Unit 2: Earth System Processes

1. Movement of Lithosphere Plates

The Earth's rigid outer layer, the lithosphere (which consists of the crust and the solid upper part of the mantle), is not one solid piece. It is broken into about a dozen major tectonic plates and many minor ones. These plates "float" on the semi-molten, flowing layer beneath it called the asthenosphere. The plates are in constant, slow motion, moving a few centimeters per year (about the speed your fingernails grow).

2. Mantle Convection and Plate Tectonics

Plate Tectonics

Plate Tectonics is the grand, unifying theory of geology. It explains that the Earth's plates move and interact with each other at their boundaries, and that these interactions are responsible for most of the Earth's major geological features, such as earthquakes, volcanoes, and mountain ranges.

Mantle Convection

This is the driving force behind plate tectonics. The mantle acts like a pot of thick, boiling soup:

The Earth's core heats the rock at the bottom of the mantle.
This hot rock becomes less dense and slowly rises.
Near the surface, it spreads out, cools, and becomes denser.
This cool, dense rock then sinks back down to be reheated.

This slow, circular flow is called a convection cell. The lithospheric plates are "dragged" along by the movement of these cells, like a conveyor belt.

Diagram: A cross-section of the Earth showing the Core, Mantle, and Crust. Draw circular arrows within the mantle (convection cells) showing hot material rising at a mid-ocean ridge and sinking at a subduction zone, dragging the plates.

3. Major Plates and Geological Activities at Plate Boundaries

Some of the major plates include the Pacific, North American, Eurasian, African, South American, Antarctic, and Indian-Australian plates. All major geological "action" happens where these plates meet.

There are three types of plate boundaries:

Crucial Exam Topic: You must be able to compare the three plate boundaries, their motion, and the features they create.

Boundary Type	Relative Motion	Geological Features	Real-World Example
Divergent	Plates move apart (Constructive)	New crust is created. Rift valleys (on land). Mid-ocean ridges (in ocean). Shallow earthquakes, volcanoes.	Mid-Atlantic Ridge; East African Rift Valley
Convergent	Plates move together (Destructive)	Crust is destroyed (subduction). Deep ocean trenches. Volcanic arcs (on land) or Island arcs (in ocean). Major mountain ranges (if continents collide). Strong earthquakes, volcanoes.	Andes Mountains (Ocean-Continent); Japan (Ocean-Ocean); Himalayas (Continent-Continent)
Transform	Plates slide past each other (Conservative)	Crust is neither created nor destroyed. Major fault lines. Only earthquakes. (No volcanoes).	San Andreas Fault, California

4. Earth’s Magnetic Field

The Earth acts like a giant bar magnet, with a magnetic field surrounding it (the magnetosphere). This field is what makes a compass point north.

Dipolar Field

The field is "dipolar," meaning it has two poles: a North Magnetic Pole and a South Magnetic Pole. These magnetic poles are located near (but not exactly at) the geographic poles (the axis on which Earth spins).

Magnetic Reversals

A key feature of the field is that it "flips" its polarity at irregular intervals (ranging from thousands to millions of years). During a magnetic reversal, the North Magnetic Pole becomes the South Magnetic Pole, and vice versa. A compass would point south instead of north. We know this has happened many times in Earth's history because of the magnetic record preserved in rocks (see Paleomagnetism).

5. Origin of the Main Geomagnetic Field

The Dynamo Theory: The Earth's magnetic field is generated by the movement of the liquid iron-nickel alloy in the Outer Core.

This process, known as the geodynamo, works like this:

The liquid outer core is a conductor (it's metal).
Convection currents (like in the mantle, but much faster) and the Earth's spin (Coriolis effect) cause this liquid metal to flow in complex patterns.
The movement of a conductor creates powerful electrical currents.
According to physics, any electrical current generates a magnetic field.

In short: The flow of liquid metal in the outer core acts as a self-sustaining electrical generator (a dynamo), producing the Earth's magnetic field.

6. Continental Drift

Continental Drift was the hypothesis, proposed by Alfred Wegener in 1912, that all the continents were once joined together in a single supercontinent called Pangaea ("all lands") and have since "drifted" apart to their current positions.

Wegener's evidence was strong:

Jigsaw Fit: The coastlines of South America and Africa fit together like puzzle pieces.
Fossil Evidence: Identical fossils (like the land-reptile Lystrosaurus and the plant Glossopteris) were found on continents now separated by vast oceans.
Rock and Mountain Matching: Mountain ranges of the same age and rock type (like the Appalachians in the US and the Caledonians in Europe) lined up when continents were reassembled.
Climate Evidence: Evidence of glaciers (glacial striations) was found in modern-day tropical areas like India and Africa, suggesting they were once near the South Pole.

Common Pitfall: Continental Drift was the *hypothesis* (the "what"). Plate Tectonics is the *theory* (the "how"). Wegener's idea was rejected at the time because he could not propose a plausible mechanism to explain *how* the continents moved. He wrongly suggested they "plowed" through the ocean floor.

7. Seafloor Spreading

Proposed by Harry Hess in the 1960s, Seafloor Spreading was the missing mechanism for continental drift. It provided the "how" and led directly to the theory of Plate Tectonics.

The process is simple:

At mid-ocean ridges (divergent boundaries), magma rises up from the mantle.
It erupts as lava, cools, and forms new oceanic crust.
This new crust pushes the older crust away from the ridge on both sides, like a giant, two-way conveyor belt.
This "spreading" of the seafloor pushes the continents apart.

This explains why oceanic crust is very young at the ridges and gets progressively older as you move away from them.

8. Paleomagnetism (The "Smoking Gun" for Plate Tectonics)

Paleomagnetism ("ancient magnetism") is the study of the Earth's magnetic field as it is recorded in rocks. It provided the final, undeniable proof for seafloor spreading.

How it works:

When lava (magma) erupts at a mid-ocean ridge, it contains iron-bearing minerals (like magnetite).
While the lava is still liquid, these minerals act like tiny compass needles and align themselves with the Earth's magnetic field at that time.
When the lava cools and solidifies into rock, this magnetic orientation is frozen in place.

The Proof:

In the 1960s, scientists towing magnetometers across the oceans found a striking pattern. The seafloor was composed of "magnetic stripes" of normal and reversed polarity.

"Normal" Polarity: Rocks that formed when the magnetic field was as it is today.
"Reversed" Polarity: Rocks that formed during a magnetic reversal (when a compass would have pointed south).

Crucially, this pattern of stripes was perfectly symmetrical on both sides of the mid-ocean ridges. This was the "smoking gun" that proved seafloor spreading was real. New crust was forming at the ridge, recording the Earth's magnetic polarity at the time, and then spreading out in both directions.

Diagram: A cross-section of a mid-ocean ridge. Show magma rising. On the seafloor to the left and right, draw a series of colored stripes (e.g., black-white-black-white) that are *symmetrical* on both sides of the ridge. Label one set "Normal Polarity" and the other "Reversed Polarity."