Earth: Our Dynamic Planet
"Terra firma." It's Latin for "solid Earth." Most of the time, at least from our perspective here on the ground, Earth seems to be just that: solid. Yet the Earth beneath our feet is actually in constant motion. It moves through time and space, of course, along with the other objects in the universe, but it moves internally as well. The powerful forces of wind, water and ice constantly erode its surface, redistributing Earth's mass in the process.
Within Earth's solid crust, faulting literally creates and then moves mountains. Hydrological changes, such as the pumping of groundwater for use by humans, cause the ground beneath us to undulate. Volcanic processes deform our planet and create new land. Landslides morph and scar the terrain. Entire continents can even rise up, rebounding from the weight of massive glaciers that blanketed the land thousands of years ago.
A Need for Indirect Measurements
Indeed, the outermost layers of the celestial blue onion that is Earth-its crust and upper mantle-aren't very solid at all. But what happens if we peel back the layers and examine what's going on deep within Earth, at its very core? Obviously, Earth's core is too deep for humans to observe directly. But scientists can use indirect methods to deduce what's going on down there.
A new study in the journal Geophysical Research Letters, by Jean Dickey of NASA's Jet Propulsion Laboratory, Pasadena, California and co-author Olivier deViron of the Institut de Physique du Globe de Paris, University Paris Diderot, Centre National de la Recherche Scientifique, Paris, has confirmed previous theoretical predictions that the churning cauldron of molten metals that make up Earth's liquid outer core is slowly being stirred by a very complex but predictable series of periodic oscillations. The findings give scientists unique insights into Earth's internal structure, the strength of the mechanisms responsible for generating Earth's magnetic field and its geology.
Understanding Earth's "Layers"
In order to better understand what's going on inside our planet, it helps to first get a lay of the land, so to speak.
Earth has several distinct layers, each with its own properties. At the outermost layer of our planet is the crust, which comprises the continents and ocean basins. Earth's crust varies in thickness from 35 to 70 kilometers (22 to 44 miles) in the continents and 5 to 10 kilometers (3 to 6 miles) in the ocean basins. The crust is mainly composed of alumino-silicates.
Next comes the mantle. The mantle is roughly solid, though very slow motion can be observed inside of it. It is about 2,900 kilometers (1,800 miles) thick, and is separated into an upper and lower mantle. It is here where most of Earth's internal heat is located. Large convective cells in the mantle circulate heat and drive the movements of Earth's tectonic plates, upon which our continents ride. The mantle is mainly composed of ferro-magnesium silicates.
Earth's innermost layer is the core, which is separated into a liquid outer core and a solid inner core. The outer core is 2,300 kilometers (1,429 miles) thick, while the inner core is 1,200 kilometers (746 miles) thick. The outer core is mainly composed of a nickel-iron alloy (liquid iron), while the inner core is almost entirely composed of a pure solid iron body.
A Magnetic Field from a Churning Core
Scientists believe Earth's magnetic field results from movements of molten iron and nickel within its liquid outer core. These flows, which are caused by interactions between Earth's core and its mantle, are neither even, nor evenly distributed. The electrical currents generated by these flows result in a magnetic field, which is similarly uneven, moves around in location and varies in strength over time. Earth's magnetic field is also slightly tilted with respect to Earth's axis. This causes Earth's geographic north and south poles to not line up with its magnetic north and south poles--they currently differ by about 11 degrees.
Magnetic Reversals
In just the last 200 million years alone, Earth's magnetic poles have actually reversed hundreds of times, with the most recent reversal taking place about 790,000 years ago. Scientists are able to reconstruct the chronology of these magnetic pole reversals by studying data on the spreading of the seafloor at Earth's mid-oceanic ridges. Unlike the doomsday scenario popularized by Hollywood in the movie "2012," however, such reversals don't occur over days, but rather on geologic timescales spanning hundreds to thousands of years-very short in geologic time but comparatively long in human time. The time span between pole reversals is even longer, ranging from 100,000 to several million years.
Earth's Magnetic "Shield"
Earth's magnetic field is essential for life on Earth. Extending thousands of kilometers into space, it serves as a shield, deflecting the constant bombardment of charged particles and radiation known as the solar wind away from Earth. These solar winds would otherwise be fatal to life on Earth. At Earth's poles, the perpendicular angle of the magnetic field to Earth there allows some of these particles to make it into our atmosphere. This results in the Northern Lights in the northern hemisphere and the Southern Lights in the southern hemisphere.
Exploiting the Magnetic Field
Here on the ground, Earth's magnetic field has many practical applications to our everyday lives. It allows people to successfully navigate on land and at sea, making it a critical tool for commerce. Hikers use it to find their way. Archaeologists use it to deduce the age of ancient artifacts such as pottery, which, when fired, assumes the magnetic field properties that were present at the time of its creation. Similarly, the field of paleomagnetism uses magnetism to give scientists glimpses into Earth's remote past. In addition, geophysicists and geologists use geomagnetism as a tool to investigate Earth's structure and changes taking place in the Earth.
