hey everyone it's nicholas i'm going to show you a bunch of incredible rock formations found here
in the united states and i want you to tell me what they all have in common ready here goes
devil's postpile california valley of fire nevada grand canyon arizona goblin valley utah shiprock
new mexico makoshika montana devils tower wyoming chimney rock nebraska monument rocks kansas
and mammoth cave kentucky ok if you said these rock formations are all beautiful well yes they
certainly are and if you said that a lot of them seem to be named after fire goblins or the devil
well yep i guess you'd be right there too but there's something about the way they were formed
that's responsible for all these astonishing monuments to time what is it well join me as
we explore the powerful forces on the surface of the earth known as weathering and erosion
let's take a look at the grand canyon arguably the most famous canyon in the world attracting
around 5 million visitors every year and appearing on nearly everyone's list of things to see before
they die if you haven't seen the grand canyon think of it as a place you haven't seen yet
because i'd like to think that one day you will and when you do there's a good chance
you'll agree with explorer john wesley powell who said the wonders of the grand canyon cannot be
adequately represented in symbols of speech nor by speech itself so how did this place
that's too wondrous to describe come to be the havasupai people who've been living in the grand
canyon for more than 800 years tell a story that explains both the creation of the grand
canyon and the origin of their people it goes like this there were two gods one of goodness
and one of evil the god of goodness had a daughter he hoped would be the mother of all living things
the god of evil didn't want that to happen so he flooded the world the good god made a boat from a
hollow tree and placed his daughter in it to save her when the flood finally receded mountains rose
up and rivers appeared one of which cut through the mountains and created the grand canyon
when the land dried the daughters set foot on a new empty earth then the sun rose up and caused
her to conceive soon she gave birth to a baby boy and later a waterfall caused her to conceive again
and she gave birth to a baby girl from the union of these two children came all the people on earth
the first of these people were the havasupai then the apache hualapai hopi paiute and navajo
all tribes with origins in and around the grand canyon now a geologist telling the origin story of
the grand canyon might say it began two billion years ago with the formation of igneous rock
which is rock formed within the earth from cooled and crystallized magma plus metamorphic rock which
is rock that's changed from its original state due to high heat and pressure along with a top layer
of sedimentary rock which is rock composed of many smaller rocks formed near the earth's surface
then between 70 and 30 million years ago the action of plate tectonics uplifted the region
and created the colorado plateau finally between five and six million years ago
the colorado river and its tributaries carved and widened the canyon through the powerful processes
of weathering and erosion which we'll talk about in a moment and which revealed the spectacular
layers of igneous metamorphic and sedimentary rock that we know today as the grand canyon
from a geographic point of view both the havasupai explanation and the modern geological explanation
have value because both help us understand the truth about this remarkable place geographers
are interested in what people in a particular area think and believe about where they live about the
origins of their place and about the process by which it became what it is today and this includes
myths legends history science art literature and everything else that makes up a place's culture
an astute geographer for example might notice similarities between the habasupai story
and the biblical stories of adam and eve and noah's ark and might therefore consider
investigating the possibility of cross-cultural influence between judeo-christian tradition
and native american tradition an astute geographer would certainly notice that both the havasupai
explanation and the modern geological explanation involved the process of water carving the grand
canyon out of the surrounding rock both stories point to the actions we know today as weathering
and erosion weathering is the breakdown of rocks soil and minerals at the earth's
surface into smaller pieces through contact with water air and biological organisms
the process of weathering over millions of years can reduce a mountain to pebbles
geologists identify two kinds of weathering mechanical and chemical mechanical weathering
is the process by which rock is physically broken down into continually smaller pieces producing
boulders stones pebbles sand dust and so on ultimately building the soil in a particular area
frost wedging is a common example of mechanical weathering when water freezes into ice and rock
crevices and then expands the crack widens and over time splits the rock frost wedging can
cause enormous parts of mountains to eventually break and fall away other examples of mechanical
weathering include tree roots growing in similar cracks rocks cracking due to forest
fires and rocks expanding and contracting in areas with extreme high and low temperatures
although mechanical weathering breaks down rocks it leaves their chemical structure unchanged this
is not the case with chemical weathering which alters iraq's chemical structure by changing
the minerals that form the rock this kind of weathering can actually change one kind of rock
into a completely different kind sometimes it can dissolve rock altogether as it did at mammoth cave
kentucky at mammoth cave carbon dioxide from soil and air combine with water to make carbonic acid
which dissolved large portions of limestone on the earth's surface and formed a massive cave system
over 405 miles long the world's longest another example of chemical weathering this one producing
very different results is acid rain which destroys forests pollutes water and eats away the surfaces
of stone structures and rock formations it occurs when chemicals produced by industrial pollution
or volcanic activity mix with water vapor and fall back to earth the leshan buddha in china
and the acropolis in athens are just two of the many world heritage sites endangered by acid rain
erosion the primary force that formed the grand canyon works hand-in-hand with weathering erosion
occurs when water wind or glaciers move weathered materials such as gravel soil and sand erosion
is critical to life on earth because it builds the soil that plants need to take root and grow
the largest canyons and valleys on earth were formed by moving water that over time cut into
rock and wore it away water also carries sediment small particles of soil sand and gravel that helps
grind away at the surface of the rocks over which it flows when the river or stream slows down the
sediment piles up and creates new landforms a broad flood plain or alluvial plain might form
on either side of the water or a low-lying plain known as a delta might form where the river enters
a lake or the ocean the mississippi river for example deposits millions of tons of sediment
each year along its enormous flood plain and in its delta where it empties into the gulf of mexico
wind is also a powerful agent of erosion particularly in areas with little water and few
plants to anchor the soil wind erosion devastated the great plains during the dust bowl of the 1930s
as the population grew and more land was plowed under huge swathes of land without plant life were
left exposed to the wind which carried away the top soil and huge dust storms leaving thousands of
american families with no food no livelihood and no choice but to migrate west to greener pastures
john steinbeck's prize-winning novel the grapes of wrath centered on the struggles of one such family
we mentioned glaciers earlier but they're worth mentioning again glaciers are huge slow-moving
sheets of ice containing dirt rocks and boulders that scrape away at terrain as the ice moves
over it during ice ages when the earth's temperature was much colder than it is now
much of the earth's water was frozen in massive glaciers that covered a third of the earth
glaciers carved out the great lakes and other smaller lakes in the northern part of the united
states and when they melted and receded left behind piles of ridge-like debris called moraines
some 50 million years ago there was a landscape of rolling hills and meandering streams in what
is now central california after a series of tectonic uplifts and the creation of a canyon by
erosion from the merced river the canyon was gouged and quarried into a u-shaped trough by
a series of glaciers when the last glacier melted some ten thousand years ago a spectacular site was
revealed yosemite valley one of the most visited and photographed natural landmarks in the world at
devil's post pile only a few miles from yosemite hot basalt lava flowing down a valley was met by a
receding glacier because of its thickness the lava cooled slowly and evenly into long symmetrical
basalt columns the glacier removed much of the rock and left behind columns bearing a polished
geometric surface that looks as if someone came through and laid perfectly hexagonal stone
flooring only no one did it appeared there as a natural result of glacial erosion as we move on in
our study of geography i want you to think about the powerful forces on the surface of the earth
that carve out landforms big and small famous and unknown over thousands of years these are the
forces that create the soil in which we grow our food the canyons and plains where we live and the
spectacular lakes and valleys where our families go to enjoy the wonders of the natural world
look around you do you see hills or valleys do you see evidence of weathering or erosion how
is the landscape formed where you live keep asking these questions and keep exploring