SOIL FORMATION

Weathering and Erosion

Soil Layers  Soil is composed of 4 layers typically:  O horizon Top layer of organic matter  A horizon Weathered rock mixed with rich organic material  B horizon Enriched by deposited minerals  C horizon Little/no organic matter

PLATE TECTONICS

Continental Drift  If you look at a map, you can see that some shapes look like they could fit together like a puzzle.  German meteorologist Alfred Wegener thought that the fit of the continents wasn’t just a coincidence.

Pangaea  Wegener suggested that all the continents were joined together at some time in the past.  In 1912, he proposed the hypothesis of continental drift.  According to this hypothesis, continents have moved slowly to their current locations.  He suggested that all continents once were connected as one large landmass that broke apart about 200 million years ago. Pangaea

A controversial idea  Wegener’s idea about continental drift were controversial.  It wasn’t until long after Wegener’s death in 1930 that his basic hypothesis was accepted.  He was unable to explain exactly how the continents drifted apart.

Fossil Clues  Besides the puzzle like fit, fossils provided support for continental drift.  Fossils of the reptile Mesosaurus have been found in South America and Africa. This swimming reptile lived in freshwater and on land How could fossils be found on land separated by huge amounts of SALT water? Wegener hypothesized that this reptile lived on both continents when they were joined.

Rock Clues  If the continents were connected at one time, then rocks that make up the continents should be the same in locations where they were joined.  Similar rock structures are found on different continents. Parts of the Appalachian Mtns are similar to those found in Greenland and Western Europe. Rock structures found from eastern South America and western Africa are similar

How could continents drift? PangaeaLaurasia and Gondwanaland

How continents drift JurassicCretaceous

How continents drift.

SEAFLOOR SPREADING

Is the seafloor spreading?  The magma carries the seafloor away from the central ridge in both directions.  New seafloor is continuously being created.  Older sea floor is pushed away from the central ridge.

Evidence for spreading  In 1968, scientists began studying rocks on the seafloor.  They took rock samples from the mid-ocean ridges.  They also took rock samples farther away from the ridge.  They found that rocks near the mid-ocean ridge were the youngest rocks.  Rocks farther away from the ridge were older.

Evidence for spreading  According to Hess’s theory of seafloor spreading, the seafloor near the ridge has formed more recently from magma.  The older seafloor is pushed away from the ridge.

THEORY OF PLATE TECTONICS

Plate Movements  In the 1960’s, scientists developed a new theory that combined continental drift and seafloor spreading.  Called the Theory of Plate Tectonics. Theory states – Earth’s crust and part of the upper mantle are broken into sections. These sections are called plates, and move on a plastic like layer of mantle. Similar to rafts on water.

Composition of Earth’s Plates  Plates are made of the crust and part of the upper mantle.  These two parts combined are the lithosphere.  This rigid layer is about 100 km thick and generally less dense than material underneath.  The plastic like layer below the lithosphere is called the asthenosphere.

Earth’s Plates

Plate Boundaries  When plates move, they interact in several ways.  They move towards each other – Converge  They pull apart from each other – Diverge  They slide alongside one another – Transform

Plate Boundaries

Divergent Boundary  Two plates that are moving apart.  Similar to seafloor spreading.

Convergent Boundary  Plates moving together.  Oceanic plates are denser than continental plates.  As the plate descends, molten rock forms and rises toward the surface, creating volcanoes.

Convergent Boundary  When two continental plates move towards each other they create mountains.  Because both plates are less dense than the asthenosphere.  The two plates collide and crumple up.

Transform Boundary  Two plates slide past each other  Move in opposite directions  OR in the same direction, just at different rates  When one plate slips and moves suddenly, an earthquake occurs.

Causes of Plate Tectonics  Many new discoveries have been made about Earth’s crust since Wegener’s day, but one question still remains.  What causes the plates to move?  Scientists now think they have a good idea.

A View of Earth

Convection Inside Earth  Soup that is cooking in a pan on the stove contains currents caused by an unequal distribution of heat in the pan.  Hot, less dense soup is force upward by the surrounding, cooler, denser soup.  As the hot soup reaches the surface, it cools and sinks back down into the pan. This entire cycle of heating, rising, cooling and sinking is called a convection current.

Convection Current  A version of this same process, occurring in the mantle is thought to be the force behind plate tectonics.

Mountains and Volcanoes  Oceanic plates are less dense than continental plates.  The pressure from the oceanic plate under the continental plate pushes magma to the surface.