Putting a name on time

-->When geologists talk about events that occurred 200 million years ago, they are referring to dates on the geological time scale.  Adding dates to the time scale was made possible with the discovery in the late 19^th century of the use of radioactive decay as a tool for determining the age of geologic materials.

The geological time scale is an evolving document, and radiometric dates are subject to refinement as technology advances.

But the geological time scale has always been an evolving document, even before dates were added. The names of the geologic eras, periods, and epochs on the geologic time scale we use today were not written in stone (so to speak!) but represent the latest version of a document that has its origins in principles enunciated almost 400 years ago.

Figuring out the order in which rock layers were deposited or emplaced was the first step to developing a time scale.

National Fossil Day

The record of life on Earth is written in stone--

National Fossil Day is a celebration organized by the National Park Service to promote public awareness and stewardship of fossils, as well as to foster a greater appreciation of their scientific and educational values.

Learn more...

Steno and stratigraphy

Nicholas Steno, born in 1638, was a renaissance era scholar, whose experience in comparative anatomy provided some of the earliest evidence recognizing fossils as the preserved remains of animals, which he famously demonstrated by comparing modern shark teeth to fossils shark teeth.

Steno is also credited with enunciating basic principles of geology that help determine the relative order of geological events that led to the development of a geological time scale.

Steno reasoned that sedimentary rock layers were formed when particles in a fluid such as water fell to the bottom, resulting in horizontal layers. Thus Steno's principle of original horizontality states that these rock layers form in the horizontal position.

Steno realized that tilted or folded rock layers, like the ones shown in the illustration here, meant that the layers were disturbed after they were deposited. It may seem obvious to us today, but this insight represented a huge step toward understanding 4.6 billion years of earth history.

Image and more information available here.

Telling time with sedimentary rock layers

Nicholas Steno is also credited with being the first to understand (and communicate to others) the most basic principle of ordering sedimentary strata, the principle of superposition, which states that in a sequence of undeformed rock strata, the oldest beds are on the bottom and the youngest are on the top. Simple, yes?

It is important to remember that this principle refers specifically to sedimentary rocks, deposited layer by layer as strata. Some metamorphic rocks may show banding that resembles layering (compare the layered gneiss show above to the sandstone), but these “layers” do not follow the principle of superposition—they reflect reorganization of grains due to stresses in the Earth’s crust associated with plate tectonics. It is sometimes difficult for aspiring geologists to tell the difference, especially in an isolated hand-sample. Familiarity with the three main types of rocks is the key to making this distinction.

The diagram, above (from here), shows how the process of superposition could happen in a lake setting.

Finding order among the rocks

When geologists refer to events that occurred 200 million years ago, they are referring to dates on the geological time scale that have been added since the discovery in the late 19^th century of the use of radioactive decay as a tool for determining the age of geologic materials.

The geological time scale is an evolving document, and radiometric dates are subject to refinement as technology advances.

But the geological time scale has always been an evolving document, even before dates were added. The names of the geologic eras, periods, and epochs on the geologic time scale we use today were not written in stone (so to speak!) but represent the latest version of a document that has its origins in principles enunciated almost 400 years ago.

Figuring out the order in which rock layers were deposited or emplaced was the first step to developing a time scale.

What is time?

How can geologists use rocks to create a chronology of Earth history?

To address this issue we need to understand the basic measure of time.

The orbit of the Earth around the Sun measures a year, the rotation of the Earth on its axis defines a day, and the vibration of the Cesium atom in an observatory near London marks seconds, but what do these different phenomena have in common?

A year is measured by the change in the position of the Earth in its orbit relative to the sun; a day is measured by change due to Earth’s rotation, and a second is measured by the change in position of a vibrating atom. Change is the fundamental measure of time.

Changes are recorded in rocks, for example, the layers of sedimentary rock record periods of deposition alternating with periods of non-deposition; metamorphic rocks record change from a previous non-metamorphosed state, igneous rocks record the change of cooling from a molten state to a crystalline state. So it’s valid to use rocks as geochronometers in sorting out the history of the Earth.

