Question

what evidence have you collected about the age of the crust alng mid-ocean ridges and ocean trenchs

Answer 1

Short answer: all independent lines of evidence show the same thing — oceanic crust is youngest at mid‑ocean ridges (essentially 0 Ma at the axis) and becomes progressively older with distance from the ridge; the oldest surviving oceanic crust is only on the order of 150–200 million years old and is typically found near subduction trenches or at the margins of ocean basins. This pattern is the cornerstone evidence for seafloor spreading and plate tectonics.

Evidence and what it shows
- Magnetic stripes (paleomagnetism)
- Basalt on the seafloor records reversals of Earth’s magnetic field. Alternating bands of normal/reversed magnetization symmetric about ridge axes can be matched to the geomagnetic polarity timescale to give ages and spreading rates (Vine–Matthews–Morley hypothesis).
- Result: age increases symmetrically away from the ridge.

- Radiometric dating of seafloor basalts
- K–Ar and Ar–Ar (and other methods) on dredged/drilled basalts give absolute ages that confirm the magnetic ages.
- Result: confirms new crust at ridges and progressively older ages outward.

- Deep-sea drilling and biostratigraphy (DSDP/ODP/IODP)
- Drilling recovers basalts and the overlying sediment. Fossils and microfossils in the sediment provide independent age constraints; sediment thickness increases with crustal age.
- Result: direct sampling validates age models and shows increasing sediment cover on older crust.

- Seafloor age maps and fracture-zone geometry
- Patterns of magnetic anomalies, fracture zones, and transform offsets produce a consistent global map of seafloor ages (used widely in plate reconstructions).
- Result: global age distribution with youngest at ridges, oldest near trenches and basin edges.

- Heat flow and thermal subsidence
- High heat flow and shallow bathymetry at ridges; heat flow decreases with age roughly as a thermal cooling model (t^−1/2). The ocean floor subsides and deepens with age.
- Result: thermal cooling model explains seafloor deepening and is consistent with measured ages.

- Seismic and gravity data
- Seismic velocity structure and gravity anomalies show differences in lithospheric thickness and rigidity as plates age and cool. Subducting slabs imaged beneath trenches confirm where older oceanic lithosphere is being consumed.
- Result: geophysical structure matches the age pattern and the process of subduction at trenches.

What this means about trenches
- Trenches are sites where oceanic lithosphere is being consumed by subduction. The plate entering a trench is typically relatively old (colder, denser) compared with the newly formed crust at ridges; thus trenches tend to be adjacent to some of the oldest oceanic crust in a given basin.
- Because oceanic crust is continuously recycled at trenches, there is a global limit to how old oceanic crust can be — roughly 150–200 Ma (much younger than continental crust).

Representative numbers and resources
- Age at ridge axis: 0 Ma (continuous creation).
- Oldest surviving oceanic crust: ≈150–200 million years (varies by basin; Atlantic oceanic crust is on the order of ~160–170 Ma at its oldest parts).
- Useful references/data sources: Vine & Matthews (1963) for magnetic stripes; DSDP/ODP/IODP drilling results; global seafloor age maps (e.g., maps by Müller et al. and NOAA/GPlates datasets).

If you want, I can:
- Show a global seafloor-age map and point out examples of youngest and oldest areas, or
- Explain in more detail how magnetic anomaly matching to the polarity timescale is done, or
- Give specific age values for a particular ridge or trench. Which would you prefer?