coal in Iowa, Missouri, Kansas.

• Amphibians such as frogs and toads first climbed on land during the Permian period (300 million years ago).

The evolution of the amniotic egg allowed later vertebrates to lay eggs away from water.

Amniotic vertebrates branched off into reptiles and synapsids.

Synapsids (such as this Dimetrodon and the modern Muskrat) possess one opening behind the eye socket, and later evolved into mammals.

• The outer membrane (chorion) of an amniotic egg allows gas exchange through the porous shell.
The amnion provides a watertight fluid-filled environment for the embryo.
The yolk sac supplies nutrients.
The allantois holds wastes secreted by the embryo.

• By the end of the Permian period 250 million years ago, the supercontinent Pangaea began to break up, eventually forming today's landscape, including the Great Plains bordered by the Rockies.

The end of the Permian also witnessed a mass extinction, where 95% of marine species and 75% of terrestrial vertebrates vanished.

The Permian was followed by the Mesozoic era - the age of reptiles.

• The Mesozoic era began 250 million years ago and comprises three periods.

The Triassic period saw the rise of dinosaurs.

The dinosaurs flourished through the Jurassic period, then became extinct by the end of the Cretaceous period.

• The extinction of dinosaurs was probably caused by the impact of a large meteorite at the end of the Cretaceous period, 65 million years ago.
The meteorite left an impact crater in the Caribbean Sea near Mexico's Yucatan Peninsula, and deposited a layer of the mineral iridium around the world.

• Starting 37 million years ago during the Tertiary period, global climate became cooler and drier.

The climate in the Great Plains of North America became even drier due to the rain shadow of the Rockies.

The dry climate was inhospitable to trees, but allowed grasses to thrive.

• The Ice Age began about 2.5 million years ago, during the Quaternary period.

Large ice sheets (glaciers) advanced down from the Arctic in episodes called glaciations, followed by warmer climate called interglacials when glaciers retreated.

The last glaciation (called Wisconsinan) began about 100,000 years ago.

Two vast ice sheets extended over much of the North American continent, with an ice-free corridor in between.

With much water locked in ice, sea levels dropped, exposing a land bridge across the Bering Strait between Siberia and Alaska.

• Advancing glaciers can push granite boulders ("erratics") from Canada down to where the ice stopped.

Other glacial features include

• moraine: eroded rock piled up by an advancing ice sheet.

• kettle (pothole): a chunk of ice sitting in a depression eventually melts, leaving behind its load of gravel.

• kame: meltwater pouring through a hole in the ice left a mound of gravel and sand.

• esker: a stream flowing through the ice left behind ridges of gravel and silt.

• loess: windblown eroded rock dust that can accumulate in thick deposits of silt.

• At its maximum about 20,000 years ago, the Wisconsinan glaciers covered much of northern Illinois.

When the glaciers finally melted 10,000 years ago due to warmer and drier climate, the surrounding vegetation of tundra and conifers retreated as well, replaced by grasses and forbs such as Prairie Smoke (Geum triflorum) .

The new landscape supported grazers such as mammoths and large predators such as saber-toothed cat, as well as migrants from Asia.

• The Bering Strait land bridge had allowed large mammals to cross over from Asia into North America.

These included several species of steppe bison, ancestors to the plains bison.

Humans may also have migrated over the land bridge from Asia, populating the continent.