*The Earth is the fifth largest of the planets in the solar system, ranking below all the gas giants, but above all the rocky planets and the ice planets.  Like all the planets except Mercury and Venus (and the possible tenth planet), Earth has a moon, and ours is larger in proportion to the planet than any other moon (except Pluto’s moon, Charon), and this affects Earth significantly, most notably by causing tides.

*Earth itself is not quite a perfect sphere; it is 24,900 miles around the equator, but 24818 miles around the poles.  This is caused, in part, by the rotation of the earth on its axis, which tends to spread the mass of the earth towards the centre as it spins.

*About 70% of the Earth is the hydrosphere (the watery part) and 30% the lithosphere (the land).  The atmosphere extends about 1000 miles from the surface, although almost all the mass of the gases in it are within 6 or 7 miles of the surface (on average—it is less so near the poles and more so near the equator).  This part, where most weather and other activity takes place, is called the troposphere.  All the life on Earth makes up the biosphere.

*Earth’s landmasses make up seven continents and thousands of islands.  Continents are cultural and historical constructs as much as they are geological.  Europe and Asia are, in many ways, one continent, but they have been historically treated as two.  Some people regard North and South America as one American continent (this is particularly popular in Latin America).  List the seven traditional continents.  Parts of all the continents extend underwater as shallow areas of sea near the coastlines.  Such an areas is called a continental shelf.

*Earth’s natural features are called landforms.  These include such things as mountains, plateaus, plains, rivers, and valleys.

*Earth’s highest mountain is Mount Everest, whose top is 29,035 feet (5½ miles) above sea level.  The lowest point on land is the shore of the Dead Sea on the border between Israel and Jordan.  It lies 1,349 feet below sea level.  The deepest point known to man is the bottom of the Mariana Trench, a vast underwater chasm, near Guam in the Pacific Ocean.  It is 35,827 feet (6¾ miles) deep.

*Landforms are created by the building up and breaking down of the Earth’s surface.

*The Earth’s surface is built up by volcanic activity and tectonic activity. 

*Volcanic activity builds up the Earth simply by depositing lava and cinders.  If this happens with enough intensity or happens for long enough, it can create mountains (like Mount Saint Helens) and even islands (like Iceland and Hawaii).

*Tectonic activity refers to the movement of vast plates of the lithosphere on top of an ocean of magma (hot liquid rock) that lies below the Earth’s surface.

*The part of the Earth that we see is just the crust, a layer of rock (with a very thin layer of life on top).  The crust is as thin as two miles thick at the bottom of the oceans, and as thick as 75 miles in some mountain chains.

*The crust mainly consists of three types of rock:  igneous (e.g. granite, obsidian), sedimentary (e.g. sandstone, shale, limestone), and metamorphic (e.g. marble).

*The crust sits on top of the mantle, a thick layer of hot, dense rock.  The elements that make up the mantle slowly shift around as those closer to the centre of the Earth heat up and those near the surface cool down.  The mantle floats on the outer core of the Earth, which due to the intense heat generated by the pressure of the mantle on tope of it, is liquid rock.

*At the very centre of the Earth is the inner core, which, although hot enough to be liquid (it is believed to be over 8,500 degrees Fahrenheit in the core), is kept solid by the intense pressure on it.

*The crust of the Earth is broken into plates that slowly shift around on a layer of magma in the mantle, as the mantle shifts around on the core.  Page 39 of the book shows the plates and their general pattern of movement.  Plates move at different velocities, The African plate moves about 25 mm per year, whereas the Australian plate moves about 60 mm per year. 

*It is believed that all the continents may have once been part of a supercontinent, known as Pangaea (Greek:  ‘all lands’), that slowly drifted apart over the past 500 million years or so.

*As the plates move, they slide and bump against each other, and sometimes slip over or under one another.  These plate tectonics create many landforms.

*Sometimes two plates collide.  Sometimes, one plate will be forced deep into the Earth, where it will melt, and the magma will be forced up in volcanic activity.  This process is called subduction, and it usually happens when a sea plate meets a land plate; the Andes were created this way. 

