Earth's first crust

As Earth cooled from its molten origin, the earliest crust probably consisted of iron and magnesium-rich minerals, minerals that can crystallize at high temperatures. This assemblage of minerals forms the igneous rock basalt.

Once the Earth had cooled sufficiently for this initial crust to form, shifting plates of basaltic crust collided, one plate sinking back into the Earth’s mantle, causing the less dense minerals in the upper mantle such as quartz and feldspar, to melt, and forming a lighter, less-dense magma that erupted at the Earth’s surface in a chain of volcanoes, or island arc.

This is the same sort of tectonic activity that formed the Aleutian Islands and the islands of Japan, so we can picture the first landforms on Earth as chains of volcanic islands.

Continents would form later, as tectonic plates carrying these island arcs collided.

Illustration of a very young Earth from here.

More on early Earth from National Geographic.

Dating the Earth

How did geologists determine that the Earth is 4 and a half billion years old when the oldest known rocks on Earth are only 3.8 billion years old?

Radiometric dating of meteorites--"free" samples of space rock--is one line of evidence for the age of the Earth and our solar system. Another line of evidence for the age of the Earth comes from closer to home-individual crystals of the mineral zircon found in ancient metamorphic rocks from Australia.

Zircon is a mineral that contains small amounts of radioactive uranium and thorium, so it can be radiometrically dated, and is formed by igneous processes that created the granitic crust of the continents. The Australia zircons give a radiometric date of about 4.2 billion years, indicating that the Earth’s granitic crust had developed by that time, very early in Earth's 4.6 billion-year history.

Illustration and more information about the Australian zircons available here.

Catching rays on earliest Earth

The atmosphere of the early Earth was probably lacking in oxygen, as oxygen atoms were likely bonded with hydrogen in water vapor rather than what is called “free oxygen” or 02, so the first lifeforms to appear were probably anaerobes, organisms that can live in the absence of oxygen.

The absence of free oxygen in the early Earth atmosphere also meant that there was no ozone—ozone is a molecule that is formed by linking three oxygen atoms (03), and it is an effective sun block, absorbing ultraviolet radiation from the sun.

In the absence of oxygen and an ozone layer, the first lifeforms must have been extremophiles, living under conditions inhospitable to life. But the fossil record suggests that life took a foothold on Earth just as soon as conditions permitted.

(to be continued)

The illustration shows non-extremophiles protected by the ozone layer of the modern Earth. That site also explains the importance of the ozone layer.

There are many links to sites about the composition of the early Earth atmosphere. Start here.

Taking a breath on earliest Earth

The Earth’s atmosphere probably had its origin in the gasses emitted by volcanoes; Earth was still very hot from its molten, impact-ridden origin, and volcanoes were abundant.

Water vapor is the most abundant component in volcanic gasses today, and this was probably the case in the early Earth as well. Ammonia, methane, and carbon dioxide are also volcanic gasses that were probably important in early Earth atmosphere.

Earth is able to retain its atmosphere because it is sufficiently large to have a gravitational force powerful enough to hang on to it, and because Earth has a magnetic field that protects it from the solar wind that would otherwise strip away the atmosphere.

The effects of not being large enough to hang onto an atmosphere can be seen on the Moon and Mercury.

For more on early Earth's atmosphere, follow this link.

Illustration is from here.

Let there be Moon

Four and a half billion years ago, when the Earth was very young, it was hardly a place conducive to life. Earth was formed from the collision of planetismals, and continued to be bombarded by the debris in the early solar system for about the first billion years of its history.

Evidence of this period of intense meteorite impacts is not preserved on Earth, whose active internal tectonic engine has recycled the crust formed during this time, but is evident on our nearest celestial neighbor, the moon.

In fact, one of the impacts during this time may have formed the moon. The collision of Earth with a Mars-sized object helps to explain the similarity of moon rocks to some Earth rocks, the slightly younger age of Moon rocks, and a major impact of a large object with the Earth can account for the 23 degree tilt of the Earth on its axis.

For an overview of this and other hypotheses of lunar formation see this site.

The illustration is from here.

Let there be light

About 5 billion years ago—8 billion years after the origin of the Universe--a star in one arm of the Milky Way galaxy collapsed in a supernova, forming a cloud of hot, interstellar debris.

Under the influence of gravity, this material flattened into a rotating disk, and in this rotating disk, particles collided, forming planetismals. This process can be compared to the formation of dust bunnies under the bed, although the force that attracts the dust particles is electrostatic attraction rather than gravity-driven collisions.

Planetismals collided with each other, eventually, about four and half billion years ago, forming the planets of our solar system, including Earth.

