Continental Drift and Plate Tectonics PDF

1.​ Continental drift Pangaea – A long time ago (about 300 million years), all the continents were joined together in one supercontinent called Pangaea. Slowly Breaking Apart – Over millions of years, Pangaea split into smaller pieces, and those pieces (our current continents) drifted...

1.​ Continental drift Pangaea – A long time ago (about 300 million years), all the continents were joined together in one supercontinent called Pangaea. Slowly Breaking Apart – Over millions of years, Pangaea split into smaller pieces, and those pieces (our current continents) drifted to where they are now. Still Moving! – The continents are still moving today, just very, very slowly (about the same speed your fingernails grow). Why Does This Happen? – The Earth's surface (the crust) sits on top of a layer of hot, flowing rock called the mantle. Heat from deep inside the Earth causes the mantle to move, which pushes the continents around like icebergs floating on water. Evidence for Continental Drift: ​ Puzzle Fit – If you look at a world map, continents (like South America and Africa) seem to fit together like puzzle pieces. ​ Fossil Clues – Fossils of the same ancient plants and animals are found on continents that are now far apart, proving they were once connected. ​ Rock and Mountain Clues – Similar rock layers and mountain ranges appear on continents that are now separated. ​ Climate Clues – Evidence of glaciers in places that are now warm (like Africa) shows that those continents were once in colder regions. Continental Drift was first proposed by Alfred Wegener in 1912, but he couldn’t explain how the continents moved. Later, the discovery of plate tectonics (which explains how Earth’s crust moves) confirmed he was right! 2.​ Seafloor Spreading Mid-Ocean Ridges – In the middle of the ocean, there are massive underwater mountain ranges called mid-ocean ridges. These are like cracks in the Earth's crust where magma (hot, melted rock) rises up. New Crust Forms – When magma reaches the surface, it cools and hardens, forming new oceanic crust. This pushes older crust away from the ridge, making the seafloor spread apart. The Ocean Floor Moves! – Over time, this process keeps repeating. More magma rises, new crust forms, and the ocean floor gradually moves outward, like a conveyor belt carrying the continents with it.  Evidence for Seafloor Spreading: ​ Magnetic Stripes – As new rock forms, it records the Earth's magnetic field. Since Earth's magnetic field flips every few hundred thousand years, scientists can see matching patterns of stripes on both sides of the ridge, proving the seafloor is spreading. ​ Age of Rocks – The rocks near the mid-ocean ridge are young, while the rocks farther away are older. This shows new rock is constantly forming and pushing the old rock aside. ​ Deep Ocean Trenches – On the other side of the ocean, old oceanic crust gets pushed under continents and melts back into the Earth at deep trenches (this process is called subduction). Why Is This Important? Seafloor Spreading is one of the main forces behind plate tectonics, which explains how continents move. It also shows that Earth's crust is constantly being recycled—new crust forms at mid-ocean ridges, and old crust is destroyed at deep trenches. 3.​ Magnetic Banding Earth’s Magnetic Field Flips – Every so often (every few hundred thousand to millions of years), the Earth's magnetic field switches direction—north becomes south, and south becomes north. New Rock Forms at Mid-Ocean Ridges – Hot, molten rock (magma) rises up from below the Earth’s crust and cools to form new oceanic crust. This rock contains tiny magnetic minerals (like a natural compass), which lock in the direction of Earth’s magnetic field at the time they cool. Stripes Form on the Ocean Floor – Over time, as more magma keeps coming up, it pushes the older rock away, forming mirror-image stripes of magnetized rock on both sides of the ridge. These stripes alternate between normal (same as today) and reversed (opposite) magnetic directions, showing a pattern of magnetic banding. Proof of Plate Tectonics – Scientists use these stripes as evidence that the seafloor is spreading, proving that continents are moving (plate tectonics in action!). 4.​ Change in the ocean floor Subduction (Old Ocean Floor Disappears) ​ On the other side of the ocean, old oceanic crust gets pushed under another plate at deep trenches (called subduction zones). ​ The old crust sinks into the mantle, where it melts and gets recycled back into the Earth. ​ This keeps the ocean floor from getting too big! Earthquakes and Volcanoes ​ As plates move, they sometimes collide, pull apart, or slide past each other, causing earthquakes. ​ Some areas where magma rises through cracks in the ocean floor form underwater volcanoes, which can create new islands (like Hawaii!). Erosion and Sediment Build-Up ​ Over time, sand, mud, and dead marine life settle on the ocean floor, forming layers of sediment. ​ Currents, waves, and underwater landslides can shift and reshape the seafloor. Rising and Falling Sea Levels ​ When sea levels rise, more of the ocean floor gets covered with water. ​ When sea levels drop, parts of the ocean floor can become exposed, forming land (like the Bering Land Bridge that once connected Asia and North America). 