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.

When the Badlands weren't bad

Early visitors to the southwest corner of South Dakota gave the name badlands to the area, as the arid, desolate, deeply eroded landscape could not support agriculture or even grazing livestock.

But 40 million years ago Badlands National Park was a very different place, where lush vegetation supported an abundance of grazing animals.

The Badlands are famous for its many fossil mammals, including Ancestral horses and camels, sheep-like oredonts and rhinoceros-like brontotheres.

About 5 million years ago the picture started to change, as tectonic forces caused uplift and facilitated erosion of the soft sedimentary rocks, creating the spectacular stark scenery which the park is known for today.

For more information, click here.

Photo is from Wikipedia Commons.

Devils Tower (a.k.a. Bear's Lodge)

Rising out of the plains of northeastern Wyoming, Devils Tower looms 867’ over low rolling hills of shale and sandstone.

The tower itself is composed of much harder igneous rock. Large crystals in the rock indicate slow cooling of magma, and give us clues to the Tower’s origin deep beneath the present-day land surface.

The sandstones and shales that surround Devils Tower were deposited in a shallow sea that covered western North America 225 million years ago. Long after the seas drained away and the sand and mud hardened into sandstone and shale tectonic forces drove magma upward.

Millenia of erosion stripped away the surrounding sandstone and shale, gradually revealing what was designated in 1906 as North America’s first national monument.

More information here. Photo by the author.

Mt. Rushmore & the Black Hills of South Dakota

As with Yosemite and Acadia, the geologic story of Mt. Rushmore comprises two chapters: the origin of the rock and how those rocks became exposed at the Earth’s surface.

About 1.6 billion years ago magma from the mantle made its way upwards through the crust and cooled deep below the surface forming the granite that is the core of the Black Hills.

The granite lay buried until about 70 million years ago when tectonic forces within the Earth caused the Black Hills region to rise.

Erosion stripped off less resistant sedimentary rocks, and as uplift continued the harder igneous rock was exposed at the surface. In an area of the Black Hills called the Needles, vertical joints in the granite resulted in the formation of thin granite spires (see photo).

And in a large block of unjointed granite, sculptor Gutzon Borglum carved his iconic tribute to 4 U.S. presidents.

More information here.

A tale of two parks

Like California’s Yosemite National Park, the predominant rock type in Acadia National Park in Maine is granite.

Acadia is geologically older; the granites of Acadia originated in the core of ancient mountains that formed from the collision of a micro-continent with eastern North America about 450 million years ago.

But both parks experienced glaciation in the last 2 million years. In both areas, glaciers polished the granite, carved U-shaped valleys, and deposited ridges of rock, sand, and gravel called moraines. In Yosemite, these Alpine glaciers originated at high elevations and flowed down pre-existing valleys. In Acadia continental ice sheets more than a mile thick originated in Canada and slowly flowed across the entire landscape. Acadia’s glaciers are long gone, but like Yosemite its landscape continues to evolve.

For more on Acadia, click here.

The other half of Yosemite's story

Although Half Dome formed deep within the Earth, near-surface processes are responsible for the Dome’s familiar profile.

The steep vertical face of Half Dome formed from vertical cracks, called joints,that developed in response to tectonic stresses in the Earth’s crust.

The spherical side is a result of exfoliation, a weathering phenomenon in which thin, concentric sheets of rock spall off the exposed surface.

Both phenomena are the result of the uplift of the granite pluton, erosion of the overlying rock layers and the accompanying release of pressure as this overburden was removed.

Ice-age glaciers flowed down the valley, clearing away the weathered debris, but the glaciers themselves did not carve out Half Dome, they just accentuated what had already been given shape by weathering.

Photo: spheroidal surface developed on granite at Yosemite

Half of the Yosemite story

For many visitors, the dramatic profile of Half Dome stands as the symbol of Yosemite National Park in northern California.

Half Dome is composed of granite, an igneous rock that forms deep within the Earth’s crust by intrusion of molten magma from below.

But how does a rock that is formed deep within the Earth’s crust come to tower over the modern landscape?

Long after the magma cooled to form large granite bodies, called plutons, tectonic forces pushed the area upwards. As the region was uplifted, erosion stripped away the less-resistant layers of rock overlying the granite plutons, eventually leaving the granite exposed to dominate the landscape.