For more information on this solar nebular hypothesis of the origin of the solar system see this link or this one.

Illustration is from this site.

At the beginning

To put the Earth’s history in context, our planet and our solar system formed about 4 and a half billion years ago, in a Universe that had its beginning at least 13 billion years ago.

In this beginning there was a tremendous expansion of all matter and energy in the Universe, in an event that astronomers call the Big Bang. From this event the first elementary particles such as quarks formed; after several hundred million years atoms, and elements such as hydrogen and helium formed, eventually, after a 100 million years, these elements condensed and formed the first stars.

Perhaps a billion years after the Big Bang galaxies began to form, including our own Milky Way Galaxy.

To be continued...

Links for more information on the Big Bang from the University of Michigan and NASA.

Big Bang timeline (illustration, above) from here.

Big numbers. Really. Big.

This week GeoLog will begin a trip through time, exploring the physical and biological changes on Earth through the more than four-and-a-half billion years of its history, as told through the rocks and fossils that are the physical evidence of these changes. But first, a few words about our frame of reference:

Geologists tend to use numbers like 4.6 billion without a second thought. In fact, the scope of what we call geologic time is ENORMOUS and extremely hard to comprehend, especially for those of us who spend our days counting seconds, minutes and hours.

After all, our own life spans are on the order of 10s to one hundred years; known civilization goes back only thousands of years, and there were no modern humans around millions of years ago.

Our ability to comprehend the vast eras of geologic time is severely challenged by the remoteness and scope of these events.

Links for some help in visualizing these large numbers:

This site measures millions and billions in pennies, and the image of one billion pennies (above) is from here.

Teachers can find an exercise for class use on big numbers here, and the geologic time scale here and here.

T. rex: Top predator for Road Kill King?

Despite its fearsome, pointed, serrated teeth and portrayal in the movies, some scientists suspect that T. rex scavenged carcasses instead of hunted down prey.

This conclusion comes from studies of modern predator/prey ecosystems, estimating the number of prey needed to support an animal of T. rex size. One group of researchers estimate that in Africa’s Serengeti grasslands, enough herbivores die daily to feed a 6,000 kg T. rex, if the dinosaur was cold-blooded, spent half a day foraging and had senses to detect carrion up to 80 meters away.

But what if T. rex were warm-blooded and had to maintain a higher metabolism? It would need more food, but it would also be able to more faster and cover more ground.

Original work: Graeme D. Ruxton and David C. Houston, 2003. could Tyrannosaurus rex have been a scavenger rather than a predator? An energetics approach. Proceedings of the Royal Society of London. B. Download their article here.

Was Sue sick?

Sue, the largest, most complete Tyrannosaurus rex found to date may have been laid low by microscopic parasites.

Paleontologists have puzzled over smooth, round holes in Sue’s jaws. Originally thought to be bite marks from another T. rex, a new analysis concludes that the holes were the result of a parasitic infection that is known to affect modern avian raptors like hawks and eagles.

The infection probably caused lesions and swelling in Sue’s mouth and throat, prompting one researcher to speculate that the infection may have killed Sue as she found it increasingly hard to swallow as the infection spread, and she may have starved to death.

It is difficult to determine the cause of death for an animal that died millions of years ago, but this finding suggests that modern birds may owe their susceptibility to this parasite to their therapod ancestors.

Research from Ewan D.S. Wolff, et al., Common avian infection plagued the tyrant dinosaurs. PLoS One

Veggie rex

Tyrannosaurus rex belongs to a group of dinosaurs called theropods. These dinosaurs generally have serrated, knife-like teeth that are typical of meat-eating carnivores.

The discovery of a small theropod dinosaur in China, however, shows that this group had more diversified dietary habits. This new dinosaur, named Incisivosaurus, for its large front teeth, shows wear marks on its teeth that are typical of tooth wear seen in plant-eaters.

Incisivosaurus teeth also lack the serrations typical of carnivore teeth. The discovery of a plant-eating theropod indicates that these dinosaurs were more diverse that previously thought, and occupied a variety of ecological roles in ecosystems 60 million years ago.

Source: Xu, X., Cheng, Y.-N. Wang, X.-L., and Chang, C.-H. (2002). "An unusual oviraptorosaurian dinosaur from China." Nature, 419: 291-293. For more information, click on the title of today's post.

Dino Dads

Analysis of adult dinosaur bones found near nests of dinosaur eggs suggests that the male dinosaur parent was caring for the eggs.

