Marine Science (9693) | Tectonic processes

🌊 Marine Science Study Notes: 2.1 Tectonic Processes 🚢

Hello future marine scientist! Welcome to the foundation of "Earth processes." Don't worry if this chapter seems like geology at first—it’s vital because the ground beneath the ocean is constantly moving, shaping the deep-sea habitats, controlling ocean chemistry, and creating dramatic features like volcanoes and trenches.

Understanding tectonic processes explains *why* the deep ocean looks the way it does, and *how* unique ecosystems, like hydrothermal vents, can exist. Let's dive into the moving planet!

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1. The Structure of the Earth (Syllabus 2.1.1)

Think of the Earth like a giant, slightly squishy boiled egg or, perhaps more accurately, an avocado. It has distinct layers.

The Layers of the Earth

The structure is limited to three main layers for this syllabus:

• Crust: The thin, solid outermost layer. This is where we live, and where the ocean sits.
• Mantle: The thick layer below the crust, made of hot, dense rock. The top part is rigid, but the deeper parts are semi-molten (plastic), allowing movement.
• Core: The very center, made of iron and nickel, generating heat and magnetic fields.

Types of Crust

The crust itself comes in two main flavors:

1. Continental Crust:

• Generally thicker (25–70 km).
• Less dense (lighter).
• Made mostly of granite.

2. Oceanic Crust:

• Generally thinner (around 5–10 km).
• More dense (heavier).
• Made mostly of basalt.

Key Takeaway: The difference in density (oceanic is heavier than continental) is critical for understanding plate movement!

2. The Theory of Plate Tectonics (Syllabus 2.1.2)

The Theory of Plate Tectonics states that the Earth’s lithosphere (the crust and the rigid uppermost mantle) is broken into large pieces called tectonic plates. These plates float and move slowly on the semi-molten layer of the mantle below (the asthenosphere), driven by heat (convection currents).

Evidence Supporting Plate Tectonics

You need to know four pieces of evidence that support this theory:

1. The Jigsaw Fit: The coastlines of different continents (most famously South America and Africa) appear to fit together perfectly, like pieces of a puzzle.
2. Distribution of Similar Fossils and Organisms: Identical non-marine fossils and currently living organisms are found on continents separated by vast oceans (e.g., specific reptiles or ferns). This suggests these landmasses were once connected.
3. Geological Matching: Distinctive rock formations, mineral deposits, and mountain ranges (geological structures) on one continent match those found exactly across the ocean on a different continent.
4. Paleomagnetic Stripes on the Ocean Floor: This is strong proof of seafloor spreading at mid-ocean ridges. As lava erupts and cools, the iron minerals align with Earth’s magnetic field. Since the Earth's magnetic field periodically reverses, this creates alternating, symmetrical stripes of magnetic orientation mirrored on both sides of a mid-ocean ridge.

3. Types of Plate Boundaries (Syllabus 2.1.3)

The movement of plates means they interact at their edges, known as boundaries. There are three types:

1. Divergent Boundaries (Moving Apart)

• Description: Plates move away from each other. Magma rises from the mantle to fill the gap, creating new oceanic crust.
• Tectonic Force: Tension (Pulling apart).
• Marine Feature Example: Mid-Ocean Ridges (like the Mid-Atlantic Ridge).

2. Convergent Boundaries (Moving Together)

• Description: Plates collide. Since the denser plate (usually oceanic crust) sinks beneath the less dense plate (usually continental crust or younger oceanic crust), this process is called subduction.
• Tectonic Force: Compression (Pushing together).
• Marine Feature Example: Ocean Trenches, volcanic arcs.

Analogy: Imagine two cars crashing head-on. If one is much heavier (denser), it forces the lighter one to crumple or fold over it.

3. Transform Boundaries (Sliding Past)

• Description: Plates slide horizontally past each other. Crust is neither created nor destroyed, but the movement causes immense friction.
• Tectonic Force: Shear (Rubbing/Tearing).
• Marine Feature Example: Major earthquakes and fault lines (e.g., parts of the San Andreas Fault continue underwater).

