Universe Today

A new crater on the Moon. Credit: ISRO/NASA/JHUAPL/LPI

Although the Earth may have had a great number of impact craters in the past, erosion and tectonic activities may have concealed many of them.

A moon crater can stay virtually intact for millions or even billions of years because the Moon does not have dynamic bodies of water on its surface. Hence, damages or distortions to such craters cannot be caused by erosion. The oldest craters are found to be over 2 billion years old.

In addition to that, the Moon also does not have active tectonic activities. Thus, alterations to the Moon's surface, like those that have formed the Earth's mountain ranges, trenches, and continents, cannot occur.

The relative ages of Moon craters can be roughly determined by comparing the number of craters in them. If a crater is very old, naturally, more recent impacts will form craters inside them.

Three small craters found near the landing site of the first Apollo mission that landed on the Moon, Apollo 11, are named after the three astronauts who belonged to that crew: Aldrin, Collins, and Armstrong, after Buzz Aldrin, Michael Collins, and Neil Armstrong, respectively.

In October 9, 2009, the LCROSS team was able to impact the Cabeus crater and reveal one of the most important discoveries of the Moon – the presence of substantial amounts of water. The Moon crater Cabeus is found near the Moon's south pole.

The presence of water on the lunar surface can greatly influence future colonization plans there. As we know, water is vital to human survival. The source of water is yet unknown and further analysis of the water ejected is still to be performed.

The largest known Moon crater is the South Pole-Aitken Basin. It measures 2,000 km wide and 13 km deep. All other large craters on the Moon are far smaller than it, leading some scientists to believe that the South Pole-Aitken Basin may have been caused by a grazing impact.

Here's a list of popular craters on Earth from Universe Today.

Come October 9, 2009, LCROSS will perform a lunar impact. Find out which crater NASA has chosen for the impact. If you want to know more about the largest crater on the Moon, NASA's got the right stuff.
There are some interesting episodes from Astronomy Cast that we'd like to recommend:
The Source of Atmospheres, the Vanishing Moon, and a Glow After Sunset
The Moon, Part 1

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Do you know that the largest verified impact craters on Earth are more than 100 km wide? And the largest of them, Vredefort Crater, being an astounding 300 km wide? Still, these giants are no match for the largest of all craters of the Moon.

So what is the largest of all craters of the Moon? The South Pole-Aitken Basin, which also happens to be the largest known crater in the Solar System, has a diameter of – hold on to your eyeballs – over 2,000 km! It's also very deep, at 13 km. That's even much deeper than the deepest point on Earth, some 11 km below the surface of the ocean – Challenger Deep.

But wait a minute. Before you start to think all craters of the Moon are nearly this large, let me assure you they're not. Most of the really large ones are not even half as large as this. They're usually only between 400 to 500 km wide. That's the same order of magnitude as our own big ones.

So if you take out the South Pole-Aitken Basin, it would be safe to say that the craters of the Moon and the craters of the Earth have more or less the same dimensions. In fact, the craters of the Moon are even about as old as those on Earth. For example, Vredefort is estimated to be at least 2 billion years old. That's about the same age as the oldest known lunar craters.

And why not? After all, if you look at them in astronomical scales, the two are definitely very close to one another. Thus, the family of rocks that may have impacted the Moon's surface may have also impacted Earth's.

The only big question that always remains is: Why then do we have so few of these craters? Aren't we supposed to have even more, considering the size of the Earth compared to the Moon?

Alright, remember your Earth Science 101 that talks about plate tectonics and how they have formed the towering Himalayas, the sinister depths of the Marianas Trench, and possibly even entire continents? No doubt therefore, these same activities may have concealed a substantial number of impact craters.

The Moon, as we know it, is not exposed to such activities. Furthermore, there is very minimal erosion there due to the absence of water on the surface. Therefore, all craters of the Moon, no matter how old, remain preserved.

Here's a list of popular craters on Earth from Universe Today.

