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Weathering and Erosion - What Happens to the Earth’s Surface Weathering is the process of either chemically or physically breaking down rock. Chemically weathered rock is chemically altered or dissolved. Air and water are often involved and form acids to dissolve and remove minerals from rocks. Carbon dioxide in the air dissolves in rainwater to form a mild acid that dissolves rocks. Limestone caverns are an example forming stalagmites and stalactites. Acids from bacteria and algae can dissolve rocks. Chemical weathering can weaken rocks and make them more susceptible to physical weathering, but these two processes do not always occur together. Physically weathered is when rocks are broken into smaller pieces through mechanical processes. Plate tectonics, frost wedging by ice, tree roots, and burrowing animals can break rocks.
Erosion is the process of moving smaller pieces of rocks or sediment from one area to another. Water, wind, ice, and gravity can move these sediments. Typically, the greatest agent, water in rivers moves sediment from upstream to downstream, and the water usually deposits or drops off that sediment into beaches, bays, gulfs, or oceans. This process is called deposition. Sediments deposit when the force of the moving water can no longer overcome the force of gravity. Weathering, erosion, and deposition can lead to changes in Earth’s surface by making landforms.
Glaciers are the primary example of ice as an agent of weathering and erosion, gouging out portions of rocks to create landforms like the Great Lakes, Nantucket Island, and valleys.
Seafloor Spreading - Earth’s Inner Heat Energy Starting It All
Seafloor spreading is one of the geological processes that is constantly changing Earth’s crust. The Theory of Plate Tectonics became widely accepted during the 1960s when scientists discovered seafloor spreading. Scientists were able to use new technology (echo sounding) to find ocean features such as trenches and mid-ocean ridges. Harry Hess proposed that at mid-ocean ridges, magma flows up from the mantle and forms new crust on both sides of the ridge. This new crust makes the ocean floor wider and pushes the continents surrounding it farther away from each other. When the two crusts meet, oceanic crust is subducted beneath the continental crust.
The mid-Atlantic ridge has been making new oceanic crust for millions of years, allowing the Atlantic Ocean to get bigger and pushing North and South America farther away from Europe and Africa. When oceanic crust and continental crust collide, the thinner yet denser oceanic crust dives below the thicker yet lighter continental crust forming a deep trench. As the oceanic crust sinks into the mantle it melts and the recycling process begins. As a result, volcanic mountain ranges form along the continental crust, and earthquakes are common in the area.
All surface features on the continent are a result of these tectonic processes. Heat in the inner Earth creates currents, or convection currents, of magma in the mantle. This movement results in seafloor spreading, plates diverging, converging, and subducting, and for the continents to move. This is the Pangea model of a supercontinent that spread out to the continents to where they are today. So we see that over time, weathering and erosion continues to reshape these surface features, while the continental crust and oceanic crust are continuing being recycled through the rock cycle. This whole process is a cycle and continues today.
Evidence for Pangea:
1. The continents match and fit by their shape.
2. The fossils, climate zones, and rock types match at the borders of the "puzzle" or plates.
3. The position of the continents today and the plates of Pangea are in the same position but "drifted apart."
Plate Tectonics Theory is supported by three great ideas:
GLOSSARY
Types of plate boundaries — There are three types of boundaries between lithospheric plates (Fig. 3):
Concepts:
Erosion is the process of moving smaller pieces of rocks or sediment from one area to another. Water, wind, ice, and gravity can move these sediments. Typically, the greatest agent, water in rivers moves sediment from upstream to downstream, and the water usually deposits or drops off that sediment into beaches, bays, gulfs, or oceans. This process is called deposition. Sediments deposit when the force of the moving water can no longer overcome the force of gravity. Weathering, erosion, and deposition can lead to changes in Earth’s surface by making landforms.
Glaciers are the primary example of ice as an agent of weathering and erosion, gouging out portions of rocks to create landforms like the Great Lakes, Nantucket Island, and valleys.
Seafloor Spreading - Earth’s Inner Heat Energy Starting It All
Seafloor spreading is one of the geological processes that is constantly changing Earth’s crust. The Theory of Plate Tectonics became widely accepted during the 1960s when scientists discovered seafloor spreading. Scientists were able to use new technology (echo sounding) to find ocean features such as trenches and mid-ocean ridges. Harry Hess proposed that at mid-ocean ridges, magma flows up from the mantle and forms new crust on both sides of the ridge. This new crust makes the ocean floor wider and pushes the continents surrounding it farther away from each other. When the two crusts meet, oceanic crust is subducted beneath the continental crust.
