Earth's Crust: Continents vs. Oceans

• The general question is how current geoscience methods allow constructing a scientific approach to terrestrial dynamics.

Earth's Crust Structure

• Observation of Earth's surface distinguishes oceanic and continental domains.
• What are the differences between continental and oceanic crusts?

Contrasts Between Continents and Oceans

Contrasted Reliefs

• Relief observations distinguish two large sets relative to sea level (=level 0).
• Positive reliefs include mountain ranges on continents.
• Average continent altitude is 300m.
• Negative reliefs are ocean floors averaging 4,000 m in depth.
• Bimodal relief distribution is due to the nature of continental and oceanic floor materials.
• Continents are granitic and less dense than basaltic oceanic floors.

Different Rocks

• Continental crust exhibits visible heterogeneity on the surface (magmatic, sedimentary, metamorphic rocks).
• Granite is the most representative rock at depth.
• Granite consists mainly of quartz, feldspars, and mica.
• Granite is entirely crystallized with a granular structure, resulting from slow magma cooling.
• Granite is a plutonic magmatic rock.
• Oceanic crust is covered by a thin sediment layer (0 to 400 m).
• Below, the crust is composed of basalt first, then gabbro.
• These are magmatic rocks from magma cooling.
• Their chemical composition is identical.
• The minerals present are the same, but their cooling mode differs, hence their texture.
• Basalt has a microlithic texture, with rare large minerals or phenocrysts (olivine, pyroxene) embedded in a matrix.
• Basalt contains uncrystallized glass and small crystals (microliths) of plagioclase.
• Basalt results from slow magma cooling, a volcanic magmatic rock.
• Gabbro has a granular texture due to large, jointed minerals.
• Magma cools slowly at depth, a plutonic magmatic rock.
• Both rocks come from the crystallization of the same magma type.
• They are less rich in silica than granite in the continental crust.
• They are rich in ferromagnesian elements (Fe, Mg).
• They contain minerals rich in these elements, such as olivine and pyroxenes, but no quartz.