Geologic Time - A Visualization and Essay

Geologists have divided the whole of geologic history into units of varying length.  Together, they compose the geologic time scale of Earth history.  The major units of the time scale were delineated during the nineteenth century, principally by scientists in Western Europe and Great Britain.  Because radiometric dating was unavailable at that time, the entire time scale was created using methods of relative dating.  It was only in the twentieth century that radiometric methods permitted numerical dates to be added.

Here is an interactive diagram of the scale of geologic time.  Click on the right columns to zoom in, the left column to zoom out.  Data from the Geological Society of America

Structure of the Time Scale

The geologic time scale subdivides the 4.6 billion year history of Earth into many different units and provides a meaningful time frame within which the events of the geologic past are arranged.   Eons represent the greatest expanses of time.  The eon that began about 542 millions years ago is the Phanerozoic, a term derived from Greek words meaning "visible life."  It is an appropriate description because the rocks and deposits  of the Phanerozoic eon contain abundant fossils that document major evolutionary trends.

Another glance at the time scale reveals that eons are divided into eras.  The Phanerozoic eon consists of the Paleozoic era (paleo = ancient, zoe = life), the Mesozoic era (meso = middle), and the Cenozoic era (ceno = recent).  As the names imply, these eras are bounded by profound worldwide changes in life forms.

Each era of the Phanerozoic eon is further divided into time units known as periods.  The Paleozoic has seven, and teh Mesozoic and Cenozoic each have three.  Each of these periods is charactered by a somewhat less profound change in life-forms as compared with the eras.

Each of the periods is divided into still smaller units called epochs.  Seven epoch have been names for the periods of the Cenozoic.  The epochs of other periods usually are simply termed early, middle, and late.

Precambrian Time

Notice that the detail of the geologic time scale does not begin until about 542 million years ago, the date for the beginning of the Cambrian period.  The nearly 4 billion years prior to the Cambrian are divided into two eons, the Archean (archaios = ancient) and the Proterzoic (proteros = before, zoe = life).  It is also common for this vast expanse of time to simply be referred to as the Precambrian.  Although it represents about 88% of Earth history, the Precambrian is not divided into nearly as many smaller time units as the Phanerozoic.

Why is the huge expanse of Precambrian time not divided into eras, periods, and epochs?  The reason is that Precambrian history is not known in great enough detail.  The quantity of information that geologists have deciphered about Earth's past is somewhat analogous to the detail of human history.  The further back we go, the less that is known.  Certainly more data and information exist about the past 10 years than for the first decade of the twentieth century; the events of the nineteenth century have been documented much better than the events of the first century AD; and so on.  So it is with Earth history.   The more recent past has the freshest, least disturbed, and most observable record.  The further back in time. a geologist goes, the more fragmented the record and clues become.  There are other reasons to explain our lack of a detailed time scale for this vast segment of Earth history.

1. The first abundant fossil evidence does not appear in the geologist record until the beginning of the Cambrian period.  Prior to the Cambrian, simple lifeforms such as algae, bacteria, and worms predominated. All of these organisms lack hard parts, an important condition favoring preservation.  For this reason, there is only a meager Precambrian fossil record.  Many exposes of Precambrian rocks have been studied in some detail, but correlation is often difficult when fossils are lacking.

2. Because Precambrian rocks are very old, most have been subjected to a great many changes.  Much of the Precambrian rock record is composed of highly distorted metamorphic rocks.  This makes the interpretation of past environments difficult because many of the clues present in the original sedimentary rocks have been destroyed.

Radiometric dating has provided a partial solution to the troublesome task of dating and correlating Precambrian rocks.  But untangling the complex Precambrian record still remains a daunting task.

Terminology and the Geologic Time Scale

There are some terms that are associated with the geologic time sale but are not "officially" recognized as being part of it.  The best known, and most common, example is "Precambrian" -- the informal name for the eons that came before the current Phanerozoic eon.  Although the term Precambrian has no formal status on teh geologic time scale, it has been traditionally used as though it does.

Hadean is another informal term that is found on some versions of the geologic time scale and is used by many geologists.  It refers to the earliest interval (eon) of Earth history -- before the oldest-known rocks.  When the term was coined in 1972, the age of Earth's oldest rocks was about 3.8 billion years.  Today that number stands at slightly greater than 4 billion, and, of course, is subject to revision.  The name Hadean derives from Hades, Greek for underworld -- a reference to the "hellish" conditions that prevailed on Earth early in its history.

Effective communication in the geosciences requires that the geologic time scale consist of standardized divisions and dates.  So, who determines which names and dates on the geologic time scale are "official"?  The organization that is largely responsible for maintaining and updating this important document is the Interanl Comittee on Straigraphy (ICS), a committee of the International Union of Geological Sciences.  Advances in teh geosciences require tha tthe scale be periodically updated to included changes in unit names and boundary age estimates.

