Earth’s Internal Processes
Wednesday, July 8, 2026 3:57 PM
Have you ever looked at a map of the world & noticed how closely the west coast of Africa mirrors the east
coast of South America? If you cut them out & push them together, their coasts fit together like the pieces
of a jigsaw puzzle
- In 1912, a German geophysicist, Alfred Wegner, presented the theory of continental drift, tracing the past
positions & motions of the continents. He suggested these continents were all once assembled into a
single super continent named Pangaea (aka "all the land" in Greek)
○ He suggested that 200 million years ago, Pangaea began to drift & break into smaller
blocks drifting apart ever since, unveiling the shape of our modern continents. He couldn’t prove
how the continental crust moved, so the theory went unaccepted until after WWII
§ What evidence he did collect, did support his theory. However he found that South America,
Africa & Australia share some of the same types of sequential layers of sedimentary rocks
§ Since sedimentary rocks are made from sediment that has been eroded from a particular area &
transported downstream or down wind, this means these continents may have been assembled
as one
□ They also possess many of the same types of fossils, indicating similar plants & animals
appeared during the same geological time periods. They show distinctive glacial scouring
marks on rocks of the same age, suggesting the continents were formerly united & covered
by a glacier some 250 million years ago
Theory of Sea Floor Spreading
- In the 1950s & 60s, better exploration of the sea floor & satellite mapping allowed the topography of the
ocean floor to be revealed. It showed features like mid-ocean ridges (sites of spreading centers) & deep
arching trenches (sites of subduction zones)
- The intense heat of earth’s interior (from decay of radioactive elements in the core & mantle) is
conducted toward the surface thru the mantle & when it reaches the asthenosphere, this heat generates
slow convection currents within the molten rock material
○ This causes continents & oceans to drift along the asthenosphere, carried by convection currents like
ice floating on a stream. This new crustal material is being pushed up along mid-ocean ridges, driven
by heat, which forces the molten magma upward, pushing the existing lithosphere away (both
continental & oceanic crusts)
- In the 60s & 70s, geologists discovered the magnetic field of earth periodically reverses itself (about every
500K yrs). As molten rock wells up from the ridge & freeze, magnetite crystals in the iron-bearing rock
sets in a certain way, lining up with the earth’s magnetic field
- Mapping of magnetic alignment of these magnetite crystals in the ocean crust showed matching sets of
rocks on each side of the spreading centers. B/c the magnetic field reverses every now & then, bands of
new material set in alternate directions, creating a ”bar code” containing the history of sea floor
spreading
○ This evidence proved the basaltic rock on the ocean floor is most recent near ridges & increases in
age w/ distance away from the ridges, on both sides. The rate of seafloor spreading varies from
1-2 inches a year, comparable to the rate of fingernail growth (1.5 inches/year)! W/ the discovery
of sea floor spreading, the Wegner's theory of continental drift is justified.
Lithospheric Plates
- It’s believed the earth’s surface is broken into 7 major plates & dozens of smaller ones, that slowly
move like ice on a river. These thin plates (40 miles deep) are thought to be floating on the
atmosphere's upper mantle, the asthenosphere is slowly deformable. The 7 major plates somewhat
correspond to the major continents or ocean basins
- These plates don’t represent one continent, they are incorporated w/ the surrounding ocean floor into
a plate. These plates collide, separate, or slip past on another. This entire process of plate formation,
movement & destruction, accepted in the late 50s early 60s, is called plate tectonics. It’s at the edges,
or boundaries of these plates where we have the most dynamic earth-shaping forces operating &
includes earthquake activity, mountain forming & volcanic activity
Three Types of Plate Boundaries:
1) Spreading Centers aka Divergence
- Plates move apart from one another at "divergent" boundaries, where new crustal material is
introduced from the asthenosphere, the most common kind of divergent plate boundary is the oceanic
ridge (mid-ocean ridge)
- New basaltic ocean floor is being created w/ rising magma, a long line of under water volcanoes form
along these ridges, Iceland is a recently formed volcanic island that sits on the mid-Atlantic ridge &
continues to spread apart, but at the same time, is being built up by active volcanic eruptions
- Sometimes, divergence occurs within continental lithosphere. When it does, land is being pulled apart
from both sides & drops downward to create a valley. This is currently happening in eastern Africa, a
region called the East African Rift Valley
○ The Arabian Peninsula/Plate, is pulling away from the rest of the continent of Africa (the African
Plate), forming the Red Sea, which continues to grow 1-2 inches wider each year! If rifting continues,