The "spiral of time" graphic (a classic!) can be found here, along with more information.

Stories pebbles tell

From their first discovery, fossils had been interpreted as stones that fell from the sky, tricks of the devil, or objects that grew in the rocks in which they are found.

Nicholas Steno’s work with modern shark teeth convinced him that the objects we recognize as fossil shark teeth looked like shark teeth because they were shark teeth, and that they must have been buried in mud or that was now dry land.

Steno also reasoned that if the fossil were a structure that had grown within solid rock, its shape would have been distorted by the enclosing rock. The pristine condition of the fossil shark teeth indicated instead that the tooth must have been buried in soft sediments which hardened later.

Steno’s realization that objects entombed within rock, like fossils or pebbles in a conglomerate were formed before the rock itself was deposited. This was another way to distinguish the relative order of events and is called the principle of included fragments.

Simply put, this principle states that the included fragment (be it a fossil in limestone or a rounded pebble in a conglomerate, shown above) is older than the rock that encloses it.

Image and more information available here.

Hawaii and the future

The submersed, volcanically inactive islands of the northwest part of the volcanic archipelago of volcanic islands of which the state of Hawai'i is a part, points to the future of the present Hawaiian islands.

As the Pacific Plate continues its cm-by-cm journey to the northwest, the Big Island of Hawai'i will move off the hot spot, its volcanic activity will cease, and erosion will eventually reduce the island to a submersed shadow of its former self.

This rather bleak view of Hawai'i’s future is tempered with the knowledge that even as erosion claims the older islands, and eventually the big island, even now the next island in the chain is growing on the seafloor off the southeast coast of Hawaii. Loihi is the name already given to this next piece of real estate in the Hawaiian chain.

Image of Loihi from here.

Hawaii and hot spots

The Hawai'i archipelago is the product of the interplay of plate tectonics—the shifting of the Earth’s outer, rigid crust—with a plume of molten material welling up from the Earth’s mantle, a hotspot.

The Hawaiian islands formed sequentially as the Pacific plate moved to the northwest over this hotspot. The upwelling magma erupted as basaltic flows on the ocean floor that eventually built up thousands of feet to breach the surface and form an island.

As the Pacific plate continued its movement to the northwest, the new island moved off the hot spot, and volcanic activity ceased. Once volcanic activity ceased, no new rock was added to the island, and surface processes, the action of wind and waves, became the dominant processes in shaping the island, and the quieted volcanic island began to succumb to erosion, eventually disappearing beneath the waves.

Photo: the southern coast of the Big Island takes a pounding from the waves.

More Aloha

The islands that comprise the state of Hawai’i are part of an extensive archipelago of volcanic islands—most of them well below sea level and most of them dormant---that stretches from the Big Island of Hawaii north and west to the Aleution Islands, another volcanic archipelago that stretches westward from Alaska to Asia.

Radiometric dates of basalts show that Hawaii’s islands grow older to the northwest; the big Island, the southernmost island in the chain, is the youngest (and indeed is still growing through active volcanic flows).

The Big Island is also the largest of the islands, thus it’s name, and the older islands are smaller.

The Big Island is the only currently volcanically active island; the older islands are not.

These three different lines of observation led geologists to formulate the hot spot theory.

Illustration from here.

Hawai’i Volcanoes National Park

Most national parks and monuments preserve the story of the park’s geological past—past climates, past tectonic history. At Hawai’i Volcanoes National Park on the Big Island of Hawai’i, Hawai’i’s past, present, and future is written in basalt—the cooled basalt that comprises the island and the still-molten basalt flows that snake their way to the ocean on the islands’ south-east shore.

The Big Island of Hawai'i is the southernmost island in the archipelago that comprises the islands in the state of Hawaii but that extends northwestward as a submersed chain of seamounts to the Aleutian archipelago of Alaska. The Big Island of Hawaii is the largest island and the only one in the chain still volcanically active.

See Google maps image to trace the Hawaiian island chain.

The US Geological Survey maintains the best site for Hawaiian volcanic activity.