The mid-Atlantic ridge has been making new oceanic crust for millions of years, allowing the Atlantic Ocean to get bigger and pushing North and South America farther away from Europe and Africa. When oceanic crust and continental crust collide, the thinner yet denser oceanic crust dives below the thicker yet lighter continental crust forming a deep trench. As the oceanic crust sinks into the mantle it melts and the recycling process begins. As a result, volcanic mountain ranges form along the continental crust, and earthquakes are common in the area.
All surface features on the continent are a result of these tectonic processes. Heat in the inner Earth creates currents, or convection currents, of magma in the mantle. This movement results in seafloor spreading, plates diverging, converging, and subducting, and for the continents to move. This is the Pangea model of a supercontinent that spread out to the continents to where they are today. So we see that over time, weathering and erosion continues to reshape these surface features, while the continental crust and oceanic crust are continuing being recycled through the rock cycle. This whole process is a cycle and continues today.
Evidence for Pangea:
1. The continents match and fit by their shape.
2. The fossils, climate zones, and rock types match at the borders of the "puzzle" or plates.
3. The position of the continents today and the plates of Pangea are in the same position but "drifted apart."
Plate Tectonics Theory is supported by three great ideas:
- Alfred Wegener with his theory that continents were once connected as a supercontinent called Pangea and drifted apart over very long periods of time.
- American geologist Harry Hess’ theory that seafloor spreading at the mid-ocean ridge cause the plates to move and appear to “drift” and "float" and
- English geologist Arthur Holmes’ suggestion that the continents sit on top of a mantle of magma that flows in convection currents due to the heat from the inner layers of the Earth.
- Weathering and erosion carries rock sediments and deposits them often in layers.
- Compacting and cementing of sediments are chemically changed to Sedimentary Rock.
- Increased pressure and heat of sedimentary rocks will transform the rock into Metamorphic Rock.
- Deeper in the Earth, metamorphic rock will experience intense heat where melting into magma, then cooling will result in Igneous Rock.
- Intrusive rock cools inside the Earth, while extrusive rock cools outside (usually from lava).
- All rock types, can undergo weathering and erosion, and continue the cycle again. This is a simplest model as we know one rock type can enter more pathways to transforming into another type.
GLOSSARY
Types of plate boundaries — There are three types of boundaries between lithospheric plates (Fig. 3):
- convergent boundary — plates converge, or come together. If a plate of oceanic lithosphere collides with thicker and less dense continental lithosphere, the denser oceanic plate will dive beneath the continent in a subduction zone (Fig. 2).
- divergent boundary — two plates diverge, or move apart and new crust or lithosphere is formed.
- transform fault boundary — plates slide past one another with no creation or destruction of crus
- asthenosphere — a portion of the mantle which underlies the lithosphere. This zone consists of easily deformed rock and in some regions reaches a depth of 700 km.
- continental drift — The first hypothesis proposing large horizontal motions of continents. This idea has been replaced by the theory of plate tectonics.
- convergent plate boundary — a boundary between two lithospheric plates that move towards each other. Such boundaries are marked by subduction, earthquakes, volcanoes, and mountain-building.
- deep-sea trenches — long, narrow, and very deep (up to 11 km) basins oriented parallel to continents and associated with subduction of oceanic lithosphere.
- divergent plate boundary — a boundary between two plates that move away from one another; new lithosphere is created between the spreading plates.
- lithosphere — the rigid, outermost layer of the Earth; includes crust and uppermost mantle and is about 100 km thick.
- mid-ocean ridge — a continuous mountain chain on the floor of all major ocean basins which marks the site where new ocean floor is created as two lithospheric plates move away from one another.
- paleomagnetism — the permanent magnetization recorded in rocks that allows reconstruction of the Earth's ancient magnetic field.
- Pangaea or Pangea — the proposed "supercontinent" that began to break apart 200 million years ago to form the present continents.
- plate tectonics — the theory that proposes that the Earth's lithosphere is broken into plates that move over a plastic layer in the mantle. Plate interactions produce earthquakes, volcanoes, and mountains.
- reversed polarity — a magnetic field with direction opposite to that of the Earth's present field.
- transform plate boundary — a boundary between lithosphere plates that slide past one another.
- sea-floor spreading — a hypothesis, proposed in the early 1960s, that new ocean floor is created where two plates move away from one another at mid-ocean ridges.
- subduction zone — a long, narrow zone where one lithospheric plate descends beneath another
Concepts:
- Patterns of change
- New seafloor is created by the upwelling of magma at mid-ocean spreading centers
- old ocean floor is destroyed by subduction at deep sea trenches.
- Magnetic fields preserved in the oceanic magma provide evidence for seafloor spreading and plate tectonics
- Convection, which results in the rise of lighter molten lava, is the mechanism for plate movement. topography of the ocean floor is similar to those of land.