the so-called "Horn of Africa" will rift away from the rest of the continent the Arabian Peninsula
formed
§ In the rift zones, magma rises through the cracks in the continental lithosphere, sometimes to
erupt and form volcanoes
○ These volcanoes (either at the ocean floor or continental lithosphere), are considered to be gently
erupting volcanoes. They arent considered life-threatening in their eruptions, the biggest threat in an
eruption of a volcano associated w/ divergent boundaries, is that the magma is so hot, it is highly
fluid
○ This means it can travel miles before it cools & hardens. It floods over the surface, destroying
anything in its path. However, it doesn't erupt violently w/ ashes & cinders. This category of
volcanism is called basaltic volcanism
- These kinds of volcanoes resulted from basaltic volcanism are called "shield" volcanoes
○ They are broad sloping mountains, due to the distance that lava spreads over the surface before it
hardens & the mountains don't build up steep sides, they look like a warrior's shield
2) Subduction Zones aka Convergence
- Plates push against each other as they come together at convergent plate boundaries. Collisions of
plates result in a compression force, pushing 2 plates together, these boundaries are responsible for
mountain ranges, highly eruptive volcanoes & deep ocean trenches. There are 3 kinds of convergent
plate boundaries, depending upon the kind of lithosphere involved:
a) ocean to continent convergent boundary
○ If we could drain the pacific ocean, we would see long, narrow curving trenches thousands of
miles long & 5-7 miles deep, cutting into the ocean floor. These trenches are created when
oceanic crust & continental crust collide. The oceanic crust (sima), is denser crustal material, is
plunged downward beneath the continental crust (sial). A deep ocean trench (the deepest part of
the ocean floor) opens up where the descending plate plunges into the asthenosphere & melts
○ Plumes of molten rock rise up back to the surface through the shattered edge of the overlying plate
as molten magma to create huge volcanic mountain ranges
- This ocean-continent collision causes the formation of major mountain belts like the Cascade Range of
the Pacific NW, where the Juan de Fuca plate (oceanic) is subducting beneath the North American
plate
○ Off the coast of South America, along the Peru-Chile trench, the Nazca oceanic plate is being pushed
under another, the South American Plate, the plate is being lifted upward, creating the Andes
Mountains & they are still rising about 15-30 feet every 1000 years. At the same time that they are
rising, they are being weathered & broken down
- This type of collision sustains many of earth’s active volcanoes, especially those around the Pacific
plate, aka the Pacific Ring of Fire. The type of volcanism associated w/ an ocean to continent collision
zone is called andesitic volcanism
○ Andesite is a type of lava that is high in the mineral silica, is gummier & thicker in its flows. This
causes the magma to hold its gases (sulfur, carbon dioxide, chlorine, methane etc) & become very
explosive. When pressures build, the volcanoes erupt in a violent explosion
- A stratovolcano (aka composite volcano)
○ A type of volcano associated w/ this type of magma at these boundaries. These are the steepest-
sided volcanoes due to the thick magma, cooling & hardening just as it runs down the volcanic
mountain, building up its sides. It’s also made up of cinders (pyroclastics = "fire pieces") which spew
out of the volcanic vent, hardening mid-air & landing on the sides of the mountain. Some famous
examples of stratovolcanoes include: Mt. St. Helens of Washington state & Mt. Shasta of California
b) ocean to ocean convergent boundary
- This type of boundary involves the collision of ocean plate boundaries w/ itself. Usually the larger one (or
the one made up entirely of oceanic lithosphere) is subducted. A deep sea trench is formed, the deepest
places found on the earth’s surface are produced in this type of collision zone. The Mariana Trench 35,839
ft. below sea level is formed as the Pacific Plate converges against the Philippine Plate
○ W/ the ocean to continent subduction, some of this crustal material convects upward to fuel
volcanoes in this zone of convergence & compression. Some of these volcanoes rise above sea
level (after many years) & become volcanic islands. Stratovolcanoes are typically strung out in
chains called island arcs (or archipelagos), which parallel the trench & are generally
curved. Examples of these include the Aleutian islands in Alaska, the islands that make up Japan,
Indonesia & the Philippines
c) continent to continent convergent boundary
- This type of boundary involves the collision of 2 continent plate boundaries. When two continental
plates collide, neither is subducted b/c continental crust (sial) is relatively light. Instead, the plates
buckle, push upwards & sideways, producing double thickness of continental crust resulting in the
formation of a high-rugged non-volcanic mountain system. The Himalayan mountain range in northern
India dramatically demonstrates this collision.