Photo: night viewing of active basalt flows, Kilauea volcano on the Big Island.

Hawai'i on a budget

If you can’t make it to Hawaii to see the effects of basaltic volcanism you might plan a visit to Craters of the Moon National Monument in Central Idaho.

The barren landscape that gives the name to the area is the result of volcanic activity 15,000-2000 years ago, and was shaped by forces similar to those that shaped the Hawaiian islands.

Craters of the Moon is a part of what is called the Great Rift volcanic zone, a 50-mile-long corridor encompassing an area roughly the size of the state of Rhode Island. The area includes 60 lava flows and 25 volcanic cones.

A rift zone is an area where the Earth’s crust is being pulled apart, allowing magma to well up from below. The cause of the rifting is still a matter of study, but, as with the basalt eruptions of Hawaii, the cause may be a hot spot in the mantle beneath the North American continent.

P.S. In 1969 Apollo 14 astronauts Alan Shepard, Edgar Mitchell, Joe Engle and Eugene Cernan visited Craters of the Moon. They explored the lava landscape in order to learn the basics of volcanic geology in preparation for future trips to the moon.

P.P.S. “The Devil's Vomit" is how one pioneer described Craters of the Moon. In the 1850's and 1860's hundreds of pioneers traveled through the area along the Oregon trail.

Photo from the National Parks Service

Oregon's trail through time

In North-Central Oregon, the John Day Fossil Beds record a succession of ecosystems over 57 million years from lush tropical jungle with crocodiles and palm trees to cooler and drier forests and grassland inhabited by horses and camels.

The dramatically shifting landscape evolved as tectonic plate collision along the nearby North American plate boundary gave rise to volcanic mountains along the west coast.

Some of the sedimentary layers at John Day are the deposits of massive mudflows generated by volcanic eruptions that engulfed entire forests. Other beds resemble those at Florissant in Wyoming—fossil lake deposits preserving fish, leaves, and insects.

Like Idaho’s Hagerman fossil beds, the youngest John Day beds preserve a pre-ice-age fauna, and the painted hills of the John Day beds recall the deeply weathered deposits of South Dakota’s Badlands.

Learn more about the John Day fossil beds here.

Illustration of the stratigraphic sequence is from here.

http://www.nps.gov/joda/naturescience/geologicformations.htm

Colorado’s fossil redwood forest

34 million years ago, in central Colorado, volcanic eruptions sent mudflows downslope, burying a forest of giant redwoods, damming a nearby river, and forming a large lake.

Subsequent eruptions deposited volcanic ash that buried insects and plants washed into the lake. The fine-grained ash preserved the smallest detail, including the antennae, legs, and sensory hairs of insects and the petals of flowers. This story is preserved in the rocks of Florissant Fossil Beds National Monument.

Florissant is famous for its abundance of fossil leaves, large tree stumps, and insects. More than 1,500 different kinds of fossil insects are found here, including dragonflies, cockroaches, grasshoppers, flies, beetles, wasps, ants and butterflies. The largest fossil tree preserved at Florissant is 13 feet in diameter and estimated to have been 300 feet tall and 1,000 years old.

A fossil photo gallery can be found here.

The image, above is from here.

Wyoming's fossil lake

50 million years ago, during the Eocene Epoch of the Cenozoic Era, a sub-tropical lake covered a portion of what is now southwestern Wyoming.

Fossil Butte National Monument preserves in its finely layered sedimentary rocks a window into the aquatic and nearby terrestrial ecosystems of this time. The deposits are famous for the diversity and abundance of its fossil fish, and other fossils include reptiles, birds, insects, plants, and the oldest known fossil bat.

Preservation of these fossils is exquisite, suggesting that conditions at the bottom of the lake were toxic, and excluded the scavengers and decomposers that otherwise would have destroyed these remains.

Workers building the Union Pacific railroad in the late 1860’s discovered the fossil fish beds near the town of Green River, Wyoming, and geologists refer to these fossil-bearing strata as the Green River Formation.