*Other times, the sea plate will be pushed under the land plate more slowly, and will pile up debris that eventually rise above sea level and add to the continent; this is called accretion.

*Rather than forcing one plate under the other, the collision of plates will sometimes simply force one or both to fold or break.  Folding helped form the Appalachian Mountains, and can still be seen in the geology around here, where the layers of sedimentary rocks were bent.

      

*Sometimes, plates pushing against each other will cause the land to break.  This is called a fault.  In reverse faulting, one slab of land is forced over another.  At other times, the pieces will slide by each other, although even this causes massive earthquakes (as along the San Andreas Fault).  This is called transform faulting.

*Other times, plates separate.  This process is simply called spreading.  As it happens, magma comes up through the cracks created in the spreading earth, and eventually forms underwater mountain ranges and even islands.  One such underwater mountain range is the Mid-Atlantic Ridge; Iceland was formed by the same forces.

*Plates can also separate at a fault; this is called normal faulting.  It is also called Horst and Graben faulting, from the German for cliffs and valleys.
 

*Faulting (and other tectonic activity) can result in earthquakes.  Subduction (and other tectonics) can allow magma to spew from the earth, creating volcanoes.  Both are very common around the edge of the Pacific plate (which is ringed with islands created by plate tectonics).  Because this area experiences so many earthquakes and volcanic eruptions, it is sometimes known as the Ring of Fire.

*It is also possible for a plate to slide over a volcano that does not move.  Such a volcano is sometimes called a hot spot.  As the plate passes over the hot spot, it periodically erupts, leaving masses of dried lava in a row.  Hawaii was formed this way.  Old Faithful in Yellowstone is also fuelled by a hot spot.

*So volcanoes and plate tectonics help build up land, but landforms are also created and shaped by things that wear down the land.  These are principally grouped as types of weathering and erosion, but sometimes the land is altered by a catastrophic change.  Mount Saint Helens eruption is an example.  It has also been suggested that at some point the Black Sea and the Mediterranean were not connected, but that a flood (possible caused by melting glaciers) caused the Mediterranean to overflow into the Black Sea, linking them (and changing the Black Sea from a freshwater lake to a saltwater sea).

*Weathering is not the same as erosion, although they are similar.  Weathering is the breakdown of rocks in place, either through expansion and contraction of ice or plant roots in them in them, or through chemical change, like water dissolving limestone.

*Erosion is the degradation of rock by things that come into contact with it, such as rain, wind, glacial ice, or the pull of gravity on it.

*Wind erosion is worst in dry places where there is not much vegetation.  The wind picks up loose dirt and dust, and blows it away.  This can destroy an area agriculturally; in the 1920s and 1930s, parts of the US Midwest, which had been farmed too much, then exposed to drought, dried up and blew away, in a phenomenon called the Dust Bowl.  On the other hand, such rich soil that is blown away can collect elsewhere.  Rich soil deposited in a new place by wind is called loess.

*Glacial erosion brings weight, friction from carried rocks, and the power of temperature change to bear in altering the landscape. 

*Draw a glacier, with all the junk in it, especially the stuff at the front.

*Glaciers carved out the Great Lakes and the many other lakes of Minnesota and Manitoba.  When they finally receded, glaciers also left great ridges of stones called moraines.  Glaciers also tend to deposit a lot of rich soil, which is what has made the US Midwest such a productive region.  In the mountains, glaciers carve out vast U-shaped valleys.

*Liquid water, however, is probably the most significant cause of erosion.  It begins when rain flows downhill.  Eventually it wears out the weakest rock and soil, and forms stream beds.  These may eventually deepen into rivers, which tend to have V-shaped valleys (making them different from glaciers). 

*Eventually most rivers flood (usually every spring), and that deposits sediment on the sides of the river.

*The best soil tends to collect in these valleys as it flows off the mountains, making bottom land some of the richest in any region.  Therefore, most of the early human civilisations grew up around major river valleys and their flood plains.