- It's estimated about 50 million years ago, India was part of the East coast of Africa. The Indian
subcontinent drifted N a few inches each year into the Indian Ocean & collided w/ the Eurasian
continent, creating the world's highest continental mountains, the Himalayas. In that range, Mt.
Everest is the world's highest mountain peak at about 29,035 feet
○ Due to this compression, it‘s still rising at about 15-30 feet every 1000 years (but at the same time,
being weathered & broken down). Behind the Himalayas lies the Tibetan Plateau, a high flat expanse
of land, also pushed up by this collision. At an elevation of 13,800 feet, the plateau rises well above
the summits of most mountains in the U.S.
3) Transform Boundary
- This is the type of boundary that results from two plates slipping past each other, laterally. They may
be moving in opposite or the same direction at different rates, most transform faults are found on the
ocean floor, offsetting the mid-ocean ridge boundaries, but few occur on land. A major example of a
transform boundary that occurs between continental plates is the boundary named the San Andreas
Fault of California. This is the boundary between the Pacific and N. American plates, which are moving
in opposite directions at a rate of about 1-2 inches per year. No volcanism results. Instead, natural
and cultural features like rivers and roads are displaced horizontally on each side of the fault.
Wednesday, July 8, 2026 3:57 PM
Have you ever looked at a map of the world & noticed how closely the west coast of Africa mirrors the east
coast of South America? If you cut them out & push them together, their coasts fit together like the pieces
of a jigsaw puzzle
- In 1912, a German geophysicist, Alfred Wegner, presented the theory of continental drift, tracing the past
positions & motions of the continents. He suggested these continents were all once assembled into a
single super continent named Pangaea (aka "all the land" in Greek)
○ He suggested that 200 million years ago, Pangaea began to drift & break into smaller
blocks drifting apart ever since, unveiling the shape of our modern continents. He couldn’t prove
how the continental crust moved, so the theory went unaccepted until after WWII
§ What evidence he did collect, did support his theory. However he found that South America,
Africa & Australia share some of the same types of sequential layers of sedimentary rocks
§ Since sedimentary rocks are made from sediment that has been eroded from a particular area &
transported downstream or down wind, this means these continents may have been assembled
as one
□ They also possess many of the same types of fossils, indicating similar plants & animals
appeared during the same geological time periods. They show distinctive glacial scouring
marks on rocks of the same age, suggesting the continents were formerly united & covered
by a glacier some 250 million years ago
Theory of Sea Floor Spreading
- In the 1950s & 60s, better exploration of the sea floor & satellite mapping allowed the topography of the
ocean floor to be revealed. It showed features like mid-ocean ridges (sites of spreading centers) & deep
arching trenches (sites of subduction zones)
- The intense heat of earth’s interior (from decay of radioactive elements in the core & mantle) is
conducted toward the surface thru the mantle & when it reaches the asthenosphere, this heat generates
slow convection currents within the molten rock material
○ This causes continents & oceans to drift along the asthenosphere, carried by convection currents like
ice floating on a stream. This new crustal material is being pushed up along mid-ocean ridges, driven
by heat, which forces the molten magma upward, pushing the existing lithosphere away (both
continental & oceanic crusts)
- In the 60s & 70s, geologists discovered the magnetic field of earth periodically reverses itself (about every
500K yrs). As molten rock wells up from the ridge & freeze, magnetite crystals in the iron-bearing rock
sets in a certain way, lining up with the earth’s magnetic field
- Mapping of magnetic alignment of these magnetite crystals in the ocean crust showed matching sets of
rocks on each side of the spreading centers. B/c the magnetic field reverses every now & then, bands of
new material set in alternate directions, creating a ”bar code” containing the history of sea floor
spreading
○ This evidence proved the basaltic rock on the ocean floor is most recent near ridges & increases in
age w/ distance away from the ridges, on both sides. The rate of seafloor spreading varies from
1-2 inches a year, comparable to the rate of fingernail growth (1.5 inches/year)! W/ the discovery
of sea floor spreading, the Wegner's theory of continental drift is justified.