Come October 9, 2009, LCROSS will perform a lunar impact. Find out which crater NASA has chosen for the impact. If you want to know more about the largest crater on the Moon, NASA's got the right stuff.
There are some interesting episodes from Astronomy Cast that we'd like to recommend:
The Source of Atmospheres, the Vanishing Moon, and a Glow After Sunset
The Moon, Part 1

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The size, shape, and land mass of the Earth has been determined over the millenia by the movement and drifting of the continental plates. This concept is based on plate tectonics and usually expressed as continental drift. The land mass of Earth started out as one massive supercontinent called Pangea. Slowly, at the rate of a few centimeters per year, the continents moved to their current positions. That doesn't mean that they are going to stay in place forever. The plates continue to shift, collide, and in general wreak havoc on our planet.

The Earth has several continental plates and oceanic plates. These plates are from 80 to 400 km thick. The names of these plates are: the Eurasian plate, Australian-Indian plate, Philippine plate, Pacific plate, Juan de Fuca plate, Nazca plate, Cocos plate, North American plate, Caribbean plate, South American plate, African plate, Arabian plate, the Antarctic plate, and the Scotia plate. These plates have been broken down into smaller sub-plates. These plates are in constant motion, often colliding and reforming the Earth's geography. The continental plats move in any of three ways: divergence, convergence, and lateral slipping.

Different things happen when the continental plates move in different ways and the results of their interactions differ because of this. Divergence happens when two plates move away from each other. This usually causes the seafloor to spread. Convergence is when two plates collide. Some of the Earth's crust is destroyed, but the end results depend on the type of plates involved. When a thin, dense oceanic plate collides with a relatively light, thick continental plate, the oceanic plate is forced under the continental plate; this phenomenon is called subduction. When two oceanic plates collide, one may be pushed under the other and magma from the mantle rises, forming volcanoes in the vicinity. When two continental plates collide, mountain ranges are created as the colliding crust is compressed and pushed upwards. Lateral slipping happens when two plates move sideways against each other, there is a tremendous amount of friction which makes the movement jerky. The plates slip, then stick as the friction and pressure build up to incredible levels. When the pressure is released suddenly, and the plates suddenly jerk apart, this is an earthquake.

There are many things that can be said about the continental plates, but most of them lead back to plate tectonics. There is a good article about plate tectonics here. Here on Universe Today we have a couple of great articles related to the continental plates: one about convergent boundaries and another about subduction. Astronomy Cast offers a good episode about plate tectonics.

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Continental plate is a term used in plate tectonics. The Earth's rocky outer crust solidified billions of years ago, soon after the Earth formed. This crust is not a solid shell; it is broken up into huge, thick plates that drift atop the soft, underlying mantle. The plates are made of rock and drift all over the globe; they move sideways and up and down. Over long periods of time, the plates also change in size. Their margins are added to, crushed together, or pushed back into the Earth's mantle. These plates are from 80 to 400 km thick.

The Earth has several continental plates and oceanic plates. Most of these have names related to their general geographic location: the Eurasian plate, Australian-Indian plate, Philippine plate, Pacific plate, Juan de Fuca plate, Nazca plate, Cocos plate, North American plate, Caribbean plate, South American plate, African plate, Arabian plate, the Antarctic plate, and the Scotia plate. These plates have been broken down into smaller sub-plates.

In order to understand the significance of a continental plate, you need a slight introduction to plate tectonics. Plate tectonics explains the movement of the Earths plates and the cause of earthquakes, volcanoes, oceanic trenches, mountain range formation, and many other geologic phenomenon. The plates are moving in different directions at a rate of 1 to 10 cm per year. When two plates collide the Earth experiences some form of seismic activity: a volcano erupts, an earthquake, etc.

There three types of continental plate movement: Divergence, Convergence, and Lateral Slipping. Divergence happens when two plates move away from each other. This usually causes the seafloor to spread. Convergence is when two plates collide. Some of the Earth's crust is destroyed, but the end results depend on the type of plates involved. When a thin, dense oceanic plate collides with a relatively light, thick continental plate, the oceanic plate is forced under the continental plate; this phenomenon is called subduction. When two oceanic plates collide, one may be pushed under the other and magma from the mantle rises, forming volcanoes in the vicinity. When two continental plates collide, mountain ranges are created as the colliding crust is compressed and pushed upwards. Lateral slipping happens when two plates move sideways against each other, there is a tremendous amount of friction which makes the movement jerky. The plates slip, then stick as the friction and pressure build up to incredible levels. When the pressure is released suddenly, and the plates suddenly jerk apart, this is an earthquake.