For exmaple, the geologic time scale shown in 

AGU's Collection of Virtual Field Trips

Spring in the northern hemisphere is the time for students pile into vans with camping gear, rock hammers, pocket transits and field notebooks, and head out to explore the marvels of the Earth’s surface. It is a wonderful part of learning Earth science. But those trips are off as another kind of science — epidemiology — takes precedence. Luckily for students today, the COVID-19 pandemic doesn’t mean they have to lose every aspect of field trips. Virtual field trips like those highlighted below are a way for students to experience geology, and at very least prime them for the real thing, when we’re all on the other side of the current societal crisis.

Check out all of the HERE

OSU Needs Massive Campus Earthquake Retrofit - Daily Barometer

We always used to joke that the CEOAS building would ironically be the first to pancake.

Great quotes from Dr. Goldfinger in here.

Oregon State University Professor of Geology and Geophysics, Chris Goldfinger, who has studied the Cascadia Subduction Zone since the 1980s, and is one of the foremost scientists on the matter, said he believes that OSU’s infrastructure is not prepared to handle such an event.

“A large number of buildings, many of them on campus, were built long before the knowledge of Cascadia, or plate tectonics for that matter, existed.  There has been little effort to retrofit them, except where major renovation required it.  This means that the town and the campus is packed with collapse-hazard buildings that will likely fail in even a modest earthquake,” Goldfinger said.

Read the article HERE

Ocean Floor off Palos Verdes

Hawaii Adventures

Interview with Fossil Hunter Michelle Barboza

The University of Florida’s Florida Museum of Natural History celebrated 100 years of inspiring people to care about life on Earth in 2017. To mark the closing of an era and the beginning of a new century, UF News profiled three Florida Museum women who are shaping the research institution’s future and breaking the cycle of stereotypes and misconceptions in the world of science. With modern tools like social media and podcasts, they continue the work of past and current museum women, who have fought for equality in their fields and for the visibility of women in science.

In this Becoming Visible video series and site, we meet Fossil Hunter Michelle Barboza, above, a Los Angeles native and graduate student who ‘accidentally’ became a scientist because of her love for the outdoors. Her podcast Femmes of STEM celebrates women who are scientists in the past with women who are scientists in the present.

DIY Squeeze Box - Observe Mountain Building - Eric Muller

Build. Observe. Play in the sand and dirt. Making your own easy-to-build squeeze box is fun for lots of different reasons, including educational ones. This Science Snacks video with Eric Muller from The Exploratorium provides instructions for building the box and experimenting with faults and folds:

Your Squeeze Box replicates geologic structures found in areas that have undergone or are undergoing compressional forces, such as regions near convergent plate boundaries…

The deformation you see in your Squeeze Box is an excellent model of what happened or is currently happening around the world due to tectonic forces. Mountain building (geologists say orogenesis) is happening in the Himalayas due to the collision of two massive continents. The Alps, Atlas, Appalachian and Rocky Mountains are all the result of compressional forces at work, uplifting mountain high into the sky. The west coast of North America, as well as all around the Pacific (the Ring of Fire), shows how compressional tectonics can plow up the ocean floor and smash land onto the edge of continents (this process is called accretion).

Study what kind of faults and folds you find, and level up the science by taking a core sample of your compressed layers with a clear straw. Learn more at Exploratorium.edu.

My Mom's Favorite Geologic Feature - Transgressive Regressive Cycles

My mom was also a geologist - I have a lot of memories as a kid of her pointing out different features in the land around us, though I didn't appreciate it very much when we would stop somewhere on vacation to pick up rocks.  How times have changed!

What are Transgressive Regressive Cycles?

When a river delivers its load of sediment to the shoreline, wave energy acting on the shoreline winnows out the fine grained mud, leaving the larger grains of sand on and near the beach. Offshore in deeper water, beyond the reach of waves, mud settles on the sea-floor. A lateral change occurs in sediment type and environment, from shoreline sand to offshore mud. Such lateral change in sediment and environment is also observed as vertical changes in ancient strata. The vertical alternation of sandstone and mudstone results from transgression and regression of a shoreline. During shoreline transgression (Figure 1, Time 1 to 4), the shoreline moves towards the land and the sedimentary environments “follow” the shoreline. As transgression continues, perhaps over a distance of tens of kilometers, offshore mud is deposited on top of sandy shoreline sediments. If a sediment core was collected from the deposited sediment, there would be a vertical change from sand at the bottom of the core into mud at the top of the core. Transgression of the shoreline occurs during sea-level rise.

Figure 1. Illustration showing how transgression and regression of the shoreline deposits a transgressive-regressive sedimentary cycle. Such cycles may extend horizontally for several tens of kilometers, and range from a few meters to a few hundred meters in thickness. 