Indirectly Measuring Earth's Magnetic Field
Since Earth's liquid core is the primary source of Earth's magnetic field, scientists can use observations of the magnetic field at Earth's surface and its variability over time to mathematically calculate and isolate the approximate motions taking place within the core.
That's what Dickey and deViron did. They combined measurements of Earth's magnetic field taken by observatory stations on land and ships at sea dating back to 1840 with those of the Danish Oersted and German CHAMP geomagnetic satellite missions, both of which were supported by NASA investments. These measurements were then used as inputs for a complex model that employs statistical time series analyses to determine how fast liquid iron is flowing within Earth's core.
"Although we do not observe the core directly, it's amazing how much we can learn about Earth's interior using magnetic field observations," said Dickey.
Visualizing the Motion in Earth's Core
In order to approximate the flow of liquid in the core, the scientists visualized its motion as a set of 20 rigid cylinders, each rotating about a common point that represents Earth's axis. "Imagine that each cylinder is slowly rotating at a different speed, and you'll get a sense of the complex churning that's taking place within Earth's core," Dickey said.
The scientists analyzed the data to identify common patterns of movement among the different cylinders. These patterns represent how momentum and energy are transferred from the liquid core-mantle interface inward through the liquid core toward the inner core with diminishing amplitudes.
Oscillations in the Core
Their analyses isolated six slow-moving oscillations, or waves of motion, occurring within the liquid core. The oscillations originated at the boundary between Earth's core and its mantle and traveled inward toward the inner core with decreasing strength. Four of these oscillations were robust, occurring at periods of 85, 50, 35 and 28 years. Since the scientist's data set goes back to 1840, the recurrence period of the longest oscillation (85 years) is less well determined than the other oscillations. The last two oscillations identified were weaker and will require further study.
The 85- and 50-year oscillations are consistent with a 1997 study by researchers Stephen Zatman and Jeremy Bloxham of Harvard University, Cambridge, Mass., who used a different analysis technique. A later purely theoretical study by Harvard researcher Jon Mound and Bruce Buffett of the University of Chicago in 2006 showed that there should be several oscillations of this type; their predicted periods agree with the first four modes identified in Dickey and deViron's study.
"Our satellite-based results are in excellent agreement with the previous theoretical and other studies in this field, providing a strong confirmation of the existence of these oscillations," said Dickey. "These results will give scientists confidence in using satellite measurements in the future to deduce long-term changes taking place deep within our restless planet."
Part I. The East African Rift System
The East African Rift System (EARS) is one the geologic wonders of the world, a place where the earth's tectonic forces are presently trying to create new plates by splitting apart old ones. In simple terms, a rift can be thought of as a fracture in the earth's surface that widens over time, or more technically, as an elongate basin bounded by opposed steeply dipping normal faults. Geologists are still debating exactly how rifting comes about, but the process is so well displayed in East Africa (Ethiopia-Kenya-Uganda-Tanzania) that geologists have attached a name to the new plate-to-be; the Nubian Plate makes up most of Africa, while the smaller plate that is pulling away has been named the Somalian Plate (Figure 1). These two plates are moving away form each other and also away from the Arabian plate to the north. The point where these three plates meet in the Afar region of Ethiopia forms what is called a triple-junction. However, all the rifting in East Africa is not confined to the Horn of Africa; there is a lot of rifting activity further south as well, extending into Kenya and Tanzania and Great Lakes region of Africa. The purpose of this paper is to discuss the general geology of these rifts are and highlight the geologic processes involved in their formation.
What is the East Africa Rift System?
The oldest and best defined rift occurs in the Afar region of Ethiopia and this rift is usually referred to as the Ethiopian Rift. Further to the South a series of rifts occur which include a Western branch, the "Lake Albert Rift" or "Albertine Rift" which contains the East African Great Lakes, and an Eastern branch that roughly bisects Kenya north-to-south on a line slightly west of Nairobi (Figure 2). These two branches together have been termed the East African Rift (EAR), while parts of the Eastern branch have been variously termed the Kenya Rift or the Gregory Rift (after the geologist who first mapped it in the early 1900's). The two EAR branches are often grouped with the Ethiopian Rift to form the East Africa Rift System (EARS). The complete rift system therefore extends 1000's of kilometers in Africa alone and several 1000 more if we include the Red Sea and Gulf of Aden as extensions. In addition there are several well-defined but definitely smaller structures, called grabens, that have rift-like character and are clearly associated geologically with the major rifts. Some of these have been given names reflecting this such as the Nyanza Rift in Western Kenya near Lake Victoria. Thus, what people might assume to be a single rift somewhere in East Africa is really a series of distinct rift basins which are all related and produce the distinctive geology and topography of East Africa.
How did these Rifts form?
The exact mechanism of rift formation is an on-going debate among geologists and geophysicists. One popular model for the EARS assumes that elevated heat flow from the mantle (strictly the asthenosphere) is causing a pair of thermal "bulges" in central Kenya and the Afar region of north-central Ethiopia. These bulges can be easily seen as elevated highlands on any topographic map of the area (Figure 1). As these bulges form, they stretch and fracture the outer brittle crust into a series of normal faults forming the classic horst and graben structure of rift valleys (Figure 3). Most current geological thinking holds that bulges are initiated by mantle plumes under the continent heating the overlying crust and causing it to expand and fracture. Ideally the dominant fractures created occur in a pattern consisting of three fractures or fracture zones radiating from a point with an angular separation of 120 degrees. The point from which the three branches radiate is called a "triple junction" and is well illustrated in the Afar region of Ethiopia (Figure 4), where two branches are occupied by the Red Sea and Gulf of Aden, and the third rift branch runs to the south through Ethiopia.