Key Takeaway: Know the relationship: Divergent = Ridge, Convergent = Trench, Transform = Friction/Earthquakes.

4. Formation of Key Marine Features (Syllabus 2.1.4)

The movement at these boundaries creates the physical landscape of the ocean floor:

Ocean Trenches

Formed at convergent boundaries where one plate subducts (sinks) beneath another. This sinking action creates a deep V-shaped depression in the ocean floor—the deepest parts of the ocean.

Mid-Ocean Ridges

Formed at divergent boundaries. As plates pull apart, magma rises and solidifies, building long underwater mountain chains where new crust is continuously formed.

Hydrothermal Vents

These are often found near mid-ocean ridges (divergent zones). They are openings in the seafloor where geothermally heated water emerges. (We cover these in detail in Section 5).

Abyssal Plains

These are the vast, flat, sediment-covered areas of the deep ocean floor, typically located between the continental rise and the mid-ocean ridge. They are formed by the slow accumulation and deposition (sedimentation) of fine particles (mostly clay and microscopic shells) carried by deep-sea currents over millions of years, covering the rugged, underlying oceanic crust.

Volcanoes and Earthquakes

• Volcanoes: Primarily formed at convergent boundaries (where subducting crust melts) or at divergent boundaries (where magma rises).
• Earthquakes and Tsunamis: Caused by sudden, massive movements of the crust, typically at transform or convergent boundaries where stress builds up and is suddenly released. A tsunami is a series of massive waves generated when a large volume of water is displaced, often by a sudden vertical movement of the seafloor during an earthquake.

5. Focus on Hydrothermal Vents (Syllabus 2.1.5, 2.1.6, 2.1.7)

Hydrothermal vents are a key feature created by tectonic activity, supporting life in the total absence of sunlight.

A. The Hydrothermal Vent Plume (2.1.5 & 2.1.6)

Water seeping through cracks in the crust near a mid-ocean ridge is heated by magma. This superheated water emerges from the vent as a "plume."

The water coming from hydrothermal vents is described as:

• Under pressure: Due to the great depth and temperature.
• Hot: Reaching temperatures of over 300 °C (it remains liquid because of the high pressure).
• Rich in dissolved nutrients: Notably sulfur compounds (like hydrogen sulfide) and other dissolved minerals and metals (e.g., iron, copper).

This emerging water forms a hydrothermal vent plume. The effects of this plume (heat, chemical signals, dissolved nutrients) can be detected over some distance away from the actual vent site. This is important because it disperses nutrients, potentially supporting organisms far from the source.

B. The Formation of Vent Chimneys (2.1.7)

Chimneys (often called 'Black Smokers' due to the dark plume they release) form because of a sudden change in solubility caused by temperature change.

Here is the step-by-step process:

1. Seepage: Cold seawater seeps down through cracks in the new oceanic crust.
2. Heating and Dissolution: The water approaches the magma chamber and is superheated (becoming highly acidic in the process). This hot, acidic water dissolves minerals and salts (particularly metal sulfides) from the surrounding rock.
3. Eruption: The hot water, rich in dissolved compounds, rises rapidly back up and out of the vent opening.
4. Precipitation: As the superheated vent water (e.g., 350 °C) meets the cold ambient seawater (e.g., 2 °C), the temperature drops instantly.
5. Insolubility: The metal sulfides and other dissolved salts, which were highly soluble in the *hot* water, become insoluble in the cold water. They immediately precipitate (crystalize) out of solution, forming solid mineral particles.
6. Building: These solid mineral particles accumulate and are deposited around the vent opening, gradually building the tall chimney structures.

Key Takeaway: Tectonic processes fuel hydrothermal vents, which support chemosynthetic ecosystems by supplying hot, nutrient-rich water.