Uplift and erosion are still active in the area today, and Yosemite’s landscape continues to evolve.

Photo of Half Dome is from here.

The more recent history of the Grand Canyon

The rocks exposed in the Grand Canyon record 2 billion years of Earth history, but the canyon itself is much younger.

Until recently most geologists estimated that the canyon formed 5 to 7 million years ago, based on radiometric dates on lava flows in the canyon.

Recently researchers from the University of New Mexico reported radiometric dates on cave deposits in the western part of the Canyon that indicated an age of 17 million years. These cave deposits are known to form near the water table. The water table dropped as the canyon carved its way downward, and caves formed early in the canyon’s history lie higher in the canyon’s walls than later-formed caves and have older deposits than caves lower in the canyon.

The new study should stimulate more research on piecing together the story of the canyon’s formation.

Source: Polyak, V., Hil, C., and Asmerom, Y., 2008. Age and evolution of the Grand Canyone revealed by U-Pb dating of water table-type speleothems. Science 319 no. 5868 p. 1377-1380. Free registration at the AAAS/Science website permits access to this article.

Or see this nice summary with great color photos (including the cave deposits shown above).

Grand Canyon: 2 billion years of Earth history, one step at a time

Nearly half of Earth’s history is exposed in the walls of the Grand Canyon in northern Arizona, USA.

Grand Canyon National Park is a natural geology textbook, and each layer of rock or stratum is a page of that history. To hike down the canyon is to walk back into time. The rocks of the upper canyon are relatively soft, flat-lying sedimentary rocks—limestone, sandstone, and shale deposited in ancient shallow seas, beaches, and swamps.

Erosion by the Colorado River and gravity-powered mass wasting have combined to create an upper gorge that is up to 18 miles across. At river level, 2 billion-year-old igneous and metamorphic rocks of the narrow inner gorge mark the remnants of ancient mountains.

Photo from here.

Yellowstone: Life in Hard Places

The geysers, mud pots, fumaroles and thermal springs that attract millions of visitors to Yellowstone National Park every year also attract a growing number of scientists.

The brilliant yellow, orange and red hues that tinge some of the hot springs are microbes that live in these boiling hot environments. Organisms that are adapted to live under extreme conditions of heat, cold, pressure, depth or chemical environment are called extremophiles.

Scientists are interested in studying extremophiles because these organisms live in conditions that are known to exist on other planets, so our search for understanding the life forms we might find on Mars can begin in our own backyard.

For more on extremophiles in Yellowstone and elsewhere, click here.

Photo of Old Faithful by the author.

Yellowstone

Yellowstone National Park, centered in the northwest corner of Wyoming, was America’s first national park, established in 1872, but the story of Yellowstone’s current landscape starts just over 2 million years ago with a series of terrific volcanic eruptions that triggered the collapse of the volcanoes and formation of calderas—large, basin-like depressions.

Yellowstone Lake occupies the youngest caldera that formed in the last big eruption about 600,000 years ago. The volcanic activity and geothermal features in Yellowstone are attributed to the presence of a hot spot, or upwelling of hot mantle material deep below the park, and as long as that subterranean heat source lasts, Yellowstone’s landscape will continue to evolve.

Photo: Yellowstone geyser. Photo by the author.

Geology of Vacation Destinations

It’s summer, and prime time for packing up and exploring North America’s favorite vacation destinations—the National Parks.

Called “the best idea we ever had”* the National parks can trace their roots to 1864 when President Abraham Lincoln deeded Yosemite Valley to the state of California. In 1872 President Ulysses S. Grant signed into law legislation that created Yellowstone Park, the first National Park. In 1906, the Antiquities Act extended protection for the country’s natural heritage to smaller features, like Devils Tower as national monuments.

The drive to preserve these areas for future generations came not just from their natural beauty but from the recognition that these areas have much to tell us about the history of the Earth.

This week: Stories from the rocks of the US National Parks and Monuments.

*By writer and historian Wallace Stegner

Darwin's pond, revisited

Darwin’s “warm little pond” scenario for the origin of life on earth may not have been hot enough to do the work of chemosynthesis, combining the elements of life—CO2, methane, and water—into more complex biological compounds, and some researchers suggest that hydrothermal vents like those found today in abyssal ocean depths are a more likely candidate for the environment in which life first evolved.

