What is the Earth’s Mantle Made Of?

The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com

Like all the other terrestrial planets, (Mercury, Venus, and Mars) the Earth is made up of many layers. This is the result of it undergoing planetary differentiation, where denser materials sink to the center to form the core while lighter materials form around the outside. Whereas the core is composed primarily of iron and nickel, Earth’s upper layer are composed of silicate rock and minerals.

This region is known as the mantle, and accounts for the vast majority of the Earth’s volume. Movement, or convection, in this layer is also responsible for all of Earth’s volcanic and seismic activity. Information about structure and composition of the mantle is either the result of geophysical investigation or from direct analysis of rocks derived from the mantle, or exposed mantle on the ocean floor.

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Is Earth Running Out Of Crust?

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Earth just doesn’t make crust like it used to… at least, not according to new research by a team of scientists in the UK.

Researchers with the Universities of Bristol, St Andrews and Portsmouth have studied elements trapped within zircon samples gathered from all over the planet to peer billions of years back in time at how Earth’s crust was being produced.

Zircon, a mineral found in granite, can be dated with precision and is thus an accurate measure for geologic timescales.

What they found was that 65% of our planet’s current crust had already existed 3 billion years ago. Since rocks older than 2.5 billion years are rare on Earth today, this means that some process began to take place that either reworked — or destroyed — a large portion of the older crust, and changed how new crust was formed.

During the first 1.5 billion years of Earth’s history, the team reports, the rate of crust formation was high — approximately 3 cubic kilometers was added to the continents each year. After that the rate dropped substantially, falling to about 0.8 cubic kilometers per year for the next 3 billion years — right up to the present day.

The cause is yet unknown, but it may be the result of the onset of plate tectonics driven by subduction — the process by which sections of Earth’s crust (“plates”) slide beneath other sections, sinking into the underlying mantle to be liquefied into magma by pressure and heat. New crust is created when the magma rises again where the plates separate… Earth’s current “conveyor belt” of crust formation.

Whatever process was in place prior to 3 billion years ago, it was much more efficient at creating crust.

“Such a sharp decrease in the crustal growth rate about 3 billion years ago indicates a dramatic change in the way the continental crust was generated and preserved,” said Dr. Bruno Dhuime of the University of Bristol’s School of Earth Sciences. “This change may in turn be linked to the onset of subduction-driven plate tectonics and discrete subduction zones as observed at the present day.  The next challenge is to determine which tectonic regime shaped the Earth’s crust in the planet’s first 1.5 billion years before this change.”

The team’s paper “A Change in the Geodynamics of Continental Growth 3 Billion Years Ago” (Bruno Dhuime, Chris J. Hawkesworth,  Peter A. Cawood, Craig D. Storey) was published March 16 in Science.

Read more on the University of Bristol’s press release here.

What is a Subduction Zone?

Transform Plate Boundary

IF you don’t know anything about plate tectonics you might be wondering about what is a subduction zone. A subduction zone is a region of the Earth’s crust where tectonic plates meet. Tectonic plates are massive pieces of the Earth’s crust that interact with each other. The places where these plates meet are called plate boundaries. Plate boundaries occur where plates separate, slide alongside each other or collide into each other. Subduction zones happen where plates collide.

When two tectonic plates meet it is like the immovable object meeting the unstoppable force. However tectonic plates decide it by mass. The more massive plate, normally a continental will force the other plate, an oceanic plate down beneath it. This is the subduction zone. When the other plate is forced down the process is called subduction. The plate enters into the magma and eventually it is completely melted. That is how the surface of the earth makes way for the crust created over time at other plate boundaries.

Subduction zones have key characteristics that help geologist and seismologist identify them. The first is mountain formation. Subduction zones always have mountain ranges caused by plate subduction. The next is volcanic activity as a plate is subducted the pressure and heat turns it into magma. These pockets of magma find paths to the surface and create volcanoes. A good example is the subduction zone near Chile. The final sign is deep marine trenches. These are the best evidence of a subduction zone as they are visible evidence of the crease formed by subduction of a plate. The most famous is the Mariana Trench.

There are some interesting theories about why Subduction occurs in the Earth’s crust. One common theory is that subduction was initiated by major impacts by asteroids or comets early in Earth’s history. This makes a lot of sense due to the geologic evidence of large impacts scattered around the world.

Understanding how subduction zones work is important because it helps scientist to identify areas of high volcanic and seismic activity. Monitoring these areas can help them warn people who live near them of imminent events and also people who could be affected by the side effects of such events such as ash clouds or tsunamis.

Subduction continues to be one of the most powerful and dynamic processes on planet Earth and as technology improves we can come to understand more about this amazing process.

We have written many articles about the subduction zone for Universe Today. For example, here is one on the Ring of Fire and plate boundaries.

You should also check out plate tectonics and subduction.

If you’d like more info on the subduction zone, check out the U.S. Geological Survey Website. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.

Sources:
http://en.wikipedia.org/wiki/Subduction
http://myweb.cwpost.liu.edu/vdivener/notes/subd_zone.htm

Subduction is a process in geology where one tectonic plates slides underneath another one and merges into the Earth’s mantle. The denser plate is the one that slips under the less dense plate; the younger plate is the less dense one. The process is not a smooth one. The tectonic plates grate against each other, which often causes earthquakes. The plate that slips under does not stay that way. Due to the heat caused by it rubbing against the other plate as well as the natural heat of the mantle, the plate melts and turns into magma. The area where subduction occurs is known as the subduction zone.

When one plate begins to slip underneath another one a trench is formed. The earthquakes that result due to the plates grinding against each other often cause magma to spill out through the trench in submarine volcanoes. Various formations such as mountain ranges, islands, and trenches are caused by subduction and the volcanoes and earthquakes it triggers. In addition to causing earthquakes, subduction can also trigger tsunamis.

When the older plate is holding a continent however, it does not sink, which is reassuring. Instead, the less dense material slips into a trench behind the denser oceanic crust where it gets stuck. The pressure continues to build until the trench flips over and the less dense plate slips underneath the one with the continent.

It is possible for a whole tectonic plate to disappear. This happens when the plate goes through subduction faster than new material can be added to the plate through seafloor spreading. The spreading pushes the plate slowly toward the subduction zone until the whole thing disappears. When this happens, the other tectonic plates rearrange to cover the area.

Subduction zones are mainly located in the Pacific Ocean. This is because seafloor spreading – the process by which new oceanic crust is created – occurs mostly in the Pacific. Thus the new material pushes the older plates outward and then they need to undergo subduction. This also explains why so many earthquakes originate in the Pacific Ocean near the Ring of Fire. That is where the subduction zones are concentrated.

Continental plates also converge, but this is not considered subduction because these plates do not have different densities and thicknesses to subduct. Landforms such as the Himalayas are formed from these convergences though.

 

Continental Crust

The crust is the top layer of the Earth’s Surface. Did you know that there are 2 types, though? One is called the Oceanic Crust, and the other, the Continental Crust. As its name suggests, the Oceanic Crust is the top layer of Earth that forms the ocean floor. The Continental Crust, however, will be our focus.

We walk on top of and dig down through the Continental Crust when we plant or drill. Even if there is an unstable surface at the very top, like sand, the deeper parts of the Crust are made of harder rocks. The large land masses, continents, have bases made from sedimentary, igneous, or metamorphic rocks, as well as any combination thereof. This shield rock is the oldest known; it’s been tested, dated, and found to have been here for 3,960,000,000 years!

Geologists, scientists who study the Earth, believe that shield rock was created when hot molten iron, known as magma cooled. If their math’s correct, it happened around the time these rocks formed, almost 4 billion years ago, right? Some of those rocks were so big it took a long time for them to cool. So, even if the rocks were formed 3.9 billion years ago, they might not have cooled for quite some time. Many estimate that the Continental Crust wasn’t completely hard for another 60,000,000 to 160,000,000 years.

The top portion of this rock has another name, platform rock. The oldest-known platform rocks are approximately 600,000,000 years old, and can be found in central North America. The sedimentary rock ranges from 1,000 to 2,000 meters thick; that is equivalent to more than a half mile to 1.25 miles. When we put the top and bottom portions of the Continental Crust together, we get what scientists call, a craton. Most cratons are stable and haven’t been damaged by earthquakes or volcanoes for hundreds of millions of years.

Around the edges are the continental margins, mostly created by sedimentary rock originally found in the oceans. How is that possible, you ask? Well, it’s due to earthquake and volcanic activity. In this case, it’s mainly due to a phenomenon called, subduction. You see, the Earth fits together like a puzzle; and, if you try to place the wrong piece into a spot where it fits, but isn’t quite right, what happens? Another piece might pop out of place. Sometimes, a continental margin works its way under the oceanic crust. When that occurs, the oceanic layer ends up on top of the continental margin. This is subduction. The most well-known place for this is along The Ring of Fire, an area that covers the edges along the Pacific Ocean. This is why so many and such violent earthquakes, volcanic eruptions, and tsunamis occur in that part of the world.

Universe Today has a wealth of information on this and other related topics. Here are just 2 of those available. The first is entitled,
Earth, Barely Habitable?.

The second is called, Interesting Facts About Planet Earth.

Universe Today also hosts Astronomy Cast, a science program that covers a variety of subjects. Episode 51: Earth, explains this subject in greater detail.

The Encyclopedia of Earth , by Michael Pidwirny has some excellent information, too.

Sources:
USGS
Science Daily