Paleontologists from Montana State University examined the adult dinosaur bones found by these nests and discovered that they lacked a distinctive layer called medullar bone, a bone type that characterizes many species of female birds and that has been found in other dinosaurs, including Tyrannosaurus and Allosaurus.

If dino dads were in fact caring for their young, this could explain the origin of male parental care that we see in the descendents of these dinosaurs—birds. The large number of eggs found in these dinosaur nests--up to 30—also compares with the clutch size of modern birds in which the dads care for the young.

A reminder that Father's Day is coming up soon!

Summary in sciencenews.org Jan. 17, 2005 (Laura Sanders). Illustration of dino dad from here. Original research article: David J. Varricchio, et al., 2008. Avian paternal care had dinosaur origin. Science 322:1826-1828.

Birds of a feather

The discovery of feathered dinosaurs led to our understanding that birds are the descendents of dinosaurs, specifically the group of dinosaurs that includes the sauropods, like Apatosaurus and the theropods like Tyrannosaurus.

However, the discovery of feather-like structures in a small dinosaur from the other major dinosaur lineage—the group that includes Triceratops and Steogsaurus—complicates things.

Tianyulong is a small herbivorous dinosaur from China with feather-like structures along its spine and tail. Paleontologists are not yet sure whether these are feathers as seen in other feathered dinosaurs, or some other sort of body covering evolutionarily unrelated to true feathers.

Scientists refer to the structures on Tianyulong as “dinofuzz”, a fitting term because this new fossil fuzzes up our picture of dinosaur-bird relationships, at least for now.

Source: Xio-ting Zheng, et al., 2009, An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures. Nature 458:333-336.

Modeling dinosaur "flight"

An unusual feathered dinosaur discovered in China raises questions about the origin of flight.

Other small dinosaurs have been found with traces of feathers on the front two legs but Microraptor had feathers on the back legs, as well. Its asymmetrical feathers indicate that the animal was capable of gliding or flying, so it assumed that Microraptor made its home in trees and glided, or flew from tree to tree like modern flying squirrels.

To test these ideas, researchers at the University of Kansas constructed a model of Microraptor, using pheasant feather for its wings, and designed a slingshot to launch it. The Microraptor model glided 24 meters, or 26 yards.

Of course, a plastic model with pheasant feathers may not be an accurate model of an extinct feathered dinosaur, but it is a first step in attempting to understand how this unique animal lived.

Summarized in Earth, May, 2009, p. 13 (Emily Lant); research by David Alexander & David Burnham, University of Kansas. Click here for a video of the model's flight!

Original report on Microraptor: Xu, X., Zhou, Z., Wang, X., Kuang, X., Zhang, F. and Du, X. (2003). "Four-winged dinosaurs from China." Nature, 421(6921): 335-340, 23 Jan 2003. http://www.nature.com/nature/journal/v421/n6921/full/nature01342.html

Bulking up on veggies

Sauropod dinosaurs, like Diplodocus and Brachiosaurus, hold the record for the largest land animals, weighing up to 80 tons, and stretching more than 60 meters from head to tail.

To support their size, these herbivorous dinosaurs must have spent most of their time eating and searching for food.

They had a mouthful of incisors, good for clipping off plants, but not designed for chewing. They had no molars for pulverizing their food, so they must have swallowed their food whole.

The giant bulk of these dinosaurs is more puzzling when considering their relatively tiny heads, but their small heads were supported by long necks, which may have been critical to the sauropod’s success.

Their long necks allowed these dinosaurs to stand in one place and browse vegetation from a large radius, allowing them to collect a lot of food without expending much energy.

Based on research by P. Martin Sander and Marcus Clauss, 2008. Sauropod Gigantism. Science 322: 20-201

Illustration from http://www.scorcher.ru/journal/art/art_pic/diplodocus_2.jpg

Thwarting T. rex

How can an herbivore protect itself from carnivorous predators?

Strategies seen today among predators and prey of the African Serengeti apparently existed among dinosaurs in ancient ecosystems.

Paleontologists from Ohio University counted growth lines in the legbones of hadrosaurs, a group of herbivivorous duck-billed dinosaurs. By counting the number and spacing of growth rings, paleontologists can determing the animals’ age and its relative growth rate—the fast growth seen in juveniles is characterized by widely-spaced growth rings; growth slows or stops at adulthood, shown by close spacing of the growth rings.

The scientists found that hadrosaurs reached their adult size by age 13. In contrast, the carnivorous Albertosaurus reached full size at 20-30 years. Maturing quickly gave hadrosaurs an advantage over their predators, as they could produce offspring at an earlier age, and their offspring grew quickly to maturity.