Water gushing from hydrothermal events tends to be alkaline from reaction with minerals in the earth’s crust. When an alkaline hydrothermal fluid (that is, a fluid with low Hydrogen ion levels) mixes with acidic seawater (which has a higher H ion concentration), the resulting pH gradient creates potential energy that can be used to power chemical reactions by the diffusion of hydrogen ions in a process called chemiosmosis.

The oldest and simplest forms of life on Earth may have used chemiosomosis as an energy source.

Source: Nicole Branan writing in Earth, May 2010, based on research by Wm Martin and others in BioEssays

Charles Darwin & the origin of life

Although best known for his ideas about the origin of species, Charles Darwin also thought about the origin of life on Earth.

In 1871 Darwin described his view of the environment in which life originated; he wrote that life may have appeared: “in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc.”.

Darwin wrote this 80 years before Stanley Miller and Harold Urey synthesized amino acids in their laboratory equivalent of Darwin’s “warm little pond”.

The Miller-Urey experiment is over 50 years old and scientists continue to revise their ideas about the conditions present in the early Earth, and the “warm little pond” scenario has been challenged by other hypotheses including one that life may have arisen under cold conditions. These scenarios will eventually sort themselves out as more research is conducted.

That’s the nature of science.

From amino acids to cells

The abiotic formation of amino acids, as demonstrated by the Miller-Urey experiment, is still a long way from manufacturing a living organism, and filling in the steps between non-living and living is an area of active research involving chemists, biologists, geologists, and physicists.

The discovery of structures called protobionts provides information on how one of the next steps took place: Protobionts are aggregates of organic molecules enclosed in a membrane. These structures can form spontaneously at Earth surface conditions, for example, through alternate wetting and drying conditions that might take place naturally on a tidal flat.

Protobionts provide an analog for the step between macromolecules and cells, and the discovery of protobionts suggest that it is only a matter of time before we discover the intermediate steps between the abiotic precursors and living things.

Helpful notes on the transition from abiotic to biotic, here.

Constructing life

Any recipe for life on Earth requires a source of essential elements, a source of energy to stimulate chemical reactions between these elements, and time for these reactions to take place.

In 1952 researchers at the University of Chicago took a mixture of ammonia, methane, and hydrogen to simulate the early Earth atmosphere and applied a spark to simulate a natural source of energy-lightning-and after 8 days found that amino acids—the building blocks of DNA-- had formed in this oxygen-free atmosphere.

In the half-century since this experiment scientists have modified their ideas about the composition of the early earth atmosphere, but the Miller-Urey experiment still stands as the first example of synthesizing organic compounds from inorganic precursors—a process that occurred in the Early Earth in some form, as our own existence is evidence that it happened.

For more on the Miller/Urey experiment, see this site.

What is life?

The oldest fossil record of life on earth is photosynthesizing cyanobacteria found in stromatolites. However, photosynthesis is a complex process, and these cyanobacteria probably had precursors that are yet unknown. What were these first hypothetical life forms like?

To answer that question we must first decide on a definition of what constitutes a living thing. This definition is not as clear-cut as it might appear.

Living things grow and reproduce, but inorganic crystals do this, too. Viruses are not regarded as living, as they require a host in order to survive and reproduce, but the existence of viruses highlights the fuzzy line between life and non-life, and reflects the fact that life must have originated from a non-living, “prebiotic” stage.

The British biologist J.B.S. Haldane concluded that “The line between living and dead matter is …somewhere between a cell and an atom.”

Photo and information on research on artificial life here.

First continents

Although we tend to think of continents as being permanent features on Earth, in fact the earliest Earth was devoid of continents.

Basaltic ocean crust formed first, as the early Earth cooled from its original molten state. Continental crust is formed through tectonic plate collisions, which involves the subduction of one plate, recycling the plate back into the Earth’s mantle, causing melting of lighter minerals and migration of this new, granitic magma upward where it solidifies, forming new continental crust.

Continental crust is less dense than ocean crust, and in a plate tectonic collision the lighter continental crust is not subducted but remains at the earth’s surface.

Continents grow by accretion of granitic crust through eons of plate collisions.

This process of continental growth and accretion continues today; much of the state of California is a mosaic of microplates that have been accreted to the west coast of North America.

Illustration from here.