You can see how the continental plate in your area has affected the substance of the Earth. There is a good article about plate tectonics here. We have an even better article about the subject here On Universe Today. Astronomy Cast offers a good episode about one of the most fearsome results of the continental plate shifting: volcanoes.

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Pangea animation

When a divergent plate boundary occurs underneath the ocean (known as the oceanic lithosphere), the rising lithosphere forms what is known as a mid-ocean ridge. Further divergent motions force the lithosphere to stretch and eventually form a deep fissure. When the fissure opens up, magma pushes itself upward, loses pressure, and then solidifies.

The longest mountain range in the planet, the Mid-Atlantic Ridge, is a result of this kind of activity. It is found underneath the Atlantic Ocean and was discovered by the HMS Challenger expedition in that ocean in 1872.

The Mid-Atlantic Ridge slices the area underneath Iceland in half. That is why Iceland is one of the favorite natural observatories for divergent plate boundary activities.

In some cases, submarine volcanoes are formed by this kind of plate tectonic activity. These volcanoes are also known as shield volcanoes. Through the course of their lifetime, such volcanoes can develop into volcanic islands.

Some of the largest volcanoes in the world are actually shield volcanoes. These kind of volcanoes include Mauna Kea, Rainbow Range, Medicine Lake Volcano, and Indian Heaven.

Divergent plate boundary activities are not confined to the ocean. Some are found on land. For example, there's the Basin and Range province in Nevada and Utah. There's also Siberia's Lake Baikal and East Africa's Lake Tanganyika.

There are actually three types of plate boundaries: transform, convergent and divergent boundaries.

Geothermal power plants are usually constructed near these boundaries, where geothermal energy can be harnessed easier than in any other region on the planet. Geothermal energy is a renewable type of energy and, along with solar, hydro, and wind, are steadily gaining popularity as people are getting more conscious about saving the environment.

It is for this reason that many geothermal power plants are found along the Pacific Ring of Fire. There, tectonic activity is very high and movement on convergent and divergent plate boundaries are frequent. Some of the largest of these plants are in the Philippines, Costa Rica, El Salvador, and California – all located along the Pacific Ring of Fire.

You can read more about divergent plate boundary activities here in Universe Today. Here are the links:

• Rising Temperatures Could Shut Down Plate Tectonics
• Continental Crust

There's more about it at USGS. Here are a couple of sources there:

• Divergent boundaries
• Hotspots: Mantle Thermal Plumes

Here are two episodes at Astronomy Cast that you might want to check out as well:

• Plate Tectonics
• Volcanoes, Hot and Cold

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Volcano Paricutin

What are volcanoes? We know of them as big mountains spewing fountains of fiery magma from their peaks. We even know of the more famous ones such as Mt. Vesuvius in Roman times and Mt. St. Helens in our recent memory. However there is more to the story than what we commonly know.

To understand volcanoes we need to understand the composition of the earth and plate tectonics. The earth can be best explained as a ball of molten rock with a cooled crust. Pretty much like a fruit pie. However the crust is the earth's surface where we live. The Earth's crust is broken up into plates. These plates float on the ocean of molten rock beneath surfaces called the mantle. Like the normal ocean magma (molten rock) in the mantle also has currents. The hotter magma rises away from the Earth's core while the cooler magma sinks back towards it. This creates the current that moves the plates of the Earth's surface making plate tectonics.

Tectonic plates are moving away, diverging or colliding, converging. These two conditions make volcanoes. A volcano is a vent or crack in the Earth crust that allows magma and hot gases to escape. When the tectonic plates diverge new magma wells ups to make the newer parts of the Earth's crust. When plates converge the common occurrence is that one plate is forced beneath the other. This is called subduction. The incredible force involved melts the rock of the subducted plate creating magma. This is the other condition under which a volcano is made.

A great way to look at is that where ever plates' boundaries are you should find a volcano. The location can be on a continent like in Europe or North America. However, the most interesting volcanoes will be in island chains. These occur in locations not at the boundaries of plates.

