Rocks and Sediment

The seafloor is made up of different types of rocks and sediment.

Rocks; The main type of rock found at the bottom of the sea is basalt, a volcanic rock that is generated at ocean ridges, where two crustal plates are being pulled apart. Most of this basalt is geologically young, being less than 200 million years old. Since this rock is below sea level, it has not been affected by erosion or metamorphism (changes in the rock due to temperature and pressure). However, it has been covered by sediment in many places. Sediment refers to small particles of rock that are transported by water, wind or ice. Sediment at the bottom of the ocean can be divided into 2 different types:

Terrigenous sediment: This type of sediment is mostly found in the continental shelf area and it consists of eroded material deposited here by rivers and glaciers on land. On the continental shelf one can find lots of ‘relict’ sediment, which was deposited here when the sea level was much lower during the Ice Age (about 12,000 years ago).

Pelagic sediment: Due to the large distance from the coastline, terrigenous sediment is less common in the deeper parts of the ocean. Here, pelagic sediment is the most common type of sediment. Pelagic sediment can be divided into 2 types: (i) ooze – covering almost 65% of the world’s ocean floor, ooze is composed of the debris of shells of plankton and foraminifera; (ii) red clay – this type of sediment contains less biological material than ooze. Its origin remains uncertain, although it is thought to be derived from dust blown across the ocean or from the deposition of chemicals in seawater. The deposition rate of red clay is extremely slow – it takes 1000 years for half a centimetre of sediment to form.