Lithospheric Plates
- It’s believed the earth’s surface is broken into 7 major plates & dozens of smaller ones, that slowly
move like ice on a river. These thin plates (40 miles deep) are thought to be floating on the
atmosphere's upper mantle, the asthenosphere is slowly deformable. The 7 major plates somewhat
correspond to the major continents or ocean basins
- These plates don’t represent one continent, they are incorporated w/ the surrounding ocean floor into
a plate. These plates collide, separate, or slip past on another. This entire process of plate formation,
movement & destruction, accepted in the late 50s early 60s, is called plate tectonics. It’s at the edges,
or boundaries of these plates where we have the most dynamic earth-shaping forces operating &
includes earthquake activity, mountain forming & volcanic activity
Three Types of Plate Boundaries:
1) Spreading Centers aka Divergence
- Plates move apart from one another at "divergent" boundaries, where new crustal material is
introduced from the asthenosphere, the most common kind of divergent plate boundary is the oceanic
ridge (mid-ocean ridge)
- New basaltic ocean floor is being created w/ rising magma, a long line of under water volcanoes form
along these ridges, Iceland is a recently formed volcanic island that sits on the mid-Atlantic ridge &
continues to spread apart, but at the same time, is being built up by active volcanic eruptions
- Sometimes, divergence occurs within continental lithosphere. When it does, land is being pulled apart
from both sides & drops downward to create a valley. This is currently happening in eastern Africa, a
region called the East African Rift Valley
○ The Arabian Peninsula/Plate, is pulling away from the rest of the continent of Africa (the African
Plate), forming the Red Sea, which continues to grow 1-2 inches wider each year! If rifting continues,
the so-called "Horn of Africa" will rift away from the rest of the continent the Arabian Peninsula
formed
§ In the rift zones, magma rises through the cracks in the continental lithosphere, sometimes to
erupt and form volcanoes
○ These volcanoes (either at the ocean floor or continental lithosphere), are considered to be gently
erupting volcanoes. They arent considered life-threatening in their eruptions, the biggest threat in an
eruption of a volcano associated w/ divergent boundaries, is that the magma is so hot, it is highly
fluid
○ This means it can travel miles before it cools & hardens. It floods over the surface, destroying
anything in its path. However, it doesn't erupt violently w/ ashes & cinders. This category of
volcanism is called basaltic volcanism
- These kinds of volcanoes resulted from basaltic volcanism are called "shield" volcanoes
○ They are broad sloping mountains, due to the distance that lava spreads over the surface before it
hardens & the mountains don't build up steep sides, they look like a warrior's shield
2) Subduction Zones aka Convergence
- Plates push against each other as they come together at convergent plate boundaries. Collisions of
plates result in a compression force, pushing 2 plates together, these boundaries are responsible for
mountain ranges, highly eruptive volcanoes & deep ocean trenches. There are 3 kinds of convergent
plate boundaries, depending upon the kind of lithosphere involved:
a) ocean to continent convergent boundary
○ If we could drain the pacific ocean, we would see long, narrow curving trenches thousands of
miles long & 5-7 miles deep, cutting into the ocean floor. These trenches are created when
oceanic crust & continental crust collide. The oceanic crust (sima), is denser crustal material, is
plunged downward beneath the continental crust (sial). A deep ocean trench (the deepest part of
the ocean floor) opens up where the descending plate plunges into the asthenosphere & melts
○ Plumes of molten rock rise up back to the surface through the shattered edge of the overlying plate
as molten magma to create huge volcanic mountain ranges
- This ocean-continent collision causes the formation of major mountain belts like the Cascade Range of
the Pacific NW, where the Juan de Fuca plate (oceanic) is subducting beneath the North American
plate