During shoreline regression (Figure 1, Time 5 to 8), the shoreline moves towards the ocean and the sedimentary environments “follow” the shoreline in a seaward direction. Shoreline sand is deposited on top of offshore mud. If a sediment core was collected of these sediments, there would be a vertical change from mud at the bottom of the core into sand at the top of the core. Regression of the shoreline usually occurs during sea-level fall. However, if there is a very high rate of sediment supply to the shoreline, such as occurs at a delta, regression may also occur during sea-level rise.
The sediment deposited during a complete transgression and regression is referred to as a transgressive-regressive cycle. Each transgressive-regressive cycle at Point Upright commences with a thin sandy interval overlain by dark grey mudstone. This is the transgressive “fining-upward” part of the cycle. The mudstone in the middle of the cycle then grades upward into sandstone and this is the regressive “coarsening-upward” part of the cycle.

Shoreline transgression and regression occurs in response to rising and falling sea level, respectively. An important mechanism of sea level change is the alternate melting and freezing of polar continental ice. Water added to the ocean derived from melting ice causes sea-level rise and shoreline transgression. On the other hand, when water is removed from the ocean as polar ice-caps grow, sea-level falls and shoreline regression occurs. The alternate melting and freezing of continental ice is caused by climate variation. The second main mechanism of sea-level change is uplift and subsidence of the sedimentary basin floor arising from plate tectonic movements. Uplift of the basin floor results in shoreline regression, while subsidence results in transgression.

Transgressive-regressive cycles can play an important role in the concentration of Earth resources such as oil, gas, and groundwater. These resources are found in the pore spaces between grains of permeable sandstone. The impermeable mudstone units that surround such sandstones act to trap these resources. Thus oil and gas traps are made up permeable sandstone (termed reservoir rock) and impermeable mudstone (seal rock). In the case of groundwater, the water-bearing sandstone is referred to as an aquifer while the surrounding impermeable mudstones are called aquicludes. In recent years, porous sandstones sealed by mudstone have been investigated as places where human-produced carbon dioxide can be injected and stored.

  

  

What Counts as a Mountain? - Tom Scott

Mountains: super tall, rocky landforms that rise high above the surrounding environment, much higher than hills. We might draw one as a single peak like Mount Fuji, or as a zig zag line of peaks representing a mountain range. Mount Everest (also known as Sagarmāthā and Chomolungma) is Earth’s highest peak above sea level, but it’s also surrounded by tall mountains in the Mahālangūr range. Do those count as some of the tallest on Earth, too?

Tom Scott asks “what counts as a mountain” on the 4,350 meter (14,271 foot) peak of Mount Evans in Colorado, North America’s 41st highest. Then he explains the answer… mostly.

Read more about topographical prominence and edge cases.

Marie Tharp: Uncovering the Secrets of the Ocean Floor

In the early 20th century, Alfred Wegener proposed a revolutionary idea: that the Earth’s continents were once joined together, and had gradually moved apart. The idea contradicted almost everything scientists thought at the time, and it took the detailed work of a brilliant cartographer to prove him right. 

Conventional ideas held that the ocean floors were flat, featureless planes. As expeditions started to go around the world collecting ocean depth measurements, Marie Tharp – not allowed to join the expeditions herself – processed the data and began to craft detailed, revealing maps of the hidden ocean depths.

She discovered that the ocean floor was in fact a complex assortment of peaks and troughs. In particular, her profiles revealed stark rift valleys, which supported Wegener’s controversial ideas. Even then, it took a long time to convince the scientific community that her findings were correct. Eventually, however, she was proved right, and Marie Tharp took her rightful place as one of history’s finest cartographers.

From The Royal Institution, narrated by physicist and oceanographer Helen Czerski, a revealing animation about American geologist and oceanographic cartographer Marie Tharp: Uncovering the Secrets of the Ocean Floor.

Zeolite Sample

Embroidered Geologic Map of California

Pangea Pop Up from TED-ed

Oregon's Lost Lake

Every spring in Oregon’s Willamette National Forest, Lost Lake begins to drain down naturally occurring lava tubes. A six foot wide lava tube hole was featured in this 2015 video from The Bulletin, which notes, “The water is most likely seeping into the subsurface below and refilling the massive aquifer that feeds springs on both sides of the Cascades.” MentalFloss explains a bit more:

Lava tubes are a common feature of Oregon’s geology. They form after a volcanic eruption, when flowing lava cools and hardens near the surface while hotter lava continues to flow down below, carving a path as it goes. Occasionally, one of these tunnels will even break through to the surface, as in the case of Lost Lake.

Western Oregon’s rainy months—beginning in the fall—yield such a massive amount of precipitation that the basin fills in at a faster rate than the tube can drain it, and the lake reappears. It freezes over in the winter months, followed by a spring thaw that leads into summer when dry weather results in a (sometimes muddy) meadow. Then the cycle begins again.

Jude McHugh, a spokeswoman for the Willamette National Forest, says it’s hard to predict when the annual drain will occur. While the video makes it seem like a single, continuous flow akin to water in a bathtub, it’s actually a much more gradual process with ebbs and flows that vary from year to year.

Here’s another Lost Lake drain video from September 2010: