Geology Reference
In-Depth Information
Birthing new seafloor at mid-ocean ridges
In Chapter 8, I briefly describe how we can date the minerals in seafloor rocks to know
how old the ocean crust is. And in Chapter 9 I describe
divergent boundaries,
where two
plates are moving away from each other. In this section, I pull these two ideas together
and explain how mantle convection creates new oceanic crust at mid-ocean ridges.
Mid-ocean ridges are mountain ranges on the seafloor. They are created by
magma welling up beneath a divergent boundary, where two plates are moving
apart. The magma erupts, cools, and forms rocky ridges of basalt. The eruption
results from the convection of heated mantle rocks as I discuss earlier in the
chapter. The production of magma is similar to what occurs at volcanic hot spots,
except that it occurs along the plate edge. Heated mantle rocks transfer heat to
the crustal rocks, and some of the minerals melt into a magma. The magma con-
tinues to rise until it erupts along the boundary edge, creating a chain of basalt
rock mountains — a mid-ocean ridge.
The magma at a mid-ocean ridge is the result of partial melting, similar to magmas at
volcanic arcs and hot spots. However, unlike basalt from plate subduction and melting,
mid-ocean ridge basalt has low amounts of silica and high amounts of magnesium, calci-
um, and iron: elements from minerals that are found in deep mantle rocks (such as
peridotite,
which I describe in Chapter 7).
The production of basalt at mid-ocean ridges tells us that heated mantle rock is moving
upward and cooling. This convection of the mantle beneath oceanic crust is the basis
for the slab-pull and ridge-push models of mantle convection and plate movement that I
introduced earlier in the chapter.
Shake, Rattle, and Roll: How Plate Move-
ments Cause Earthquakes
Whatever initiates mantle convection, we know that convection continually drives
crustal plates toward one another. It seems like the earth's crust is pretty solid and the
continental plates are packed on pretty tightly, so when a plate or two start moving,
they really shake things up!