Ocean volcanoes are important because they help to make new islands. There are certain places that are not tectonic boundaries. These are regions of the Earth's mantle called hot spots. They are places where rising magma is highly focused and melt the crust. This creates a new volcanic mountain on the ocean floor. Over time the volcano grows in stature until an island is born above the ocean's surface. If a plate passes over a hot spot, it can over time create island chains such as the Hawaiian Island chain.

If you enjoyed this article there are several others on Universe Today that you will enjoy. There is a great article about extinct volcanoes. There is also a great article on three types of volcanoes.

There is also great resources online. For example you can check out United States Geological Survey webpage on volcanoes. There is also the Oregon State University Volcano World web page

You can also listen to Astronomy Cast. Episode 141 talks about volcanoes on Earth and other rocky planets.

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gleebruk indonesia after tsunami dec 04

A Tsunamis is big wave caused by a major Earthquake or some other massive movement. When an earthquake happens the vibrations travel through deep ocean waters until it approaches a shore line. The shallow depths cause the waves created by the earthquake or major movement to build upon each other making the massive waves we know about.

While we are most familiar with the one huge wave, a tsunamis can also be a series of devastating waves. This can often be more damaging than the one huge wave.

The strength of a tsunamis relies on several factors. The first is the earthquake or landslide that started it. The stronger the earthquake or the bigger the landslide the more powerful the tsunamis is likely to be. Another thing to look for is how close the tsunamis wave's origin is to a shoreline. This can be tricky. If right next to the land, the shore will not likely see a tsunamis. However if it is relatively close the power of the tsunamis can dramatically decrease.

Most Tsunamis occur in the Pacific and Indian Ocean in the region of volcanic activity called the Ring of Fire. This region of the world has a lot of earthquakes and volcanoes so it is place that has a high occurrence of Tsunamis. However, most of them while they cause property damage do little harm to human life even the more powerful ones. Especially with advanced warning. However, there have been a few that are now considered to be the most deadly in recorded history.

The deadliest occurred recently in 2004 in Indonesia. It is called the Christmas Tsunamis of 2004. The tsunamis was created by a powerful earthquake near Indonesia. The series of waves caused devastation all around the Indian ocean hitting places as far as East Africa. The death toll ended up being in the hundreds of thousands.

To prevent such disasters many governments such as the United States have monitoring systems to track earthquakes and warn of possible tsunamis. This advanced warning helps to prevent loss of life by giving residents enough time to evacuate to higher and safe locations.

Tsunamis help to remind us that there are many natural forces that we still can not control.

If you enjoyed this article you will like some of the others on Universe Today. There is a great article about Tsunamis and another about the effects of Tsunamis.

There are also some great resources online. The University of Washington has a great Tsunamis web page You can also look at the Savage earth "Waves of Destruction" web page.

You can also listen to Astronomy Cast. Episode 51 is about the Earth and various natural forces.

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The current tectonic plates.

The theory of seafloor spreading was proposed by Harry H. Hess, an American geophysicist, in 1960.  According to Hess, seafloor spreading is where two tectonic plates move apart and the ocean floor spreads out. When two tectonic plates spread apart they break. Magma then forces its way up through the cracks in an underwater volcano. When the magma hits the water, it cools and forms ridges along the plates that are pulling apart. In a normal volcano, the magma would pour down the sides and build up, but the since the plates are pulling apart underwater, the magma is more like forming a bridge. However, this bridge has valleys and mountains and is known as an oceanic ridge. The material being formed is known as constructive because a new ocean floor is actually being formed as a result of the process.

This process is thought to be the reason for the drifting of the continents. Continents next to the Atlantic Ocean are thought to be moving one or two centimeters away from the Mid-Atlantic Ridge every year.

Since the ocean floor is actually growing, you would think that the ocean floor would start encroaching on the land. However, as certain plates get larger others get smaller. This process is called subduction. At subduction zones, the old crust is melted back into the Earth. Subduction is very frequent in the Pacific Ocean, especially in certain areas. The subduction in the Pacific Ocean is what causes the volcanic activity in the Ring of Fire area of the Pacific Ocean.  Plates in the Pacific Ocean are also spreading much more rapidly than the Mid-Atlantic ones are.