The stretching process associated with rift formation is often preceded by huge volcanic eruptions which flow over large areas and are usually preserved/exposed on the flanks of the rift. These eruptions are considered by some geologists to be "flood basalts" - the lava is erupted along fractures (rather than at individual volcanoes) and runs over the land in sheets like water during a flood. Such eruptions can cover massive areas of land and develop enormous thicknesses (the Deccan Traps of India and the Siberian Traps are examples). If the stretching of the crust continues, it forms a "stretched zone" of thinned crust consisting of a mix of basaltic and continental rocks which eventually drops below sea level, as has happened in the Red Sea and Gulf of Aden. Further stretching leads to the formation of oceanic crust and the birth of a new ocean basin.
Part II. The East African Rift
If the rifting process described occurs in a continental setting, then we have a situation similar to what is now occurring in Kenya where the East African/Gregory Rift is forming. In this case it is referred to as "continental rifting" (for obvious reasons) and provides a glimpse into what may have been the early development of the Ethiopian Rift.
As mentioned in Part I, the rifting of East Africa is complicated by the fact that two branches have developed, one to the west which hosts the African Great Lakes (where the rift filled with water) and another nearly parallel rift about 600 kilometers to the east which nearly bisects Kenya north-to-south before entering Tanzania where it seems to die out (Figure 2). Lake Victoria sits between these two branches. It is thought that these rifts are generally following old sutures between ancient continental masses that collided billions of years ago to form the African craton and that the split around the Lake Victoria region occurred due to the presence of a small core of ancient metamorphic rock, the Tanzania craton, that was too hard for the rift to tear through. Because the rift could not go straight through this area, it instead diverged around it leading to the two branches that can be seen today.
As is the case in Ethiopia, a hot spot seems to be situated under central Kenya, as evidenced by the elevated topographic dome there (Figure 1). This is almost exactly analogous to the rift Ethiopia, and in fact, some geologists have suggested that the Kenya dome is the same hotspot or plume that gave rise to the initial Ethiopian rifting. Whatever the cause, it is clear that we have two rifts that are separated enough to justify giving them different names, but near enough to suggest that they are genetically related.
Other Points of Interest:
What else can we say about the Ethiopian and Kenya Rifts? Quite a lot actually; even though the Eastern and Western branches were developed by the same processes they have very different characters. The Eastern Branch is characterized by greater volcanic activity while the Western Branch is characterized by much deeper basins that contain large lakes and lots of sediment (including Lakes Tanganyika, the 2nd deepest lake in the world, and Malawi).
Recently, basalt eruptions and active crevice formation have been observed in the Ethiopian Rift which permits us to directly observe the initial formation of ocean basins on land. This is one of the reasons why the East African Rift System is so interesting to scientists. Most rifts in other parts of the world have progressed to the point that they are now either under water or have been filled in with sediments and are thus hard to study directly. The East African Rift System however, is an excellent field laboratory to study a modern, actively developing rift system.
This region is also important for understanding the roots of human evolution. Many hominid fossil finds occur within the rift, and it is currently thought that the rift's evolution may have played an integral role in shaping our development. The structure and evolution of the rift may have made East Africa more sensitive to climate changes which lead to many alternations between wet and arid periods. This environmental pressure could have been the drive needed for our ancestors to become bipedal and more brainy as they attempted to adapt to these shifting climates (see Geotimes 2008 articles: Rocking the Cradle of Humanity by Beth Christensen and Mark Maslin, and Tectonic Hypotheses of Human Evolution by M. Royhan Gani and Nahid DS Gani).
Conclusions:
The East African Rift System is a complicated system of rift segments which provide a modern analog to help us understand how continents break apart. It is also a great example of how many natural systems can be intertwined - this unique geological setting may have altered the local climate which may have in turn caused our ancestors to develop the skills necessary to walk upright, develop culture and ponder how such a rift came to be. Just like the Grand Canyon, the East African Rift System should be high on any geologist's list of geologic marvels to visit.
One of my blog posts from a number of weeks ago prompted this inquiry:
"About 15 miles south of Holbrook, AZ there are extremely deep fissures (referred to locally as "the Cracks") They range from just a few inches wide to several feet wide, but some seem to go down for a hundred feet. When you drop a rock in the crack you can hear it bouncing down the hole for an extremely long distance. Are these formed from seizmic [sic] activity?"
Actually, this area is pretty quiet seismically. But it is underlain by extensive salt and other evaporate minerals. As these deposits were dissolved over time, the overlying rocks dropped down, creating a broad anticlinal fold. This resulted in extension cracks over the top of the folded layers which have eroded into The Sinks and other similar features in the region.
We have a publication that describes the fractures and related topics with striking photos including the one above.