More information from the National Parks Service, here.

http://www.nps.gov/fobu/naturescience/naturalfeaturesandecosystems.htm

Dinosaur National Monument

In 1909 Earl Douglass, a vertebrate paleontologist from the Carnegie Museum of Pittsburgh, Pennsylvania, mounted an expedition to northeastern Utah to prospect for dinosaurs for his Museum.

Douglass chose his prospecting site carefully, knowing that dinosaurs were likely to be found in the preserved sediments of ancient river systems. The expedition was a resounding success, and in 1915 President Woodrow Wilson proclaimed the Douglass quarry Dinosaur National Monument.

The Monument preserves the remains of more than 1500 fossil bones in place in the sandstone in which they were preserved. Thanks to tectonics, the originally flat-lying sedimentary rock layers were tilted upwards so that the fossil bone bed forms a wall of the visitor’s center, a kind of paleontological bas relief.

More information from the National Parks Service here.

White Sands

To most people the word sand means the stuff that beaches and deserts are covered with and to many of us that means sand-sized grains of the mineral quartz, the most resistant mineral at the Earth’s surface.

Quartz is insoluble and slow to weather, and so it is the stuff of many deserts and beaches. However, in a desert where the little water that falls every year creates temporary lakes, as those lakes evaporate, the minerals dissolved in the water precipitate, forming crystals of evaporite minerals like salt and gypsum.

As the winds whip across these dried up lakebeds, the crystals are swept up and piled into great glistening dunes of white gypsum sand. This is White Sands National Monument in New Mexico.

Photo from here.

More info on the park here.

Carlsbad Caverns

The desert that surrounds Carlsbad Caverns National Park in southeastern New Mexico is not an environment conducive to cave formation and cannot account for the over 100 caves dissolved from the areas’ limestone bedrock; most of the cave formation in this area took place during the last ice age when the climate here was wetter and pine forests covered the landscape above the cave.

Unlike Mammoth Cave, which was formed by slightly acidic groundwater percolating down from the surface dissolving and enlarging fractures, at Carlsbad Caverns water enriched with hydrogen-sulfide from oil and gas deposits buried in sedimentary layers beneath the limestone migrated up from below and mixed with rainwater to form sulfuric acid, dissolving the rock and but leaving behind delicate cave deposits made of gypsum.

More information on Carlsbad from the National Parks Service.

http://www.nps.gov/cave/naturescience/cave.htm

Beneath the rolling hills of south-central Kentucky, subterranean chambers coalesce over 400 square miles to form the largest cave system in the world, the aptly named Mammoth Cave.

The vastness of Mammoth Cave belies its formation by literally one drop of water at a time.

Groundwater percolating through the fractured limestone bedrock dissolves the rock a molecule at a time, slowly enlarging the fractures.

Once a cave is formed, as mineral-laden groundwater drips down from the roof of the cave, crystals of calcium carbonate precipitate out and adhere to the cave roof—the beginning of a stalactite, icicle-like formations that hold tight to the ceiling. On the cave floor beneath the dripping water, calcium carbonate crystals may accumulate in a stalagmite; remember it just might reach the cave roof.

More info on Mammoth Cave National park here.

Photo from here.

Petrified Forest

Standing in the blazing sun of the eastern Arizona desert, you would not suspect that 200 million years ago towering sequoias nearly 200 feet tall and 9 feet in diameter grew here in lush forests among broad rivers.

The evidence of this vanished ecosystem, of course, lies scattered on the ground—the fossilized trunks of these trees that give the name to Petrified Forest National Park.

Over the millennia, climates changed, shallow seas covered the region, depositing marine sediments and burying the fossil forest deeper until about 60 million years ago when tectonic forces pushed upward the entire area we call the Colorado Plateau. Erosion followed, stripping off the younger sedimentary layers and eventually bringing the fossil forest to once more to light.

More information here.

http://www.nps.gov/pefo/naturescience/fossils.htm

P.S., In addition to the plants, this Late Triassic ecosystem supported insects, amphibians, and reptiles, including crocodile-like phytosaurs and Coelophysis, an early dinosaur.