Source: Drew Lee, Royal Society London B, Aug 5, 2008

Filling an empty niche

Despite the occupation by reptiles of almost every ecological niche during the Mesozoic Era, there were no large planktivorous marine reptiles, the niche filled today by baleen whales.

Recent discoveries in museum drawers may hold the answer to this gap in Mesozoic reptile ecology. Fossils that had lain unstudied or incorrectly identified have been newly identified as suspension-feeding pachycormids, a group of giant bony fish.

These fish were previously thought to have been a short-lived group, limited to the Jurassic Period. Mesozoic marine reptiles may have been excluded from the large-bodied, suspension-feeding trophic niche by these supersized fish.

The pachycormids were extinct by the end of the Cretaceous Period, opening up the planktivorous niche to a new group--the whales.

Matt Friedman, et al., 100-Million-year dynasty of giant planktivorous bony fishes in the Mesozoic Seas. Science 327

A new spin on an ancient predator

More than 100 years after its discovery the Middle Cambrian Burgess Shale of British Columbia Canada, continues to offer up new insight into the history of life.

One of the many enigmatic soft-bodied animals of the 500 million year old Burgess Shale of British Columbia is Nectocaris (fossil shown above) long thought to be a shrimp-like arthropod (reconstruction, above, left), but a recent study shows that the animal is most likely a cephalopod, ancestral to the group includes modern squid, octopus, and the pearly Nautilus (reconstruction above, right).

This re-classification of Nectocaris extends the geologic range of the cephalopods back 30 million years and dramatically changes hypotheses of cephalopod evolutionary history.

Nectocaris does not have an external shell, as did other ancient fossil cephalopods, and this discovery scuttles previous hypotheses that cephalopods evolved the ability to float and then swim after the evolution of their chambered shell. Nectocaris shows that cephalopods shells evolved later in cephalopod evolution, possibly in response to increased predation during the Late Cambrian.

Martin Smith and Jean-Bernard Caron, Nature 2010. Photos by the authors. Reconstructions from Discover Magazine blog

Burgess Shale redux

The Middle Cambrian Burgess Shale of British Columbia, Canada, is famous for the preservation of bizarre and distinctive animal fossils, like Anomalocaris, Hallucigenia, and Marella (shown at right).

Since its discovery over 100 years ago, other Burgess Shale faunas have been found in strata of similar age around the world, but the fauna appeared to have died out by the end of the Middle Cambrian.

The recent discovery of a Burgess Shale type fauna in Morocco from rocks millions years younger than the Burgess Shale breathes life into a fauna that was thought to be long extinct--including animals like Marella, above, left.

The apparent extinction of the Burgess Shale animals was probably a result of the rarity of the exceptional circumstances required to preserve soft-bodied organisms. The discovery opens the door to finding other, younger Burgess Shale type faunas around the world.

Source and photo credit: Peter Van Roy, et al., 2010, Ordovician faunas of Burgess Shale type. Nature 465:215-218.

Happy Anniversary, GeoLog!

365 days and 211 scripts ago, WPRR, public radio in Grand Rapids, Michigan, broadcast the first "GeoLog" program. GeoLog Blog reproduces the scripts along with a photo and links to the original source of the news item or other related website.

Mp3 files of GeoLog programs are now available on iTunes. Check them out at

http://www.publicrealityradio.org/programs/geolog

Tell your local radio station to pick up the program--each piece is only 1 minute long, short enough to run as a PSA (public service announcement).

Spread the word--Earth history rocks!

"Dark Earth"

Astronomers estimate that the sun was up to 30% dimmer early in its history (before its internal fusion engine was up to full power), and as a consequence put out less heat. Therefore, any water on the young Earth should have been frozen.

However, rocks formed during this period of Earth history, the Archean Eon, about 3.8-2.5 billion years ago, give evidence of deposition in liquid water. This discrepancy has been called the “faint young sun paradox”.

Now there is a new explanation for a warmer-than-expected early earth: Continents were smaller and more of the surface of the Earth was covered by ocean, giving the early Earth a darker, less-reflective surface, or low albedo.

Accordingly, the Earth’s surface absorbed more of the sun’s energy than it would have with a more reflective surface, enough to keep its oceans liquid, and keeping the planet hospitable for the development of early lifeforms.

Source: Rosing, Minik T.; Bird, Dennis K.; Sleep, Norman H.; Bjerrum, Christian J., 2010, No climate paradox under the faint early Sun. Nature 464: 744–747. Also reported in Science News April 24, 2010.

Photo credit and more info on the "faint young sun paradox": http://www.astrosociety.org/pubs/mercury/35_06/paradox.html