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  • What is the largest delta on Earth, formed by confluence of three major rivers?

    Posted on November 16th, 2010 admin No comments

    First correct answer was from @paulsmoffett:  The Ganges Delta is the largest delta on Earth.

    The Ganges Delta is formed by confluence of the Ganges, Brahmaputra, and Meghna rivers, and some smaller streams.  Depending on how you look at it, the Ganges and Hooghly might look like one river, and the Brahmaputra and Meghna as one other river.  The Meghna River is actually a combination of a major distributary of the Ganges and a major distributary of the Brahmaputra.  It’s all tangled.  The Ganges and Brahmaputra have clearly separate headwaters, but as they approach the Bay of Bengal, they seem to turn into one huge braided river with many different names.  And they all empty into the denser braids of the Ganges Delta.

    One reason the Ganges Delta is so large is that so many rivers contribute to it.  Another is that each river has a heavy silt load compared to most rivers on Earth.  Most of the rivers feeding the Ganges Delta drain large areas of the rapidly eroding Himalayas, providing rich silt to the delta.

    The Ganges Delta encompasses huge wilderness areas, including the largest mangrove swamp in the world, the Sundarbans.  It also supports nearly 150 million people with its rich soil and fisheries.

    The Ganges Delta comprises much of the nation of Bangladesh, and much of the state of West Bengal, India.

    The Ganges Delta is subject to frequent flooding due to the hydrology of the wide, shallow Bay of Bengal directing typhoons to it.  Even lesser storms on the Bay of Bengal can cause serious flooding.  Flooding in the Ganges Delta is likely to become much worse in the future.  Not only is the sea rising due to global warming, the land around the Ganges Delta is subsiding as a result of the continuing tectonic collision of the Indian Subcontinent with Asia.

    More about the Ganges Delta:

  • Term for a large intrusive mass of igneous rock which solidified deep under the surface? e.g. Stone Mountain, GA.

    Posted on September 17th, 2010 admin No comments

    Hint 1:  “Large” means bigger than a dike.  2:  @mlv’s laccolith is one type, not the more general name.

    There are two acceptable answers to this.  The first was from @mlv:  A large intrusive mass of igneous rock which solidified deep within earth is called a “pluton.”  The second correct answer ws from @hdrx_engr:  It can also be called a “batholith.”

    The term “pluton” is a bit more general.  it includes laccoliths, but batholith does not.  Typically, a “laccolith” is a mountain, but a “batholith” may be a mountain range.  Also, a laccolith is a very particular type of intrusion, usually not very far below the surface, where magma intruded between two older layers of rock and deformed the one above into a dome.  The resulting pluton is more or less flat on the bottom and domed on top.

    The term “batholith” is generally reserved for formations aggregated from more than one intrusion event.  Many laccoliths, dikes, and other single-intrusion formations may merge together to form a gigantic batholith.

    Examples of well known batholiths and/or plutons include Georgia’s Stone Mountain and the vast Sierra Nevada Batholith (of which Yosemite is part).  As these examples illustrate, we become aware of the existence of batholith/plutons when the overlying layers of rock erode away and expose the igneous rock.  There are certainly many, many undiscovered plutons buried within the earth even now.

    A few tweeters responding to this seemed a bit confused by the difference between the formation versus the substance of which it is formed.  Magma cools within the earth to form a “batholith” composed of “intrusive igneous rock.”  “Batholith” (or “pluton”) is the formation, and “intrusive igneous rock” is what it’s made of.

    More about plutons and batholiths:

    About Stone Mountain:\

    About the Sierra Nevada Batholith:

  • What is the term for a circular formation of intrusive igneous rock surrounding an ancient volcano?

    Posted on September 15th, 2010 admin No comments

    No answers quite correct on this one.  A circular formation of intrusive igneous rock surrounding an ancient volcano is a ring dike.

    A ring dike forms as a volcano “runs out of gas,” and magma penetrates cracks around it but does not reach the surface.  A ring dike is often exposed when the softer extrusive igneous rock erodes away.  Well known examples of ring dikes include New Hampshire’s Ossipee Mountains and Pawtuckaway Mountains.

    Honorable mention to @mlv.  A laccolith is intrusive igneous rock, often circular, located near ancient volcanos, but not “surrounding” the volcano.

    A laccolith forms when magma intrudes between two horizontal layers of rock, lifting the upper layer but spreading flat on the lower layer.

    Several people answered “caldera.”  The caldera is the hole atop a volcano.  Perhaps they meant the rim of the caldera.  The rim of a caldera is extrusive igneous rock, not intrusive.

    The rim of a caldera is relatively recent, and nothing of the caldera remains of a truly ancient volcano on earth.  It erodes away.  A caldera may endure for eons on a body like the moon, where there is very little erosion.

