A contour map that displays the variation in time between two seismic events or reflections.

Explain what causes the paleomagnetic patterns on the seafloor. As new ocean crust forms at ridges, it records the Earth’s magnetic field at that time. He showed rocks with similar features were on continents now separated by oceans. Fossils of land animals that could not fly or swim were found on different continents.

What do deep-sea trenches reveal about Earth’s crust? They indicate where crust is moving together and diving down deeper into the mantle.

Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere. Older rocks will be found farther away from the spreading zone while younger rocks will be found nearer to the spreading zone.

Seafloor spreading, theory that oceanic crust forms along submarine mountain zones, known collectively as the mid-ocean ridge system, and spreads out laterally away from them. Rising magma assumes the polarity of Earth’s geomagnetic field before it solidifies into oceanic crust.

Several types of evidence from the oceans supported Hess’s theory of sea-floor spreading-evidence from molten material, magnetic stripes, and drilling samples.

Abundant evidence supports the major contentions of the seafloor-spreading theory. First, samples of the deep ocean floor show that basaltic oceanic crust and overlying sediment become progressively younger as the mid-ocean ridge is approached, and the sediment cover is thinner near the ridge.

Seafloor spreading occurs at divergent plate boundaries. As tectonic plates slowly move away from each other, heat from the mantle’s convection currents makes the crust more plastic and less dense. The less-dense material rises, often forming a mountain or elevated area of the seafloor.

Several types of evidence supported Hess’s theory of sea-floor spreading: eruptions of molten material, magnetic stripes in the rock of the ocean floor, and the ages of the rocks themselves.

Features of the ocean include the continental shelf, slope, and rise. The ocean floor is called the abyssal plain. Below the ocean floor, there are a few small deeper areas called ocean trenches. Features rising up from the ocean floor include seamounts, volcanic islands and the mid-oceanic ridges and rises.

Terms in this set (7)

1. Magma comes out of the rift valley.
2. Magma cools to rock and hardens.
3. Rock is pushed away as new rock is formed at MOR.
4. Oceanic crust and continental crust meet at the trench.
5. Oceanic crust bends down under the continental crust.
6. Gravity pulls rock towards mantle.
7. Rock melts to mantle.

Harry Hess’s hypothesis about seafloor spreading had collected several pieces of evidence to support the theory. So, the distribution of ice sheets across the planet is not evidence of sea-floor spreading.

mid-ocean ridges

The oldest seafloor is comparatively very young, approximately 280 million years old. It is found in the Mediterranean Sea and is a remnant of an ancient ocean that is disappearing between Africa and Europe.

The youngest crust of the ocean floor can be found near the seafloor spreading centers or mid-ocean ridges. As the plates split apart, magma rises from below the Earth’s surface to fill in the empty void.

Understanding plate motions [This Dynamic Earth, USGS] Divergent boundaries — where new crust is generated as the plates pull away from each other. Convergent boundaries — where crust is destroyed as one plate dives under another.

They found that the youngest rocks on the seafloor were at the mid-ocean ridges. The rocks get older with distance from the ridge crest. The scientists were surprised to find that the oldest seafloor is less than 180 million years old.

The heat from radioactive processes within the planet’s interior causes the plates to move, sometimes toward and sometimes away from each other. This movement is called plate motion, or tectonic shift.

They move at a rate of one to two inches (three to five centimeters) per year.

In this lesson, we explore the causes of plate movement, including thermal convection, ridge push and slab pull.

When the plates move they collide or spread apart allowing the very hot molten material called lava to escape from the mantle. When collisions occur they produce mountains, deep underwater valleys called trenches, and volcanoes. The Earth is producing “new” crust where two plates are diverging or spreading apart.

Over millions of years, continents drift across Earth’s surface, going from one climate zone to another. Without plate tectonics, Earth would not have its diverse geography, which provides a wide range of habitats. Plate tectonics is also responsible for hydrothermal vents on the ocean floor.

If two tectonic plates collide, they form a convergent plate boundary. Usually, one of the converging plates will move beneath the other, a process known as subduction. The new magma (molten rock) rises and may erupt violently to form volcanoes, often building arcs of islands along the convergent boundary.

Plate tectonics affects humans in several important ways. What would Earth be like without plate tectonics? We’d have many fewer earthquakes and much less volcanism, fewer mountains, and probably no deep-sea trenches. In other words, the Earth would be a much different place.

A dozen cold, rigid plates slowly slip and slide atop Earth’s hot inner mantle, diving beneath one another and occasionally colliding. “Plate tectonics is what modulates our atmosphere at the longest timescales. You need that to be able to keep water here, to keep it warm, to keep life chugging along.”

Three good things that plate tectonics provide for humans are: fertile soils, ore deposits and fossil fuels.

seven

lithosphere