One major reason for the theory of seafloor spreading is the presence of the Mid-Atlantic Ridge. In the 1960’s, scientists discovered the Mid-Atlantic Ridge, which is a huge network of undersea oceans in the Atlantic Ocean is just part of a mountain system that circles the globe. Proponents of the seafloor spreading theory have a good deal of evidence to back up their theory. For one, scientists can see that the rock on the ocean floor gets younger as you approach the Mid-Atlantic Ridge, and the sediment covering is also significantly lighter. Another piece of evidence is that the rock that makes up the ocean floor is much younger than rock on the continents. This suggests that the older ocean bed truly does absorb back into the Earth’s crust.

Universe Today has articles on tectonic plates and underwater volcanoes.

For more information check out plate tectonics and seafloor spreading.

Astronomy Cast has episodes on plate tectonics.

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Image shows the crates of Grábrók near Borgarnes in the west of Iceland.

Iceland possesses one of the highest concentrations of active volcanoes in our planet. This is believed to be mainly due to its nearness to the boundaries of the Eurasian and North American Plates – or specifically, the Mid-Atlantic Ridge. In fact, the boundary actually slices Iceland underneath.

Observations totally agree with theory, as most of the active Iceland volcanoes are found near the boundary. Rifting processes due to the separating plates have resulted in subglacial and regional fissure eruptions.

Another reason may be that Iceland lies right above a volcanic hotspot. A volcanic hotspot is a region described to have experienced severe volcanic activity for a long period of time. It is believed to be due to an upward convection of hot mantle passing through a narrow passage.

Among the 130 Iceland volcanoes, 18 have undergone eruption since settlers started moving in during the early 870's. But that's not all. It is estimated that, for the last 500 years, Iceland volcanoes have spewed forth one-third of the total lava output of the Earth.

Perhaps the post popular volcano in Iceland – well, it certainly is the most active – is Hekla. Rising up to an altitude of 1,491 meters Hekla has erupted at least 20 times since the settling in 874. Its fiery nature earned it the title "Gateway to Hell" during the Middle Ages. Hekla alone has produced one of the biggest volumes of lava in the world.

Not to be outdone is another one of the Iceland volcanoes – the infamous Laki. The Laki eruption of 1783 is one of the most devastating in recent history. Its dust cloud even reached Asia and Africa.

Since the mid-ocean ridge peculiarly rises above sea level in Iceland, it is a favorite spot for researchers who have taken interest in these ridges.

Because of the high activity of the Iceland volcanoes, there is an abundance of geothermal energy in the country. Hence, electric bills are quite cheap here. However, considering the geological instability of the place, I'm sure many of you would rather stay where you are and opt to pay higher electric bills.

We have some articles in Universe Today that are related to Iceland volcanoes. Here are two of them:

• List of Volcanoes
• Active Volcano

Iceland volcano articles brought to you by NASA, here are the links:

• Icelandic Volcanoes Help Researchers Understand Potential Effects of Eruptions
• Volcano and Aurora in Iceland

Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:

• Volcanoes, Hot and Cold
• Plate Tectonics

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In Plate Tectonic Theory, the lithosphere is broken into tectonic plates, which undergo some large scale motions. The boundary regions between plates are aptly called plate boundaries. Based upon their motions with respect to one another, these plate boundaries are of three kinds: divergent, convergent, and transform.

Divergent Boundaries

Divergent boundaries are those that move away from one another. When they separate, they form what is known as a rift. As the gap between the two plates widen, the underlying layer may be soft enough for molten lava underneath to push its way upward. This upward push results in the formation of volcanic islands. Molten lava that succeeds in breaking free eventually cools and forms part of the ocean floor.

Some formations due to divergent plate boundaries are the Mid-Atlantic Ridge and the Gakkel Ridge. On land, you have Lake Baikal in Siberia and Lake Tanganyika in East Africa.

Convergent Boundaries

Convergent boundaries are those that move towards one another. When they collide, subduction usually takes place. That is, the denser plate gets subducted or goes underneath the less dense one. Sometimes, the plate boundaries also experience buckling. Convergent boundaries are responsible for producing the deepest and tallest structures on Earth.

Among those that have formed due to convergent plate boundaries are K2 and Mount Everest, the tallest peaks in the world. They formed when the Indian plate got subducted underneath the Eurasian plate. Another extreme formation due to convergent boundaries is the Mariana Trench, the deepest region on Earth.