    To understand the difference between a ring dike and the rim of a caldera, it is important to understand the difference between “intrusive” and “extrusive” igneous rock.  Extrusive igneous rock solidifies from lava cooling on the surface.  What we see of a geologically recent volcano is extrusive rock.  Intrusive igneous rock solidifies from magma cooling within the earth without reaching the surface.  It becomes visible due to the erosion of overlying rock strata.  Because extrusive rock cools quickly, its crystals tend to be finer.  Or, it may have no crystal structure at all, forming volcanic glass.  Slow-cooling intrusive rock has coarse crystals, e.g. granite.

    More about ring dikes:

    And some examples of ring dikes:

    And more about laccoliths:

  • Why was so little plant material decomposed during the Carboniferous Period, yielding today’s fossil fuels?

    Posted on August 12th, 2010 admin No comments

    First correct answer was from @rozberk:  In the Carboniferous Period, plants had evolved lignin (wood resin), but bacteria had not yet evolved to digest it.

    During the Carboniferous Period, plants did not produce wood, but many had very thick lignin-rich bark which provided support for tall plants.  This lignin-rich material forms much of the petroleum, coal, and natural gas that we burn today.

    Many modern plants, of course, produce wood that can support their weight internally.  The bark of a modern tree is just for protection from the elements, insects, and infection.  It does not support the weight of the tree.

    Even today, some 360 million years after the first appearance of lignin, few animals or bacteria can digest it.  However, enough can digest it that there are no longer such huge deposits of undecayed plant material as there were in the Carboniferous Period.  Yes, in certain environments today, dead plants can become peat and possibly even coal, but the huge deposits of the past will never happen again.

    More about lignin and its importance in the formation of fossil fuels: and

  • Why are the peaks in the northern Appalachian Mountains generally lower than those in southern Appalachians?

    Posted on August 11th, 2010 admin No comments

    No correct answers to this one:  Peaks are higher in the Southern Appalachians because ice age glaciers never reached there.

    Peaks in the Southern Appalachians were not subject to erosion by continental ice sheets during the last 2 million years of ice ages.

    The tallest peak in the northeastern United States is Mount Washington in New Hampshire, at 6,288 feet above sea level.  The tallest peak in the eastern U.S. is Mount Mitchell in North Carolina, at 6,684 ft.  That might not seem like much of a difference, but consider this:  Northeast of Virginia, only Mountt Washington exceeds 6,000 feet.  South of there, more than 6 peaks exceed 6,000 feet.  The Southern Appalachians also have more 5,000 footers and more 4,000 footers than the Northern Appalachians.

    About 12,000 years ago, Mount Mitchell towered above the Southern Appalachians much as it does today.  At the same time, Mount Washington was completely hidden under a featureless sheet of ice.  Some estimates say that the ice sheet was as much as 4,000 feet higher than the height of Mt. Washington today, though other estimates suggest that the highest peaks of the Northern Appalachians may have risen above the ice.

    Also, there may have been montaigne glaciers in the Southern Appalachians during the coolest parts of the glacial periods, but the continental ice sheets never reached south of Virginia.

    More about the Appalachians: and

  • New World/Old World monkeys are said to have diverged when some drifted across Atlantic on vegetation. How possible?

    Posted on July 19th, 2010 admin No comments

    No answers to yesterday’s question.  Primitive monkeys survived drifting across the Atlantic Ocean 40 million years ago because Atlantic was smaller.

    When the divergence of New World/Old World monkeys occurred  about 40 million years ago, the Atlantic was nowhere near as wide as it is now.  Granted, it was no picnic for monkeys on drifting clumps of mangrove trees to drift across a thousand miles of open ocean, but a few survived.  Certainly many more monkeys and other animals died drifting in the early Atlantic than survived to cross, but it only takes a few survivors to start a new population, and that is what scientists believe happened.

    Depending on whose estimate of seafloor spreading rates you believe, the Atlantic is a good 15 feet wider now than when I was born.  I wouldn’t want to cross those extra 15 feet drifting on a clump of vegetation.  Now, I have crossed the Atlantic a couple of times on a U.S. Navy ship, …  That wasn’t exactly a picnic, either.

    More about how monkeys crossed the Atlantic:

  • A sand dune erodes in the wind revealing a tube of glass, smooth inside, rough sand grains outside. what is it?

    Posted on July 8th, 2010 admin No comments

    First correct answer was from @rozberk:  Natural glass tubes in sand are caused by lightning.  @rozberk was also the first with the name:  They’re called fulgurites.