○ Off the coast of South America, along the Peru-Chile trench, the Nazca oceanic plate is being pushed
under another, the South American Plate, the plate is being lifted upward, creating the Andes
Mountains & they are still rising about 15-30 feet every 1000 years. At the same time that they are
rising, they are being weathered & broken down
- This type of collision sustains many of earth’s active volcanoes, especially those around the Pacific
plate, aka the Pacific Ring of Fire. The type of volcanism associated w/ an ocean to continent collision
zone is called andesitic volcanism
○ Andesite is a type of lava that is high in the mineral silica, is gummier & thicker in its flows. This
causes the magma to hold its gases (sulfur, carbon dioxide, chlorine, methane etc) & become very
explosive. When pressures build, the volcanoes erupt in a violent explosion
- A stratovolcano (aka composite volcano)
○ A type of volcano associated w/ this type of magma at these boundaries. These are the steepest-
sided volcanoes due to the thick magma, cooling & hardening just as it runs down the volcanic
mountain, building up its sides. It’s also made up of cinders (pyroclastics = "fire pieces") which spew
out of the volcanic vent, hardening mid-air & landing on the sides of the mountain. Some famous
examples of stratovolcanoes include: Mt. St. Helens of Washington state & Mt. Shasta of California
b) ocean to ocean convergent boundary
- This type of boundary involves the collision of ocean plate boundaries w/ itself. Usually the larger one (or
the one made up entirely of oceanic lithosphere) is subducted. A deep sea trench is formed, the deepest
places found on the earth’s surface are produced in this type of collision zone. The Mariana Trench 35,839
ft. below sea level is formed as the Pacific Plate converges against the Philippine Plate
○ W/ the ocean to continent subduction, some of this crustal material convects upward to fuel
volcanoes in this zone of convergence & compression. Some of these volcanoes rise above sea
level (after many years) & become volcanic islands. Stratovolcanoes are typically strung out in
chains called island arcs (or archipelagos), which parallel the trench & are generally
curved. Examples of these include the Aleutian islands in Alaska, the islands that make up Japan,
Indonesia & the Philippines
c) continent to continent convergent boundary
- This type of boundary involves the collision of 2 continent plate boundaries. When two continental
plates collide, neither is subducted b/c continental crust (sial) is relatively light. Instead, the plates
buckle, push upwards & sideways, producing double thickness of continental crust resulting in the
formation of a high-rugged non-volcanic mountain system. The Himalayan mountain range in northern
India dramatically demonstrates this collision.
- It's estimated about 50 million years ago, India was part of the East coast of Africa. The Indian
subcontinent drifted N a few inches each year into the Indian Ocean & collided w/ the Eurasian
continent, creating the world's highest continental mountains, the Himalayas. In that range, Mt.
Everest is the world's highest mountain peak at about 29,035 feet
○ Due to this compression, it‘s still rising at about 15-30 feet every 1000 years (but at the same time,
being weathered & broken down). Behind the Himalayas lies the Tibetan Plateau, a high flat expanse
of land, also pushed up by this collision. At an elevation of 13,800 feet, the plateau rises well above
the summits of most mountains in the U.S.
3) Transform Boundary
- This is the type of boundary that results from two plates slipping past each other, laterally. They may
be moving in opposite or the same direction at different rates, most transform faults are found on the
ocean floor, offsetting the mid-ocean ridge boundaries, but few occur on land. A major example of a
transform boundary that occurs between continental plates is the boundary named the San Andreas
Fault of California. This is the boundary between the Pacific and N. American plates, which are moving
in opposite directions at a rate of about 1-2 inches per year. No volcanism results. Instead, natural
and cultural features like rivers and roads are displaced horizontally on each side of the fault.