Transform Boundaries

Transform boundaries are those that slide alongside one another. Lest you imagine a slippery, sliding motion, take note that the surfaces involved are exposed to huge amounts of stress and strain and are momentarily held in place. As a result, when the two plates finally succeed in moving with respect to one another, huge amounts of energy are released. This causes earthquakes.

The San Andreas fault in North America is perhaps the most popular transform boundary. Transform boundary is also known as transform fault or conservative plate boundary.

Movements of the plates are usually just a few centimeters per year. However, due to the huge masses and forces involved, they typically result in earthquakes and volcanic eruptions. If the interactions between plate boundaries involve only a few centimeters per year, you could just imagine the great expanse of time it had to take before the land formations we see today came into being.

You can read more about plate boundaries here in Universe Today. Here are the links:

• Tectonic Plates
• Lithosphere

Here are the links of two more articles from USGS:

• Main Types of Plate Boundaries
• Understanding Plate Motions

Here are two episodes at Astronomy Cast that you might want to check out as well:

• Plate Tectonics
• Volcanoes, Hot and Cold

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Earthquakes are among the most devastating forces of nature. What we have are seven of the world’s most famous earthquakes, chronologically listed below. Not all included here are necessarily the strongest (in terms of magnitude) but they made the headlines when they hit. Here they are:

Shaanxi Earthquake of 1556

– This was the deadliest quake ever recorded. It claimed the lives of about 830,000 people. At that time, most inhabitants in the affected areas were living in Yaodongs or artificial caves. They were buried alive when the huge tremors caused the cliffs in which these caves were located in, to collapse.

San Francisco Earthquake of 1906

– Although its tremors were also felt in Southern Oregon, it is the resulting fire in San Francisco that had a more devastating impact on the economy. Is has been often compared recently to Hurricane Katrina because of its similar economic bearing.

The Great Chilean Earthquake of 1960

– Like the one that hit Asia in 2004, this 9.5-rated quake resulted in a massive tsunami reaching up to as high as 10.7 meters. This magnitude is the highest recorded ever. Although the tsunami originated in Cañete, Chile, the waves raced north-westward to Japan and the Philippines, wreaking havoc there.

Great Alaska Earthquake of 1964

– With a magnitude of 9.2, it is the second strongest earthquake to be ever recorded. It caused tsunamis, landslides, and resulted in major landscape changes. Some places near Kodiak is said to have been raised 9.1 meters high, while those near Portage were dropped by 2.4 meters.

Great Tangshan Earthquake of 1976

– This is the deadliest quake of the 20th Century, with the number of deaths hitting somewhere near 250,000. Weak building structures and the time this disaster struck (4 am) contributed a lot to that sickening number.

Bam Earthquake of 2003

– The death toll in this tremor reached over 26,000. Like the one in Tangshan, the use of poor construction materials was one of the leading culprits for the deaths. Most of the affected buildings were made of mud bricks.

Indian Ocean Earthquake of 2004

– The resulting tsunami that killed 230,000 people was caused by a subduction between the India and Burma plate. Its 30 m-high waves destroyed virtually everything in its path, making this quake not only one of the most famous earthquakes but also one of the famous natural disasters in history.

Excluding poor building infrastructure, we can see that high death tolls in these famous earthquakes result when the tremors are accompanied by tsunamis. This happens when the quake’s epicenter is found at the bottom of the ocean.

You can read more about famous earthqueakes here in Universe Today. Here are the links:

• Satellites Show How Earth Moved During Earthquake
• Earthquake Should Show a Gravity Scar

There's more about it at USGS. Here are a couple of sources there:

• Indian Ocean Earthquake Triggers Deadly Tsunami
• The Great 1906 San Francisco Earthquake

Here are two episodes at Astronomy Cast that you might want to check out as well:

• Plate Tectonics
• Volcanoes, Hot and Cold

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Pangea animation

When tectonic plates start to diverge, the linear feature formed is called a rift. Sometimes, the gap widens and sometimes it stops. When the gap eventually widens, it then evolves into a rift valley. Divergent boundaries that occur between oceanic plates produce mid-oceanic ridges.