    Fulgurites are caused by lightning fusing the sand into glass as it passes through.  The blast of expanding air and vaporized sand leaves the fulgurite hollow.  The outer surface retains the texture of sand grains which did not melt completely in the rapid formation of the fulgurite.

    I find surprisingly few good pictures of fulgurites online.  It’s not just a short tube of sand, it’s frozen lightning!  When lightning reaches water table under sand, the fulgurite branches out into level “feet” that the fulgurite can stand on once it is excavated from the sand.

    The Academy of Natural Sciences Natural History Museum in Philadelphia has an excellent collection of fulgurites, or had when I was a kid.  There were several specimens that stood over 2 feet tall on their crooked, branching feet, and I vaguely recall one or two standing over five feet tall.

    The pictures of fulgurites I find online are pathetic compared to my memory of those fulgurites in Philadelphia’s Natural History Museum.

    Next time I’m in Philadelphia, I’ll make a point of getting pictures of the fulgurites in the Natural History Museum and posting them online.

    The best time to find fulgurites is not after thunderstorm, but after windstorm.  Many might form during a thunderstorm, but they’re hidden within the sand dunes.  After a windstorm erodes the dune, the fulgurite is revealed.

    More about fulgurites:

  • Type of metamorphic rock similar to granite, but with various minerals arranged in bands rather than random crystals.

    Posted on June 1st, 2010 admin No comments

    First correct answer was from @paulsmoffett.  Metamorphic rock similar to granite but with banded minerals is gneiss (homonym of “nice”).

    Gneiss may be metamorphosed from igneous rock, such as granite, or from sedimentary rocks.  Some coarse-grained granite gneiss contains “eyes” (German “augen”), which are coarse grains of feldspar mingled in with the bands.

    Much of the bedrock in my area (NH) is ancient augen gneiss, and the glacial till includes all kinds of rock, including some gneiss.

    Gneiss boulder with clear black and white bands

    Gneiss boulder with clear black and white bands

    The boulder pictured above has very neatly defined bands of black and white, especially visible near the lower right.  (The wide dark band perpendicular to the others is the shadow of the footbridge I was standing on.)  This boulder is not “native” to this place, but part of the glacial debris.  It is located along the Arethusa Falls Trail in Crawford Notch State Park, New Hampshire.

    Elephant Head is a gneissose granite outcrop

    Elephant Head is a gneissose granite outcrop

    Elephant Head, pictured above, is a large outcrop of gneissose granite at the top of Crawford Notch in Crawford Notch State Park, New Hampshire.  It’s also a great hike for small children.

    More about gneiss:

  • What type of seismic event is most often recorded by the United States Geological Survey?

    Posted on May 26th, 2010 admin No comments

    No correct answers to this one.  Guess it was too obscure.  The type of seismic event most often recorded by USGS is mine blasts.

    Mine blasts that register on seismometers include planned explosions, planned and unplanned mine roof collapses, and unplanned rockbursts.  U.S. Geological Survey registers as many as 50 mine blasts every day.

    More about mine blasts:

    And an audio, with transcript, about non-earthquake seismicity:

  • In terms of plate tectonics, volcanoes form at 3 (arguably 4) kinds of places. What are they?

    Posted on May 18th, 2010 admin No comments

    First correct answer was from @rivrchik:  Volcanoes form at hotspots, subduction zones, and mid-oceanic ridges.

    “Hot spots” are also known as mantle plumes.  Examples:  Yellowstone, Hawaii.

    Subduction zones are also known as convergent plate boundaries.  Examples:  All Pacific “Ring of Fire” volcanoes.

    Mid-oceanic ridges are examples of divergent plate boundaries.  Example:  Mid-Atlantic Ridge.

    Sometimes “mid-oceanic ridges” are classed separately from rifting zones within continental plates, sometimes together.  The thought behind classifying them together is that the continental plate involved is in the process of breaking into two plates.  Other than the few examples of continental plates in the process of breaking up, there are no current examples on earth of a divergent plate boundary between two continental plates.  All boundaries between well-distinguished continental plates are convergent (e.g India crashing into Asia, Africa crashing into Europe).

    Examples of rifting zones within continental plates, where one continental plate is breaking into two or more include the  East African Rift (generating volcanoes such as Kilimanjaro, Mt. Kenya, Ngorongoro Crater), and the Baikal Rift.

    Volcanoes at rifting zones within continents are called “non-hotspot intraplate volcanism.”

    Subduction zone volcanoes tend to be taller, high-viscosity “stratovolcanoes” which often explode violently.  Mantle plumes and divergent plate boundary volcanoes tend to be low-viscosity “shield volcanoes” which are not very explosive by comparison.

    Iceland, uniquely, is thought to be a mantle plume superimposed on a divergent plate boundary.

    More about volcanoes: