The Earth’s crust is the upper layer, ranging from 5-10 km for oceanic crust and 30-50 km for continental crust. It was formed 4.6-3.9 billion years ago and is composed of tectonic plates floating on top of the mantle. Oceanic crust is denser and constantly being subducted, while life on Earth is responsible for the current chemical composition of the crust.
The earth’s crust is its upper layer, with a thickness of 5 km (3 mi) to 10 km (6 mi) for oceanic crust and 30 km (20 mi) to 50 km (30 mi) for continental crust. This is less than 1% of the Earth’s entire depth. The crust is differentiated into an oceanic portion, composed of denser rocks such as basalt, diabase and gabbro, and a continental crust portion, composed of lighter rocks such as granite.
The earth’s crust was initially formed in the Hadean Eon, between 4.6 and 3.9 billion years ago. The Earth started out as a ball of molten rock, but over the course of 100-150 million years the surface cooled and hardened. The heavier elements, such as iron and nickel, mostly sank into the earth’s core, leaving the lighter elements on top. Today, the oldest known rocks are the Canadian Shield zircons dated to 4.4 billion years ago. Today’s crust itself ranges in age, from more than 3 billion years to less than a hundred million years for oceanic crust.
Both continental and oceanic crust are composed of tectonic plates floating on top of the underlying molten layer, the mantle. Separating the earth’s crust and mantle is a boundary called the Mohorovicic discontinuity, defined as a change in rock plasticity and seismic velocity. Much of what we know about the Earth’s internal structure was learned by measuring the velocities of seismic waves passing through it.
Oceanic crust, being denser than continental crust, is constantly being subducted, pulled under other tectonic plates, and pushed into the mantle, where it melts. Large rift valleys form where plates move apart, while mountains form where they collide. In these rift valleys, magma rises from below the surface to replace crust lost to subduction. The entire oceanic crust is recycled every two hundred million years.
Life on Earth is mainly responsible for the current chemical composition of the earth’s crust. Because oxygen is a byproduct of photosynthesis and photosynthesizing organisms evolved over 2 billion years ago, large amounts of oxygen have been released throughout the Earth’s history, and over 99% of the crust is composed of various oxides. Some exceptions are chlorine, sulfur and fluorine. Silicon oxide (SiO2) is the most abundant oxide, making up about 60% of the crust, followed by aluminum oxide (Al2O3) making up 15%, with calcium oxide, magnesium oxide, iron oxide , titanium oxide and a few others that make the rest.
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