In places where molten lava is able to move up and fill the gap, volcanic islands are eventually formed. Molten lava that rises eventually cools and forms part of the ocean floor.

One divergent boundary is the Mid-Atlantic Ridge, found at the bottom of the Atlantic and is the longest mountain range in the world. That's right, the longest mountain range is hidden from our view. Imagine how astonished crew members of the HMS Challenger were when they discovered the massive rise underneath them. The Challenger expedition was dedicated to scientific discoveries the became foundations of oceanography. The Mid-Atlantic Ridge was observed by the HMS Challenger in 1872.

The record for the slowest divergent boundary in the world goes to Gakkel Ridge between the North American Plate and the Eurasian Plate in the Arctic Ocean. Its annual rate of separation is less than one centimeter – that's about half as fast the rate your fingernails grow. Robotic submersibles belonging to the AGAVE expedition discovered microbial communities of over a dozen new species on this ridge.

Although not as common, rift valleys can also be formed on land. One example is the Basin and Range province in Nevada and Utah. The world's largest freshwater lakes such as Siberia's Lake Baikal and East Africa's Lake Tanganyika are found in rift valleys.

One of the favorite natural laboratories for the study of divergent boundaries is Iceland. The Mid-Atlantic Ridge runs beneath Iceland and as the North American Plate moves westward while the Eurasian Plate moves eastward, Iceland will slowly be sliced in half. When water rushes in to fill the widening gap, this huge island of ice will form two smaller islands.

How far can divergent boundaries go? Well if we look at a time frame of 100 to 200 million years, we can easily spot the Atlantic Ocean. What is believed to have been a tiny inlet of water between the formerly merged Europe, Africa, and Americas has now evolved into this vast expanse of water.

You can read more about divergent boundaries here in Universe Today. Here are the links:

• Rising Temperatures Could Shut Down Plate Tectonics
• Continental Crust

There's more about it at USGS. Here are a couple of sources there:

• Divergent boundaries
• Hotspots: Mantle Thermal Plumes

Here are two episodes at Astronomy Cast that you might want to check out as well:

• Plate Tectonics
• Volcanoes, Hot and Cold

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Interesting Crater in Meridiani Planum. Credit: NASA/JPL/University of Arizona

Although the movement can only go by a few centimeters per year, the sheer mass involved introduces tremendous pressures and frictional forces. These in turn can allow some plate material to melt. It is therefore not surprising to find volcanoes in these regions. Furthermore, these parts are also very susceptible to earthquakes.

The Himalayas, which include our planet's tallest peaks – K2 and Mt. Everest, were formed when parts of the Indian Plate was subducted below part of the Eurasian plate. Another bi-product of this particular convergence is the Tibetan Plateau a.k.a. "the roof of the world".

If K2 and Mt. Everest are our world's tallest natural peaks, its deepest part is the Mariana Trench in the Pacific Ocean. While humans have repeatedly conquered Mt. Everest and K2, the usual conquerors of the Mariana Trench are only deep sea robots. The trench is formed due to the subduction of part of the Pacific Plate, the world's largest plate, under the much smaller Mariana Plate.

Subduction typically happens when one of the approaching plates is denser than the other. Thus, the usual convergent boundaries are those portions of plates made of dense oceanic crusts and those less dense ones such as those of continental or other oceanic crusts. The denser plate of course gets subducted underneath the other.

Subduction can occur between an oceanic and continental plate, two continental plates, or two oceanic plates. When the two plates collide, their boundaries naturally buckle or one delves under the other. Hence, deep trenches and tall mountain ranges are formed.

Convergent boundaries, like its exact opposite – divergent boundaries, are just products of plate tectonics. It is believed that the lithosphere is subdivided into what are known as tectonic plates. You can imagine these plates as parts of a global jig-saw puzzle moving very slowly.

Right now, there are 8 major plates: the African, Antarctic, Australian, Indian, Eurasian, North American, South American, and Pacific plates. There is a greater number of minor plates with the most notable being the Arabian, Caribbean, Juan de Fuca, Cocos, Nazca, Philippine Sea, and Scotia plates.

