Geology of Missouri  The geology of Missouri includes deep Precambrian basement rocks formed within the last two billion years and overlain by thick sequences of marine sedimentary rocks, interspersed with igneous rocks by periods of volcanic activity. Missouri is a leading producer of lead from minerals formed in Paleozoic dolomite. The oldest rocks in Missouri are igneous and metamorphic crystalline basement rocks formed during the Proterozoic through the accretion of volcanic island arcs to the southern shore of the proto-North American continent of Laurentia. Rocks formed from 1.7 to 1.6 billion years ago are primarily known from deep boreholes in the north of the state, but are universally deeply buried by thick layers of sedimentary rock. Younger igneous rocks formed from magma in the south between 1.5 and 1.4 billion years ago. In places, magma intruded preexisting basement rock and then erupted at the surface as volcanoes along the Laurentian coastline. Granite and lava flows remain in the St. Francois Mountains from this period. Middle Proterozoic basement rocks are cut by the Central Missouri Tectonic Zone and Grand River fault zone, both of which are hypothesized to be the remnants of boundaries between small plates. Magma often moves along these fault zones. Between the two zones is the Missouri Gravity Low, or MGL, a mass of low density granite including the Missouri batholith up to 370 miles long and 60 miles wide, identified in gravity surveys. Igneous activity ended around 1.3 billion years ago, with the intrusion of numerous dikes and sills into newly crystallized rhyolite and granite. Although not directly impacting the current boundaries of Missouri, the Midcontinent Rift System formed from 1.2 to one billion years ago as mafic lave erupted in a rift zone spanning Lake Superior through Iowa to Kansas. Laurentia was included in the supercontinent Rodinia from one billion to 541 million years ago and no new rock formation took place in Missouri. Failed rifting of the continent produced the Reelfoot Rift, which extends beneath the Mississippi Embayment to the southeast lowlands of the state and intersects the Missouri Gravity Low, creating the New Madrid Seismic Zone. Sloss sequences—long-running changes in sea level, described by American geologist Lawrence Sloss—played an important role in sediment deposition during the Paleozoic. The Sauk sequence formed during the Cambrian, followed by the Tippecanoe in the Ordovician, the Kaskaskia between the full immersion of the Silurian and the early Mississippian.