🌐TOEFL Listening 2026

Transcript — part of an earth-science lecture. Lecturer: We now understand continents move — but remember, until the 1960s most geologists believed continents were fixed. The decisive evidence came from the sea floor. Magnetic minerals in cooling lava align with Earth's magnetic field, and the field reverses irregularly, so the ocean crust records those reversals as a striped pattern on either side of mid-ocean ridges. When research vessels mapped these stripes, they found them symmetric about the ridge — mirror images, equidistant, equally old on both sides. That symmetry is the signature of sea-floor spreading: new crust forms at the ridge and moves apart at roughly the rate fingernails grow, carrying the stripes away with it. The same spreading explains why the ocean floor is geologically young everywhere — nowhere older than about two hundred million years — while continental rocks can exceed four billion: ocean crust is continually created and destroyed at trenches, but continents are too buoyant to sink. What did research vessels actually map to produce this evidence?

Gratis: 10 preguntas/día · 0/10