You can read more about convergent boundaries here in Universe Today. Here are the links:

• Rising Temperatures Could Shut Down Plate Tectonics
• Mars Once Had Plate Tectonics

There's more about it at USGS. Here are a couple of sources there:

• Understanding Plate Motions
• Plate Tectonics – Juan de Fuca Ridge – Juan de Fuca Subduction

Here are two episodes at Astronomy Cast that you might want to check out as well:

• Plate Tectonics
• Volcanoes, Hot and Cold

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The current tectonic plates.

The reason that tectonic plates are able to move is the Earth's lithosphere has a higher strength and lower density than the underlying asthenosphere. Their movement is dictated by heat dissipation from the Earth's mantle. Lateral density variations in the mantle result in convection, which is transferred into tectonic plate motion through some combination of frictional drag, downward suction at the subduction zones, and variations in topography and density of the crust that result in differences in gravitational forces.

Tectonic plates consist of lithospheric mantle overlain by one of two types of crust material: oceanic crust (sima) and continental crust (sial). Average oceanic lithosphere is typically 100 km thick; its thickness is a function of its age: as time passes, it conductively cools and becomes thicker. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is a function of its distance from the mid-ocean ridge where it was formed. For a typical distance oceanic lithosphere must travel before being subducted, the thickness varies…6 km thick at mid-ocean ridges to greater than 100 km at subduction zones; for shorter or longer distances, the subduction zone thickness becomes smaller or larger, respectively. Typical continental lithosphere is typically 200 km thick, though this also varies considerably between basins, mountain ranges, and stable cratonic interiors of continents. The two types of crust also differ in thickness, with continental crust being considerably thicker than oceanic. The difference being 35 km vs. 6 km.

The tectonic plates have played a major role in the formation of the Earth as we know it and will continue to do so for the entire existence of this planet. At the same time those plates have wreaked havoc on human dwellings and lifestyle by causing earthquakes, tsunamis, etc. Ah, mother Earth is our sustenance and our bane.

There is a very in depth article on plate tectonics here. Universe Today has a good article on the effects that global warming may have on the tectonic plates and another about the plate tectonics of Mars. Astronomy Cast offers a great episode on the tectonic plates.

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A geovent

Geothermal heat is a growing market throughout developed countries. It accounts for .07% of the worlds energy. The market will continue to grow rapidly due to recent advances in geothermal heat pump technology. While geothermal accounts for a very low percentage of total world usage, it has many applications. In Iceland, geothermal heat is pumped under roads and walkways to melt the snow. The same application is used in Boise, Idaho. In a process known as district heating, entire groups of buildings are heated by the same geothermal field.

Some parts of the world, including substantial portions of the western USA, are underlain by relatively shallow geothermal resources. In these areas, water or steam may be captured from natural hot springs and piped directly into heat exchangers. Alternatively, the heat may come from waste heat supplied by co-generation from a geothermal electrical plant or from deep wells into hot aquifers. Direct geothermal heating is far more efficient than geothermal electricity generation and has less demanding temperature requirements, so it is viable over a large geographical range. If the shallow ground is hot but dry, air or water may be circulated through earth tubes or downhole heat exchangers which act as heat exchangers with the ground.

Geothermal heat can be extracted for relatively cold ground using geothermal heat pumps. Closed loop geothermal heat pumps circulate a carrier fluid (usually a water/antifreeze mix) through pipes buried in the ground. As the fluid circulates underground it absorbs heat from the ground and, on its return, the now warmer fluid passes through the heat pump which uses electricity to extract the heat from the fluid. The re-chilled fluid is sent back through the ground thus continuing the cycle. The heat extracted and that generated by the heat pump appliance as a byproduct is used to heat the house. The addition of the ground heating loop in the energy equation means that more heat is generated than if electricity alone had been used directly for heating. Switching the direction of heat flow, the same system can be used to circulate the cooled water through the house for cooling in the summer months. The heat is exhausted to the same relatively cool soil (or groundwater) rather than delivering it to the hot outside air as an air conditioner does. As a result, the heat is pumped over a greater temperature difference and this leads to higher efficiency and lower energy use.

The many uses of geothermal heat boggles the minds of people who are interested in “green” energy and nearly everyone else who thinks about it. There is some more information about plate tectonics on Astronomy Cast as well as an interesting article on the effects of global warming on plate tectonics here on Universe Today.

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