5.​ Divergent Boundary A Divergent Boundary is where two tectonic plates are moving apart from each other, creating new crust. Imagine it like a crack in the Earth's surface where magma rises up and pushes the plates away—kind of like a slowly growing rip in a piece of fabric. Plates Pull Apart – The Earth's outer layer (the lithosphere) is made of large pieces called tectonic plates. At a divergent boundary, these plates slowly move away from each other. Magma Rises Up – As the plates separate, magma (hot melted rock) from beneath the Earth’s surface rises up to fill the gap. New Crust Forms – When the magma cools, it hardens into new rock, creating fresh ocean floor or land. Where Does This Happen? ​ In the Ocean → Mid-Ocean Ridges ○​ Most divergent boundaries are underwater, forming mid-ocean ridges (like the Mid-Atlantic Ridge).  ○​ As magma rises and cools, it pushes the seafloor apart, causing seafloor spreading. ​ On Land → Rift Valleys ○​ If a divergent boundary happens on land, it creates a rift valley, like the East African Rift. ○​ Over millions of years, these valleys can split apart completely, letting ocean water in and forming new seas. What Happens Because of This? ​ Earthquakes – As the plates move, the crust cracks and shifts, causing small earthquakes. ​ Volcanic Activity – Magma rising to the surface can create underwater volcanoes or volcanic islands. ​ New Ocean Formation – If the rift keeps growing, it can eventually split continents apart and create a new ocean! Real-Life Example: ​ The Mid-Atlantic Ridge is a huge underwater mountain range where the Atlantic Ocean is slowly getting wider as the seafloor spreads. 6.​ Convergent Boundary A Convergent Boundary is where two tectonic plates collide or push into each other. Imagine two cars crashing in slow motion—one might slide under the other, or both might crumple and push upward. This is how mountains, volcanoes, and deep ocean trenches are formed! What Happens at a Convergent Boundary? Plates Move Toward Each Other – Two plates slowly push against each other over millions of years. Something Has to Give – Depending on the type of plates, the collision can cause different things to happen: ○​ One plate sinks under the other (Subduction) → Creates trenches & volcanoes. ○​ Both plates push up (Collision) → Creates mountains. Types of Convergent Boundaries Oceanic-Continental Convergence (Ocean Plate vs. Land Plate) ​ The denser oceanic plate sinks under the lighter continental plate. ​ This process, called subduction, creates a deep ocean trench and a line of volcanoes on land. ​ Example: The Andes Mountains & the Peru-Chile Trench (where the Pacific Plate sinks under South America). Oceanic-Oceanic Convergence (Two Ocean Plates Collide) ​ One oceanic plate sinks under the other, forming a deep ocean trench and a chain of volcanic islands. ​ Example: The Mariana Trench (deepest part of the ocean) and the Mariana Islands. Continental-Continental Convergence (Two Land Plates Collide) ​ Since both plates are light and can’t sink, they crumple and push upward, creating huge mountains. ​ Example: The Himalayas, where India’s plate is crashing into Asia, forming the tallest mountains in the world (including Mount Everest). What Does This Cause? ​ Earthquakes – The intense pressure between the plates causes big earthquakes. ​ Volcanoes – When one plate sinks and melts, magma rises up to form volcanoes. ​ Mountains & Trenches – Convergent boundaries create massive mountain ranges and the deepest parts of the ocean. 7.​ Causes of plate motion The Earth's tectonic plates aren't just sitting still—they’re constantly moving, even though it happens super slowly (a few centimeters per year). The causes of plate motion come from deep inside the Earth and are driven by heat and gravity. What Makes Plates Move? Convection Currents ​ Inside the Earth, the mantle (a hot, semi-liquid layer) moves in circulating currents called convection currents. How it works: ○​ Deep inside the Earth, hot rock rises because heat makes it less dense. ○​ When it reaches the surface, it cools down and sinks back down. ○​ This cycle of rising and sinking slowly pushes and pulls the tectonic plates above it.  Ridge Push ​ At mid-ocean ridges (where new ocean crust is formed), magma rises and creates new rock. ​ This new rock pushes the older rock away, causing plates to move apart. ​ Think of it like toothpaste coming out of a tube—when new material is added, the rest gets pushed forward. Slab Pull ​ At subduction zones, where one plate sinks under another, the sinking plate pulls the rest of the plate down with it. ​ Gravity helps pull the plate deeper into the Earth. ​ This is one of the strongest forces driving plate movement! 8.​ The downhill theory Slab Pull (Gravity pulls tectonic plates downhill at subduction zones)